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Biochemistry_Lippincott_583	Biochemistry_Lippinco	There are six major types of GAG, including chondroitin 4-and 6-sulfates, keratan sulfate, dermatan sulfate, heparin, heparan sulfate, and hyaluronic acid. All GAG, except hyaluronic acid, are found covalently attached to a core protein, forming proteoglycan monomers. Many proteoglycan monomers associate with a molecule of hyaluronic acid to form proteoglycan aggregates. GAG are synthesized in the Golgi. The polysaccharide chains are elongated by the sequential addition of alternating acidic and amino sugars, donated by their UDP derivatives. D-Glucuronate may be epimerized to L-iduronate. The last step in synthesis is sulfation of some of the amino sugars. The source of the sulfate is 3′-phosphoadenosyl-5′-phosphosulfate (PAPS). The completed proteoglycans are secreted into the extracellular matrix (ECM) or remain associated with the outer surface of cells. GAG are degraded by lysosomal acid hydrolases. They are first broken down to oligosaccharides, which are degraded sequentially	Biochemistry_Lippinco. There are six major types of GAG, including chondroitin 4-and 6-sulfates, keratan sulfate, dermatan sulfate, heparin, heparan sulfate, and hyaluronic acid. All GAG, except hyaluronic acid, are found covalently attached to a core protein, forming proteoglycan monomers. Many proteoglycan monomers associate with a molecule of hyaluronic acid to form proteoglycan aggregates. GAG are synthesized in the Golgi. The polysaccharide chains are elongated by the sequential addition of alternating acidic and amino sugars, donated by their UDP derivatives. D-Glucuronate may be epimerized to L-iduronate. The last step in synthesis is sulfation of some of the amino sugars. The source of the sulfate is 3′-phosphoadenosyl-5′-phosphosulfate (PAPS). The completed proteoglycans are secreted into the extracellular matrix (ECM) or remain associated with the outer surface of cells. GAG are degraded by lysosomal acid hydrolases. They are first broken down to oligosaccharides, which are degraded sequentially
Biochemistry_Lippincott_584	Biochemistry_Lippinco	matrix (ECM) or remain associated with the outer surface of cells. GAG are degraded by lysosomal acid hydrolases. They are first broken down to oligosaccharides, which are degraded sequentially from the nonreducing end of each chain. A deficiency of any one of the hydrolases results in a mucopolysaccharidosis. These are hereditary disorders in which GAG accumulate in tissues, causing symptoms such as skeletal and ECM deformities and intellectual disability. Examples of these genetic diseases include Hunter (X-linked) and Hurler syndromes. Glycoproteins are proteins to which oligosaccharides (glycans) are covalently attached. They differ from the proteoglycans in that the length of the glycoprotein’s carbohydrate chain is relatively short (usually two to ten sugar residues long, although it can be longer), may be branched, and does not contain serial disaccharide units. Membrane-bound glycoproteins participate in a broad range of cellular phenomena, including cell-surface recognition	Biochemistry_Lippinco. matrix (ECM) or remain associated with the outer surface of cells. GAG are degraded by lysosomal acid hydrolases. They are first broken down to oligosaccharides, which are degraded sequentially from the nonreducing end of each chain. A deficiency of any one of the hydrolases results in a mucopolysaccharidosis. These are hereditary disorders in which GAG accumulate in tissues, causing symptoms such as skeletal and ECM deformities and intellectual disability. Examples of these genetic diseases include Hunter (X-linked) and Hurler syndromes. Glycoproteins are proteins to which oligosaccharides (glycans) are covalently attached. They differ from the proteoglycans in that the length of the glycoprotein’s carbohydrate chain is relatively short (usually two to ten sugar residues long, although it can be longer), may be branched, and does not contain serial disaccharide units. Membrane-bound glycoproteins participate in a broad range of cellular phenomena, including cell-surface recognition
Biochemistry_Lippincott_585	Biochemistry_Lippinco	it can be longer), may be branched, and does not contain serial disaccharide units. Membrane-bound glycoproteins participate in a broad range of cellular phenomena, including cell-surface recognition (by other cells, hormones, and viruses), cell-surface antigenicity (such as the blood group antigens), and as components of the ECM and of the mucins of the gastrointestinal and urogenital tracts, where they act as protective biologic lubricants. In addition, almost all of the globular proteins present in human plasma are glycoproteins. Glycoproteins are synthesized in the rough endoplasmic reticulum (RER) and the Golgi. The precursors of the carbohydrate components of glycoproteins are nucleotide sugars. O-Linked glycoproteins are synthesized in the Golgi by the sequential transfer of sugars from their nucleotide carriers to the hydroxyl group of a serine or threonine residue in the protein. N-Linked glycoproteins are synthesized by the transfer of a preformed oligosaccharide from its	Biochemistry_Lippinco. it can be longer), may be branched, and does not contain serial disaccharide units. Membrane-bound glycoproteins participate in a broad range of cellular phenomena, including cell-surface recognition (by other cells, hormones, and viruses), cell-surface antigenicity (such as the blood group antigens), and as components of the ECM and of the mucins of the gastrointestinal and urogenital tracts, where they act as protective biologic lubricants. In addition, almost all of the globular proteins present in human plasma are glycoproteins. Glycoproteins are synthesized in the rough endoplasmic reticulum (RER) and the Golgi. The precursors of the carbohydrate components of glycoproteins are nucleotide sugars. O-Linked glycoproteins are synthesized in the Golgi by the sequential transfer of sugars from their nucleotide carriers to the hydroxyl group of a serine or threonine residue in the protein. N-Linked glycoproteins are synthesized by the transfer of a preformed oligosaccharide from its
Biochemistry_Lippincott_586	Biochemistry_Lippinco	from their nucleotide carriers to the hydroxyl group of a serine or threonine residue in the protein. N-Linked glycoproteins are synthesized by the transfer of a preformed oligosaccharide from its RER membrane lipid carrier, dolichol pyrophosphate, to the amide N of an asparagine residue in the protein. They contain varying amounts of mannose. A deficiency in the phosphotransferase that phosphorylates mannose residues at carbon 6 in N-linked glycoprotein enzymes destined for the lysosomes results in I-cell disease. Glycoproteins are degraded in lysosomes by acid hydrolases. A deficiency of any one of these enzymes results in a lysosomal glycoprotein storage disease (oligosaccharidosis), resulting in accumulation of partially degraded structures in the lysosome.	Biochemistry_Lippinco. from their nucleotide carriers to the hydroxyl group of a serine or threonine residue in the protein. N-Linked glycoproteins are synthesized by the transfer of a preformed oligosaccharide from its RER membrane lipid carrier, dolichol pyrophosphate, to the amide N of an asparagine residue in the protein. They contain varying amounts of mannose. A deficiency in the phosphotransferase that phosphorylates mannose residues at carbon 6 in N-linked glycoprotein enzymes destined for the lysosomes results in I-cell disease. Glycoproteins are degraded in lysosomes by acid hydrolases. A deficiency of any one of these enzymes results in a lysosomal glycoprotein storage disease (oligosaccharidosis), resulting in accumulation of partially degraded structures in the lysosome.
Biochemistry_Lippincott_587	Biochemistry_Lippinco	Choose the ONE best answer. 4.1. Mucopolysaccharidoses are hereditary lysosomal storage diseases. They are caused by: A. defects in the degradation of glycosaminoglycans. B. defects in the targeting of enzymes to lysosomes. C. an increased rate of synthesis of the carbohydrate component of proteoglycans. D. an insufficient rate of synthesis of proteolytic enzymes. E. the synthesis of abnormally small amounts of core proteins. F. the synthesis of heteropolysaccharides with an altered structure. Correct answer = A. The mucopolysaccharidoses are caused by deficiencies in any one of the lysosomal acid hydrolases responsible for the degradation of glycosaminoglycans (not proteins). The enzyme is correctly targeted to the lysosome, so blood levels of the enzyme do not increase, but it is nonfunctional. In these diseases, synthesis of the protein and carbohydrate components of proteoglycans is unaffected, in terms of both structure and amount.	Biochemistry_Lippinco. Choose the ONE best answer. 4.1. Mucopolysaccharidoses are hereditary lysosomal storage diseases. They are caused by: A. defects in the degradation of glycosaminoglycans. B. defects in the targeting of enzymes to lysosomes. C. an increased rate of synthesis of the carbohydrate component of proteoglycans. D. an insufficient rate of synthesis of proteolytic enzymes. E. the synthesis of abnormally small amounts of core proteins. F. the synthesis of heteropolysaccharides with an altered structure. Correct answer = A. The mucopolysaccharidoses are caused by deficiencies in any one of the lysosomal acid hydrolases responsible for the degradation of glycosaminoglycans (not proteins). The enzyme is correctly targeted to the lysosome, so blood levels of the enzyme do not increase, but it is nonfunctional. In these diseases, synthesis of the protein and carbohydrate components of proteoglycans is unaffected, in terms of both structure and amount.
Biochemistry_Lippincott_588	Biochemistry_Lippinco	4.2. The presence of the following compound in the urine of a patient suggests a deficiency in which one of the enzymes listed below? A. Galactosidase B. Glucuronidase C. Iduronidase D. Mannosidase E. Sulfatase Correct answer = E. Degradation of glycoproteins follows the rule: last on, first off. Because sulfation is the last step in the synthesis of this sequence, a sulfatase is required for the next step in the degradation of the compound shown. 4.3. An 8-month-old boy with coarse facial features, skeletal abnormalities, and delays in both growth and development is diagnosed with I-cell disease based on his presentation and on histologic and biochemical testing. I-Cell disease is characterized by: A. decreased production of cell surface O-linked glycoproteins. B. elevated levels of acid hydrolases in the blood. C. inability to N-glycosylate proteins. D. increased synthesis of proteoglycans. E. oligosaccharides in the urine.	Biochemistry_Lippinco. 4.2. The presence of the following compound in the urine of a patient suggests a deficiency in which one of the enzymes listed below? A. Galactosidase B. Glucuronidase C. Iduronidase D. Mannosidase E. Sulfatase Correct answer = E. Degradation of glycoproteins follows the rule: last on, first off. Because sulfation is the last step in the synthesis of this sequence, a sulfatase is required for the next step in the degradation of the compound shown. 4.3. An 8-month-old boy with coarse facial features, skeletal abnormalities, and delays in both growth and development is diagnosed with I-cell disease based on his presentation and on histologic and biochemical testing. I-Cell disease is characterized by: A. decreased production of cell surface O-linked glycoproteins. B. elevated levels of acid hydrolases in the blood. C. inability to N-glycosylate proteins. D. increased synthesis of proteoglycans. E. oligosaccharides in the urine.
Biochemistry_Lippincott_589	Biochemistry_Lippinco	B. elevated levels of acid hydrolases in the blood. C. inability to N-glycosylate proteins. D. increased synthesis of proteoglycans. E. oligosaccharides in the urine. Correct answer = B. I-Cell disease is a lysosomal storage disease caused by deficiency of the phosphotransferase needed for synthesis of the mannose 6phosphate signal that targets acid hydrolases to the lysosomal matrix. This results in secretion of these enzymes from the cell and accumulation of materials within the lysosome because of impaired degradation. None of the other choices relates to I-cell disease or lysosomal function. Oligosaccharides in the urine are characteristic of the muco-and polysaccharidoses but not I-cell disease (a type II mucolipidosis). 4.4. An infant with corneal clouding has dermatan sulfate and heparan sulfate in his urine. Decreased activity of which of the enzymes listed below would confirm the suspected diagnosis of Hurler syndrome? A. α-L-Iduronidase B. α-Glucuronidase	Biochemistry_Lippinco. B. elevated levels of acid hydrolases in the blood. C. inability to N-glycosylate proteins. D. increased synthesis of proteoglycans. E. oligosaccharides in the urine. Correct answer = B. I-Cell disease is a lysosomal storage disease caused by deficiency of the phosphotransferase needed for synthesis of the mannose 6phosphate signal that targets acid hydrolases to the lysosomal matrix. This results in secretion of these enzymes from the cell and accumulation of materials within the lysosome because of impaired degradation. None of the other choices relates to I-cell disease or lysosomal function. Oligosaccharides in the urine are characteristic of the muco-and polysaccharidoses but not I-cell disease (a type II mucolipidosis). 4.4. An infant with corneal clouding has dermatan sulfate and heparan sulfate in his urine. Decreased activity of which of the enzymes listed below would confirm the suspected diagnosis of Hurler syndrome? A. α-L-Iduronidase B. α-Glucuronidase
Biochemistry_Lippincott_590	Biochemistry_Lippinco	A. α-L-Iduronidase B. α-Glucuronidase C. Glycosyltransferase D. Iduronate sulfatase Correct answer = A. Hurler syndrome, a defect in the lysosomal degradation of glycosaminoglycans (GAG) with corneal clouding, is due to a deficiency in αL-iduronidase. β-Glucuronidase is deficient in Sly syndrome, and iduronate sulfatase is deficient in Hunter syndrome. Glycosyltransferases are enzymes of GAG synthesis. 4.5. Distinguish between glycoproteins and proteoglycans. Glycoproteins are proteins to which short, branched, structurally diverse oligosaccharide chains (glycans) are attached. Proteoglycans consist of a core protein to which long, unbranched, glycosaminoglycan (GAG) chains are attached. GAG are large complexes of negatively charged heteropolysaccharides composed of repeating [acidic sugar-amino sugar]n disaccharide units. For additional ancillary materials related to this chapter, please visit thePoint. I. OVERVIEW	Biochemistry_Lippinco. A. α-L-Iduronidase B. α-Glucuronidase C. Glycosyltransferase D. Iduronate sulfatase Correct answer = A. Hurler syndrome, a defect in the lysosomal degradation of glycosaminoglycans (GAG) with corneal clouding, is due to a deficiency in αL-iduronidase. β-Glucuronidase is deficient in Sly syndrome, and iduronate sulfatase is deficient in Hunter syndrome. Glycosyltransferases are enzymes of GAG synthesis. 4.5. Distinguish between glycoproteins and proteoglycans. Glycoproteins are proteins to which short, branched, structurally diverse oligosaccharide chains (glycans) are attached. Proteoglycans consist of a core protein to which long, unbranched, glycosaminoglycan (GAG) chains are attached. GAG are large complexes of negatively charged heteropolysaccharides composed of repeating [acidic sugar-amino sugar]n disaccharide units. For additional ancillary materials related to this chapter, please visit thePoint. I. OVERVIEW
Biochemistry_Lippincott_591	Biochemistry_Lippinco	I. OVERVIEW Lipids are a heterogeneous group of water-insoluble (hydrophobic) organic molecules (Fig. 15.1). Because of their insolubility in aqueous solutions, body lipids are generally found compartmentalized, as in the case of membrane-associated lipids or droplets of triacylglycerol in adipocytes, or transported in blood in association with protein, as in lipoprotein particles (see p. 227) or on albumin. Lipids are a major source of energy for the body, and they also provide the hydrophobic barrier that permits partitioning of the aqueous contents of cells and subcellular structures. Lipids serve additional functions in the body (for example, some fat-soluble vitamins have regulatory or coenzyme functions, and the prostaglandins and steroid hormones play major roles in the control of the body’s homeostasis). Deficiencies or imbalances of lipid metabolism can lead to some of the major clinical problems encountered by physicians, such as atherosclerosis, diabetes, and obesity.	Biochemistry_Lippinco. I. OVERVIEW Lipids are a heterogeneous group of water-insoluble (hydrophobic) organic molecules (Fig. 15.1). Because of their insolubility in aqueous solutions, body lipids are generally found compartmentalized, as in the case of membrane-associated lipids or droplets of triacylglycerol in adipocytes, or transported in blood in association with protein, as in lipoprotein particles (see p. 227) or on albumin. Lipids are a major source of energy for the body, and they also provide the hydrophobic barrier that permits partitioning of the aqueous contents of cells and subcellular structures. Lipids serve additional functions in the body (for example, some fat-soluble vitamins have regulatory or coenzyme functions, and the prostaglandins and steroid hormones play major roles in the control of the body’s homeostasis). Deficiencies or imbalances of lipid metabolism can lead to some of the major clinical problems encountered by physicians, such as atherosclerosis, diabetes, and obesity.
Biochemistry_Lippincott_592	Biochemistry_Lippinco	II. DIGESTION, ABSORPTION, SECRETION, AND UTILIZATION The average daily intake of lipids by U.S. adults is ~78 g, of which >90% is triacylglycerol ([TAG], formerly called triglyceride [TG]), that consists of three fatty acids (FA) esterified to a glycerol backbone (see Fig. 15.1). The remainder of the dietary lipids consists primarily of cholesterol, cholesteryl esters, phospholipids, and nonesterified (free) FA (FFA). The digestion of dietary lipids begins in the stomach and is completed in the small intestine. The process is summarized in Figure 15.2. A. Digestion in the stomach	Biochemistry_Lippinco. II. DIGESTION, ABSORPTION, SECRETION, AND UTILIZATION The average daily intake of lipids by U.S. adults is ~78 g, of which >90% is triacylglycerol ([TAG], formerly called triglyceride [TG]), that consists of three fatty acids (FA) esterified to a glycerol backbone (see Fig. 15.1). The remainder of the dietary lipids consists primarily of cholesterol, cholesteryl esters, phospholipids, and nonesterified (free) FA (FFA). The digestion of dietary lipids begins in the stomach and is completed in the small intestine. The process is summarized in Figure 15.2. A. Digestion in the stomach
Biochemistry_Lippincott_593	Biochemistry_Lippinco	A. Digestion in the stomach Lipid digestion in the stomach is limited. It is catalyzed by lingual lipase that originates from glands at the back of the tongue and gastric lipase that is secreted by the gastric mucosa. Both enzymes are relatively acid stable, with optimal pH values of 4 to 6. These acid lipases hydrolyze FA from TAG molecules, particularly those containing short-or medium-chainlength (≤12 carbons) FA such as are found in milk fat. Consequently, these lipases play a particularly important role in lipid digestion in infants for whom milk fat is the primary source of calories. They also become important digestive enzymes in individuals with pancreatic insufficiency such as those with cystic fibrosis (CF). Lingual and gastric lipases aid these patients in degrading TAG molecules (especially those with short-to medium-chain FA) despite a near or complete absence of pancreatic lipase (see Section D.1. below). B. Cystic fibrosis	Biochemistry_Lippinco. A. Digestion in the stomach Lipid digestion in the stomach is limited. It is catalyzed by lingual lipase that originates from glands at the back of the tongue and gastric lipase that is secreted by the gastric mucosa. Both enzymes are relatively acid stable, with optimal pH values of 4 to 6. These acid lipases hydrolyze FA from TAG molecules, particularly those containing short-or medium-chainlength (≤12 carbons) FA such as are found in milk fat. Consequently, these lipases play a particularly important role in lipid digestion in infants for whom milk fat is the primary source of calories. They also become important digestive enzymes in individuals with pancreatic insufficiency such as those with cystic fibrosis (CF). Lingual and gastric lipases aid these patients in degrading TAG molecules (especially those with short-to medium-chain FA) despite a near or complete absence of pancreatic lipase (see Section D.1. below). B. Cystic fibrosis
Biochemistry_Lippincott_594	Biochemistry_Lippinco	B. Cystic fibrosis CF is the most common lethal genetic disease in Caucasians of Northern European ancestry and has a prevalence of ~1:3,300 births in the United States. CF is an autosomal-recessive disorder caused by mutations to the gene for the CF transmembrane conductance regulator (CFTR) protein that functions as a chloride channel on epithelium in the pancreas, lungs, testes, and sweat glands. Defective CFTR results in decreased secretion of chloride and increased uptake of sodium and water. In the pancreas, the depletion of water on the cell surface results in thickened mucus that clogs the pancreatic ducts, preventing pancreatic enzymes from reaching the intestine, thereby leading to pancreatic insufficiency. Treatment includes replacement of these enzymes and supplementation with fat-soluble vitamins. [Note: CF also causes chronic lung infections with progressive pulmonary disease and male infertility.] C. Emulsification in the small intestine	Biochemistry_Lippinco. B. Cystic fibrosis CF is the most common lethal genetic disease in Caucasians of Northern European ancestry and has a prevalence of ~1:3,300 births in the United States. CF is an autosomal-recessive disorder caused by mutations to the gene for the CF transmembrane conductance regulator (CFTR) protein that functions as a chloride channel on epithelium in the pancreas, lungs, testes, and sweat glands. Defective CFTR results in decreased secretion of chloride and increased uptake of sodium and water. In the pancreas, the depletion of water on the cell surface results in thickened mucus that clogs the pancreatic ducts, preventing pancreatic enzymes from reaching the intestine, thereby leading to pancreatic insufficiency. Treatment includes replacement of these enzymes and supplementation with fat-soluble vitamins. [Note: CF also causes chronic lung infections with progressive pulmonary disease and male infertility.] C. Emulsification in the small intestine
Biochemistry_Lippincott_595	Biochemistry_Lippinco	C. Emulsification in the small intestine The critical process of dietary lipid emulsification occurs in the duodenum.	Biochemistry_Lippinco. C. Emulsification in the small intestine The critical process of dietary lipid emulsification occurs in the duodenum.
Biochemistry_Lippincott_596	Biochemistry_Lippinco	Emulsification increases the surface area of the hydrophobic lipid droplets so that the digestive enzymes, which work at the interface of the droplet and the surrounding aqueous solution, can act effectively. Emulsification is accomplished by two complementary mechanisms, namely, use of the detergent properties of the conjugated bile salts and mechanical mixing due to peristalsis. Bile salts, made in the liver and stored in the gallbladder, are amphipathic derivatives of cholesterol (see p. 224). Conjugated bile salts consist of a hydroxylated sterol ring structure with a side chain to which a molecule of glycine or taurine is covalently attached by an amide linkage (Fig. 15.3). These emulsifying agents interact with the dietary lipid droplets and the aqueous duodenal contents, thereby stabilizing the droplets as they become smaller from peristalsis and preventing them from coalescing. [Note: See p. 225 for a more complete discussion of bile salt metabolism.]	Biochemistry_Lippinco. Emulsification increases the surface area of the hydrophobic lipid droplets so that the digestive enzymes, which work at the interface of the droplet and the surrounding aqueous solution, can act effectively. Emulsification is accomplished by two complementary mechanisms, namely, use of the detergent properties of the conjugated bile salts and mechanical mixing due to peristalsis. Bile salts, made in the liver and stored in the gallbladder, are amphipathic derivatives of cholesterol (see p. 224). Conjugated bile salts consist of a hydroxylated sterol ring structure with a side chain to which a molecule of glycine or taurine is covalently attached by an amide linkage (Fig. 15.3). These emulsifying agents interact with the dietary lipid droplets and the aqueous duodenal contents, thereby stabilizing the droplets as they become smaller from peristalsis and preventing them from coalescing. [Note: See p. 225 for a more complete discussion of bile salt metabolism.]
Biochemistry_Lippincott_597	Biochemistry_Lippinco	D. Degradation by pancreatic enzymes The dietary TAG, cholesteryl esters, and phospholipids are enzymatically degraded (digested) in the small intestine by pancreatic enzymes, whose secretion is hormonally controlled. 1.	Biochemistry_Lippinco. D. Degradation by pancreatic enzymes The dietary TAG, cholesteryl esters, and phospholipids are enzymatically degraded (digested) in the small intestine by pancreatic enzymes, whose secretion is hormonally controlled. 1.
Biochemistry_Lippincott_598	Biochemistry_Lippinco	Triacylglycerol degradation: TAG molecules are too large to be taken up efficiently by the mucosal cells (enterocytes) of the intestinal villi. Therefore, they are hydrolyzed by an esterase, pancreatic lipase, which preferentially removes the FA at carbons 1 and 3. The primary products of hydrolysis are, thus, a mixture of 2-monoacylglycerol (2-MAG) and FFA (see Fig. 15.2). [Note: Pancreatic lipase is found in high concentrations in pancreatic secretions (2%–3% of the total protein present), and it is highly efficient catalytically, thus insuring that only severe pancreatic deficiency, such as that seen in CF, results in significant malabsorption of fat.] A second protein, colipase, also secreted by the pancreas, binds the lipase at a ratio of 1:1 and anchors it at the lipid–aqueous interface. Colipase restores activity to lipase in the presence of inhibitory substances like bile salts that bind the micelles. [Note: Colipase is secreted as the zymogen, procolipase, which is activated	Biochemistry_Lippinco. Triacylglycerol degradation: TAG molecules are too large to be taken up efficiently by the mucosal cells (enterocytes) of the intestinal villi. Therefore, they are hydrolyzed by an esterase, pancreatic lipase, which preferentially removes the FA at carbons 1 and 3. The primary products of hydrolysis are, thus, a mixture of 2-monoacylglycerol (2-MAG) and FFA (see Fig. 15.2). [Note: Pancreatic lipase is found in high concentrations in pancreatic secretions (2%–3% of the total protein present), and it is highly efficient catalytically, thus insuring that only severe pancreatic deficiency, such as that seen in CF, results in significant malabsorption of fat.] A second protein, colipase, also secreted by the pancreas, binds the lipase at a ratio of 1:1 and anchors it at the lipid–aqueous interface. Colipase restores activity to lipase in the presence of inhibitory substances like bile salts that bind the micelles. [Note: Colipase is secreted as the zymogen, procolipase, which is activated
Biochemistry_Lippincott_599	Biochemistry_Lippinco	Colipase restores activity to lipase in the presence of inhibitory substances like bile salts that bind the micelles. [Note: Colipase is secreted as the zymogen, procolipase, which is activated in the intestine by trypsin.] Orlistat, an antiobesity drug, inhibits gastric and pancreatic lipases, thereby decreasing fat absorption, resulting in weight loss.	Biochemistry_Lippinco. Colipase restores activity to lipase in the presence of inhibitory substances like bile salts that bind the micelles. [Note: Colipase is secreted as the zymogen, procolipase, which is activated in the intestine by trypsin.] Orlistat, an antiobesity drug, inhibits gastric and pancreatic lipases, thereby decreasing fat absorption, resulting in weight loss.
Biochemistry_Lippincott_600	Biochemistry_Lippinco	2. Cholesteryl ester degradation: Most dietary cholesterol is present in the free (nonesterified) form, with 10%–15% present in the esterified form. Cholesteryl esters are hydrolyzed by pancreatic cholesteryl ester hydrolase (cholesterol esterase), which produces cholesterol plus FFA (see Fig. 15.2). Activity of this enzyme is greatly increased in the presence of bile salts. 3.	Biochemistry_Lippinco. 2. Cholesteryl ester degradation: Most dietary cholesterol is present in the free (nonesterified) form, with 10%–15% present in the esterified form. Cholesteryl esters are hydrolyzed by pancreatic cholesteryl ester hydrolase (cholesterol esterase), which produces cholesterol plus FFA (see Fig. 15.2). Activity of this enzyme is greatly increased in the presence of bile salts. 3.
Biochemistry_Lippincott_601	Biochemistry_Lippinco	3. Phospholipid degradation: Pancreatic juice is rich in the proenzyme of phospholipase A2 that, like procolipase, is activated by trypsin and, like cholesteryl ester hydrolase, requires bile salts for optimum activity. Phospholipase A2 removes one FA from carbon 2 of a phospholipid, leaving a lysophospholipid. For example, phosphatidylcholine (the predominant phospholipid of digestion) becomes lysophosphatidylcholine. The remaining FA at carbon 1 can be removed by lysophospholipase, leaving a glycerylphosphoryl base (for example, glycerylphosphorylcholine, see Fig. 15.2) that may be excreted in the feces, further degraded, or absorbed.	Biochemistry_Lippinco. 3. Phospholipid degradation: Pancreatic juice is rich in the proenzyme of phospholipase A2 that, like procolipase, is activated by trypsin and, like cholesteryl ester hydrolase, requires bile salts for optimum activity. Phospholipase A2 removes one FA from carbon 2 of a phospholipid, leaving a lysophospholipid. For example, phosphatidylcholine (the predominant phospholipid of digestion) becomes lysophosphatidylcholine. The remaining FA at carbon 1 can be removed by lysophospholipase, leaving a glycerylphosphoryl base (for example, glycerylphosphorylcholine, see Fig. 15.2) that may be excreted in the feces, further degraded, or absorbed.
Biochemistry_Lippincott_602	Biochemistry_Lippinco	4. Control: Pancreatic secretion of the hydrolytic enzymes that degrade dietary lipids in the small intestine is hormonally controlled (Fig. 15.4). Cells in the mucosa of the lower duodenum and jejunum produce the peptide hormone cholecystokinin (CCK), in response to the presence of lipids and partially digested proteins entering these regions of the upper small intestine. CCK acts on the gallbladder (causing it to contract and release bile, a mixture of bile salts, phospholipids, and free cholesterol) and on the exocrine cells of the pancreas (causing them to release digestive enzymes). It also decreases gastric motility, resulting in a slower release of gastric contents into the small intestine (see p. 353). Other intestinal cells produce another peptide hormone, secretin, in response to the low pH of the chyme entering the intestine from the stomach. Secretin causes the pancreas to release a solution rich in bicarbonate that helps neutralize the pH of the intestinal contents,	Biochemistry_Lippinco. 4. Control: Pancreatic secretion of the hydrolytic enzymes that degrade dietary lipids in the small intestine is hormonally controlled (Fig. 15.4). Cells in the mucosa of the lower duodenum and jejunum produce the peptide hormone cholecystokinin (CCK), in response to the presence of lipids and partially digested proteins entering these regions of the upper small intestine. CCK acts on the gallbladder (causing it to contract and release bile, a mixture of bile salts, phospholipids, and free cholesterol) and on the exocrine cells of the pancreas (causing them to release digestive enzymes). It also decreases gastric motility, resulting in a slower release of gastric contents into the small intestine (see p. 353). Other intestinal cells produce another peptide hormone, secretin, in response to the low pH of the chyme entering the intestine from the stomach. Secretin causes the pancreas to release a solution rich in bicarbonate that helps neutralize the pH of the intestinal contents,
Biochemistry_Lippincott_603	Biochemistry_Lippinco	to the low pH of the chyme entering the intestine from the stomach. Secretin causes the pancreas to release a solution rich in bicarbonate that helps neutralize the pH of the intestinal contents, bringing them to the appropriate pH for digestive activity by pancreatic enzymes.	Biochemistry_Lippinco. to the low pH of the chyme entering the intestine from the stomach. Secretin causes the pancreas to release a solution rich in bicarbonate that helps neutralize the pH of the intestinal contents, bringing them to the appropriate pH for digestive activity by pancreatic enzymes.
Biochemistry_Lippincott_604	Biochemistry_Lippinco	E. Absorption by enterocytes	Biochemistry_Lippinco. E. Absorption by enterocytes
Biochemistry_Lippincott_605	Biochemistry_Lippinco	FFA, free cholesterol, and 2-MAG are the primary products of lipid digestion in the jejunum. These, plus bile salts and fat-soluble vitamins (A, D, E, and K), form mixed micelles (that is, disc-shaped clusters of a mixture of amphipathic lipids that coalesce with their hydrophobic groups on the inside and their hydrophilic groups on the outside). Therefore, mixed micelles are soluble in the aqueous environment of the intestinal lumen (Fig. 15.5). These particles approach the primary site of lipid absorption, the brush border membrane of the enterocytes. This microvilli-rich apical membrane is separated from the liquid contents of the intestinal lumen by an unstirred water layer that mixes poorly with the bulk fluid. The hydrophilic surface of the micelles facilitates the transport of the hydrophobic lipids through the unstirred water layer to the brush border membrane where they are absorbed. Bile salts are absorbed in the terminal ileum, with <5% being lost in the feces. [Note:	Biochemistry_Lippinco. FFA, free cholesterol, and 2-MAG are the primary products of lipid digestion in the jejunum. These, plus bile salts and fat-soluble vitamins (A, D, E, and K), form mixed micelles (that is, disc-shaped clusters of a mixture of amphipathic lipids that coalesce with their hydrophobic groups on the inside and their hydrophilic groups on the outside). Therefore, mixed micelles are soluble in the aqueous environment of the intestinal lumen (Fig. 15.5). These particles approach the primary site of lipid absorption, the brush border membrane of the enterocytes. This microvilli-rich apical membrane is separated from the liquid contents of the intestinal lumen by an unstirred water layer that mixes poorly with the bulk fluid. The hydrophilic surface of the micelles facilitates the transport of the hydrophobic lipids through the unstirred water layer to the brush border membrane where they are absorbed. Bile salts are absorbed in the terminal ileum, with <5% being lost in the feces. [Note:
Biochemistry_Lippincott_606	Biochemistry_Lippinco	the hydrophobic lipids through the unstirred water layer to the brush border membrane where they are absorbed. Bile salts are absorbed in the terminal ileum, with <5% being lost in the feces. [Note: Relative to other dietary lipids, cholesterol is only poorly absorbed by the enterocytes. Drug therapy (for example, with ezetimibe) can further reduce cholesterol absorption in the small intestine.] Because short-and medium-chain FA are water soluble, they do not require the assistance of mixed micelles for absorption by the intestinal mucosa.	Biochemistry_Lippinco. the hydrophobic lipids through the unstirred water layer to the brush border membrane where they are absorbed. Bile salts are absorbed in the terminal ileum, with <5% being lost in the feces. [Note: Relative to other dietary lipids, cholesterol is only poorly absorbed by the enterocytes. Drug therapy (for example, with ezetimibe) can further reduce cholesterol absorption in the small intestine.] Because short-and medium-chain FA are water soluble, they do not require the assistance of mixed micelles for absorption by the intestinal mucosa.
Biochemistry_Lippincott_607	Biochemistry_Lippinco	F. Triacylglycerol and cholesteryl ester resynthesis	Biochemistry_Lippinco. F. Triacylglycerol and cholesteryl ester resynthesis
Biochemistry_Lippincott_608	Biochemistry_Lippinco	The mixture of lipids absorbed by the enterocytes migrates to the smooth endoplasmic reticulum (SER) where biosynthesis of complex lipids takes place. The long-chain FA are first converted into their activated form by fatty acyl coenzyme A (CoA) synthetase (thiokinase), as shown in Figure 15.6. Using the fatty acyl CoA derivatives, the 2-MAG absorbed by the enterocytes are converted to TAG through sequential reacylations by two acyltransferases, acyl CoA:monoacylglycerol acyltransferase and acyl CoA:diacylglycerol acyltransferase. Lysophospholipids are reacylated to form phospholipids by a family of acyltransferases, and cholesterol is acylated primarily by acyl CoA:cholesterol acyltransferase (see p. 232). [Note: Virtually all long-chain FA entering the enterocytes are used in this fashion to form TAG, phospholipids, and cholesteryl esters. Short-and medium-chain FA are not converted to their CoA derivatives and are not reesterified to 2-MAG. Instead, they are released into the	Biochemistry_Lippinco. The mixture of lipids absorbed by the enterocytes migrates to the smooth endoplasmic reticulum (SER) where biosynthesis of complex lipids takes place. The long-chain FA are first converted into their activated form by fatty acyl coenzyme A (CoA) synthetase (thiokinase), as shown in Figure 15.6. Using the fatty acyl CoA derivatives, the 2-MAG absorbed by the enterocytes are converted to TAG through sequential reacylations by two acyltransferases, acyl CoA:monoacylglycerol acyltransferase and acyl CoA:diacylglycerol acyltransferase. Lysophospholipids are reacylated to form phospholipids by a family of acyltransferases, and cholesterol is acylated primarily by acyl CoA:cholesterol acyltransferase (see p. 232). [Note: Virtually all long-chain FA entering the enterocytes are used in this fashion to form TAG, phospholipids, and cholesteryl esters. Short-and medium-chain FA are not converted to their CoA derivatives and are not reesterified to 2-MAG. Instead, they are released into the
Biochemistry_Lippincott_609	Biochemistry_Lippinco	fashion to form TAG, phospholipids, and cholesteryl esters. Short-and medium-chain FA are not converted to their CoA derivatives and are not reesterified to 2-MAG. Instead, they are released into the portal circulation, where they are carried by serum albumin to the liver.]	Biochemistry_Lippinco. fashion to form TAG, phospholipids, and cholesteryl esters. Short-and medium-chain FA are not converted to their CoA derivatives and are not reesterified to 2-MAG. Instead, they are released into the portal circulation, where they are carried by serum albumin to the liver.]
Biochemistry_Lippincott_610	Biochemistry_Lippinco	G. Lipid malabsorption Lipid malabsorption, resulting in increased lipid (including the fat-soluble vitamins and essential FA, see p. 182) in the feces, a condition known as steatorrhea, can be caused by disturbances in lipid digestion and/or absorption (Fig. 15.7). Such disturbances can result from several conditions, including CF (causing poor digestion) and short bowel syndrome (causing decreased absorption). The ability of short-and medium-chain FA to be taken up by enterocytes without the aid of mixed micelles has made them important in medical nutrition therapy for individuals with malabsorption disorders. H. Secretion from enterocytes	Biochemistry_Lippinco. G. Lipid malabsorption Lipid malabsorption, resulting in increased lipid (including the fat-soluble vitamins and essential FA, see p. 182) in the feces, a condition known as steatorrhea, can be caused by disturbances in lipid digestion and/or absorption (Fig. 15.7). Such disturbances can result from several conditions, including CF (causing poor digestion) and short bowel syndrome (causing decreased absorption). The ability of short-and medium-chain FA to be taken up by enterocytes without the aid of mixed micelles has made them important in medical nutrition therapy for individuals with malabsorption disorders. H. Secretion from enterocytes
Biochemistry_Lippincott_611	Biochemistry_Lippinco	The newly resynthesized TAG and cholesteryl esters are very hydrophobic and aggregate in an aqueous environment. Therefore, they must be packaged as particles of lipid droplets surrounded by a thin layer composed of phospholipids, nonesterified cholesterol, and a molecule of the protein apolipoprotein (apo) B-48 (see p. 228). This layer stabilizes the particle and increases its solubility, thereby preventing multiple particles from coalescing. [Note: Microsomal triglyceride transfer protein is essential for the assembly of all TAG-rich apo B–containing particles in the ER (see p. 228).] The lipoprotein particles are released by exocytosis from enterocytes into the lacteals (lymphatic vessels in the villi of the small intestine). The presence of these particles in the lymph after a lipid-rich meal gives it a milky appearance. This lymph is called chyle (as opposed to chyme, the name given to the semifluid mass of partially digested food that passes from the stomach to the duodenum),	Biochemistry_Lippinco. The newly resynthesized TAG and cholesteryl esters are very hydrophobic and aggregate in an aqueous environment. Therefore, they must be packaged as particles of lipid droplets surrounded by a thin layer composed of phospholipids, nonesterified cholesterol, and a molecule of the protein apolipoprotein (apo) B-48 (see p. 228). This layer stabilizes the particle and increases its solubility, thereby preventing multiple particles from coalescing. [Note: Microsomal triglyceride transfer protein is essential for the assembly of all TAG-rich apo B–containing particles in the ER (see p. 228).] The lipoprotein particles are released by exocytosis from enterocytes into the lacteals (lymphatic vessels in the villi of the small intestine). The presence of these particles in the lymph after a lipid-rich meal gives it a milky appearance. This lymph is called chyle (as opposed to chyme, the name given to the semifluid mass of partially digested food that passes from the stomach to the duodenum),
Biochemistry_Lippincott_612	Biochemistry_Lippinco	meal gives it a milky appearance. This lymph is called chyle (as opposed to chyme, the name given to the semifluid mass of partially digested food that passes from the stomach to the duodenum), and the particles are named chylomicrons. Chylomicrons follow the lymphatic system to the thoracic duct and are then conveyed to the left subclavian vein, where they enter the blood. The steps in the production of chylomicrons are summarized in Figure 15.6. [Note: Once released into blood, the nascent (immature) chylomicrons pick up apolipoproteins E and C-II from high-density lipoproteins and mature. (For a more detailed description of chylomicron structure and metabolism, see p. 227.)]	Biochemistry_Lippinco. meal gives it a milky appearance. This lymph is called chyle (as opposed to chyme, the name given to the semifluid mass of partially digested food that passes from the stomach to the duodenum), and the particles are named chylomicrons. Chylomicrons follow the lymphatic system to the thoracic duct and are then conveyed to the left subclavian vein, where they enter the blood. The steps in the production of chylomicrons are summarized in Figure 15.6. [Note: Once released into blood, the nascent (immature) chylomicrons pick up apolipoproteins E and C-II from high-density lipoproteins and mature. (For a more detailed description of chylomicron structure and metabolism, see p. 227.)]
Biochemistry_Lippincott_613	Biochemistry_Lippinco	I. Use by the tissues Most of the TAG contained in chylomicrons is broken down in the capillary beds of skeletal and cardiac muscle and adipose tissue. The TAG is degraded to FFA and glycerol by lipoprotein lipase (LPL). This enzyme is synthesized and secreted primarily by adipocytes and muscle cells. Secreted LPL is anchored to the luminal surface of endothelial cells in the capillaries of muscle and adipose tissues. [Note: Familial chylomicronemia (type I hyperlipoproteinemia) is a rare, autosomal-recessive disorder caused by a deficiency of LPL or its coenzyme apo C-II (see p. 228). The result is fasting chylomicronemia and severe hypertriacylglycerolemia, which can cause pancreatitis.] 1.	Biochemistry_Lippinco. I. Use by the tissues Most of the TAG contained in chylomicrons is broken down in the capillary beds of skeletal and cardiac muscle and adipose tissue. The TAG is degraded to FFA and glycerol by lipoprotein lipase (LPL). This enzyme is synthesized and secreted primarily by adipocytes and muscle cells. Secreted LPL is anchored to the luminal surface of endothelial cells in the capillaries of muscle and adipose tissues. [Note: Familial chylomicronemia (type I hyperlipoproteinemia) is a rare, autosomal-recessive disorder caused by a deficiency of LPL or its coenzyme apo C-II (see p. 228). The result is fasting chylomicronemia and severe hypertriacylglycerolemia, which can cause pancreatitis.] 1.
Biochemistry_Lippincott_614	Biochemistry_Lippinco	Fate of free fatty acids: The FFA derived from the hydrolysis of TAG may either directly enter adjacent muscle cells and adipocytes or be transported in the blood in association with serum albumin until they are taken up by cells. [Note: Human serum albumin is a large protein secreted by the liver. It transports a number of primarily hydrophobic compounds in the circulation, including FFA and some drugs.] Most cells can oxidize FA to produce energy (see p. 190). Adipocytes can also reesterify FFA to produce TAG molecules, which are stored until the FA are needed by the body (see p. 188). 2. Fate of glycerol: Glycerol released from TAG is taken up from the blood and phosphorylated by hepatic glycerol kinase to produce glycerol 3phosphate, which can enter either glycolysis or gluconeogenesis by oxidation to dihydroxyacetone phosphate (see p. 101) or be used in TAG synthesis (see p. 189). 3.	Biochemistry_Lippinco. Fate of free fatty acids: The FFA derived from the hydrolysis of TAG may either directly enter adjacent muscle cells and adipocytes or be transported in the blood in association with serum albumin until they are taken up by cells. [Note: Human serum albumin is a large protein secreted by the liver. It transports a number of primarily hydrophobic compounds in the circulation, including FFA and some drugs.] Most cells can oxidize FA to produce energy (see p. 190). Adipocytes can also reesterify FFA to produce TAG molecules, which are stored until the FA are needed by the body (see p. 188). 2. Fate of glycerol: Glycerol released from TAG is taken up from the blood and phosphorylated by hepatic glycerol kinase to produce glycerol 3phosphate, which can enter either glycolysis or gluconeogenesis by oxidation to dihydroxyacetone phosphate (see p. 101) or be used in TAG synthesis (see p. 189). 3.
Biochemistry_Lippincott_615	Biochemistry_Lippinco	3. Fate of chylomicron remnants: After most of the TAG has been removed, the chylomicron remnants (which contain cholesteryl esters, phospholipids, apolipoproteins, fat-soluble vitamins, and a small amount of TAG) bind to receptors on the liver (apo E is the ligand; see p. 229) and are endocytosed. The intracellular remnants are hydrolyzed to their component parts. Cholesterol and the nitrogenous bases of phospholipids (for example, choline) can be recycled by the body. [Note: If removal of remnants by the liver is decreased because of impaired binding to their receptor, they accumulate in the plasma. This is seen in the rare type III hyperlipoproteinemia (also called familial dysbetalipoproteinemia or broad beta disease, see p. 231).] III. CHAPTER SUMMARY	Biochemistry_Lippinco. 3. Fate of chylomicron remnants: After most of the TAG has been removed, the chylomicron remnants (which contain cholesteryl esters, phospholipids, apolipoproteins, fat-soluble vitamins, and a small amount of TAG) bind to receptors on the liver (apo E is the ligand; see p. 229) and are endocytosed. The intracellular remnants are hydrolyzed to their component parts. Cholesterol and the nitrogenous bases of phospholipids (for example, choline) can be recycled by the body. [Note: If removal of remnants by the liver is decreased because of impaired binding to their receptor, they accumulate in the plasma. This is seen in the rare type III hyperlipoproteinemia (also called familial dysbetalipoproteinemia or broad beta disease, see p. 231).] III. CHAPTER SUMMARY
Biochemistry_Lippincott_616	Biochemistry_Lippinco	III. CHAPTER SUMMARY Dietary lipid digestion begins in the stomach and continues in the small intestine (Fig. 15.8). Cholesteryl esters, phospholipids, and triacylglycerols (TAG) containing long-chain-length fatty acids (FA) are degraded in the small intestine by pancreatic enzymes. The most important of these enzymes are cholesterol esterase, phospholipase A2, and pancreatic lipase.	Biochemistry_Lippinco. III. CHAPTER SUMMARY Dietary lipid digestion begins in the stomach and continues in the small intestine (Fig. 15.8). Cholesteryl esters, phospholipids, and triacylglycerols (TAG) containing long-chain-length fatty acids (FA) are degraded in the small intestine by pancreatic enzymes. The most important of these enzymes are cholesterol esterase, phospholipase A2, and pancreatic lipase.
Biochemistry_Lippincott_617	Biochemistry_Lippinco	In cystic fibrosis, thickened mucus prevents these enzymes reaching the intestine. In contrast, TAG in milk fat contain short-to medium-chainlength FA and are degraded in the stomach by acid lipases (lingual lipase and gastric lipase). The hydrophobic nature of lipids requires that dietary lipids be emulsified for efficient degradation. Emulsification occurs in the small intestine using peristaltic action (mechanical mixing) and bile salts (detergents). The primary products of dietary lipid degradation are 2monoacylglycerol, nonesterified (free) cholesterol, and free FA. These compounds, plus the fat-soluble vitamins, form mixed micelles that facilitate dietary lipid absorption by intestinal mucosal cells (enterocytes). These cells use activated long-chain FA to regenerate TAG and cholesteryl esters and also synthesize protein (apolipoprotein [apo] B-48), all of which are then assembled with the fat-soluble vitamins into lipoprotein particles called chylomicrons. Short-and	Biochemistry_Lippinco. In cystic fibrosis, thickened mucus prevents these enzymes reaching the intestine. In contrast, TAG in milk fat contain short-to medium-chainlength FA and are degraded in the stomach by acid lipases (lingual lipase and gastric lipase). The hydrophobic nature of lipids requires that dietary lipids be emulsified for efficient degradation. Emulsification occurs in the small intestine using peristaltic action (mechanical mixing) and bile salts (detergents). The primary products of dietary lipid degradation are 2monoacylglycerol, nonesterified (free) cholesterol, and free FA. These compounds, plus the fat-soluble vitamins, form mixed micelles that facilitate dietary lipid absorption by intestinal mucosal cells (enterocytes). These cells use activated long-chain FA to regenerate TAG and cholesteryl esters and also synthesize protein (apolipoprotein [apo] B-48), all of which are then assembled with the fat-soluble vitamins into lipoprotein particles called chylomicrons. Short-and
Biochemistry_Lippincott_618	Biochemistry_Lippinco	and cholesteryl esters and also synthesize protein (apolipoprotein [apo] B-48), all of which are then assembled with the fat-soluble vitamins into lipoprotein particles called chylomicrons. Short-and medium-chain FA enter blood directly. Chylomicrons are first released into the lymph and then enter the blood, where their lipid core is degraded by lipoprotein lipase (with apo C-II as the coenzyme) in the capillaries of muscle and adipose tissues. Thus, dietary lipids are made available to the peripheral tissues. Fat maldigestion or malabsorption causes steatorrhea (lipid in the feces). A deficiency in the ability to degrade chylomicron components, or remove chylomicron remnants after TAG has been degraded, results in accumulation of these particles in blood.	Biochemistry_Lippinco. and cholesteryl esters and also synthesize protein (apolipoprotein [apo] B-48), all of which are then assembled with the fat-soluble vitamins into lipoprotein particles called chylomicrons. Short-and medium-chain FA enter blood directly. Chylomicrons are first released into the lymph and then enter the blood, where their lipid core is degraded by lipoprotein lipase (with apo C-II as the coenzyme) in the capillaries of muscle and adipose tissues. Thus, dietary lipids are made available to the peripheral tissues. Fat maldigestion or malabsorption causes steatorrhea (lipid in the feces). A deficiency in the ability to degrade chylomicron components, or remove chylomicron remnants after TAG has been degraded, results in accumulation of these particles in blood.
Biochemistry_Lippincott_619	Biochemistry_Lippinco	Choose the ONE best answer. 5.1. Which one of the following statements about lipid digestion is correct? A. Large lipid droplets are emulsified (have their surface area increased) in the mouth through the act of chewing (mastication). B. The enzyme colipase facilitates the binding of bile salts to mixed micelles, maximizing the activity of pancreatic lipase. C. The peptide hormone secretin causes the gallbladder to contract and release bile. D. Patients with cystic fibrosis have difficulties with digestion because their pancreatic secretions are less able to reach the small intestine, the primary site of lipid digestion. E. Formation of triacylglycerol-rich chylomicrons is independent of protein synthesis in the intestinal mucosa.	Biochemistry_Lippinco. Choose the ONE best answer. 5.1. Which one of the following statements about lipid digestion is correct? A. Large lipid droplets are emulsified (have their surface area increased) in the mouth through the act of chewing (mastication). B. The enzyme colipase facilitates the binding of bile salts to mixed micelles, maximizing the activity of pancreatic lipase. C. The peptide hormone secretin causes the gallbladder to contract and release bile. D. Patients with cystic fibrosis have difficulties with digestion because their pancreatic secretions are less able to reach the small intestine, the primary site of lipid digestion. E. Formation of triacylglycerol-rich chylomicrons is independent of protein synthesis in the intestinal mucosa.
Biochemistry_Lippincott_620	Biochemistry_Lippinco	E. Formation of triacylglycerol-rich chylomicrons is independent of protein synthesis in the intestinal mucosa. Correct answer = D. Patients with cystic fibrosis, a genetic disease resulting in a deficiency of a functional chloride transporter, have thickened mucus that impedes the flow of pancreatic enzymes into the duodenum. Emulsification occurs through peristalsis, which provides mechanical mixing, and bile salts that function as detergents. Colipase restores activity to pancreatic lipase in the presence of inhibitory bile salts that bind the micelles. Cholecystokinin is the hormone that causes contraction of the gallbladder and release of stored bile, and secretin causes release of bicarbonate. Chylomicron formation requires synthesis of apolipoprotein B-48. 5.2. Which one of the following statements about lipid absorption from the intestine is correct? A. Dietary triacylglycerol must be completely hydrolyzed to free fatty acids and glycerol before absorption.	Biochemistry_Lippinco. E. Formation of triacylglycerol-rich chylomicrons is independent of protein synthesis in the intestinal mucosa. Correct answer = D. Patients with cystic fibrosis, a genetic disease resulting in a deficiency of a functional chloride transporter, have thickened mucus that impedes the flow of pancreatic enzymes into the duodenum. Emulsification occurs through peristalsis, which provides mechanical mixing, and bile salts that function as detergents. Colipase restores activity to pancreatic lipase in the presence of inhibitory bile salts that bind the micelles. Cholecystokinin is the hormone that causes contraction of the gallbladder and release of stored bile, and secretin causes release of bicarbonate. Chylomicron formation requires synthesis of apolipoprotein B-48. 5.2. Which one of the following statements about lipid absorption from the intestine is correct? A. Dietary triacylglycerol must be completely hydrolyzed to free fatty acids and glycerol before absorption.
Biochemistry_Lippincott_621	Biochemistry_Lippinco	A. Dietary triacylglycerol must be completely hydrolyzed to free fatty acids and glycerol before absorption. B. The triacylglycerol carried by chylomicrons is degraded by lipoprotein lipase, producing fatty acids that are taken up by muscle and adipose tissues and glycerol that is taken up by the liver. C. Fatty acids that contain ≤12 carbon atoms are absorbed and enter the circulation primarily via the lymphatic system. D. Deficiencies in the ability to absorb fat result in excessive amounts of chylomicrons in the blood.	Biochemistry_Lippinco. A. Dietary triacylglycerol must be completely hydrolyzed to free fatty acids and glycerol before absorption. B. The triacylglycerol carried by chylomicrons is degraded by lipoprotein lipase, producing fatty acids that are taken up by muscle and adipose tissues and glycerol that is taken up by the liver. C. Fatty acids that contain ≤12 carbon atoms are absorbed and enter the circulation primarily via the lymphatic system. D. Deficiencies in the ability to absorb fat result in excessive amounts of chylomicrons in the blood.
Biochemistry_Lippincott_622	Biochemistry_Lippinco	D. Deficiencies in the ability to absorb fat result in excessive amounts of chylomicrons in the blood. Correct answer = B. The triacylglycerols (TAG) in chylomicrons are degraded to fatty acids (FA) and glycerol by lipoprotein lipase on capillary endothelial surfaces in muscle and adipose tissue, thus providing a source of FA to these tissues for degradation or storage and providing glycerol for hepatic metabolism. In the duodenum, TAG are degraded to one 2-monoacylglycerol + two free FA that get absorbed. Medium-and short-chain FA enter directly into blood (not lymph), and they neither require micelles nor get packaged into chylomicrons. Because chylomicrons contain dietary lipids that were digested and absorbed, a defect in fat absorption would result in decreased production of chylomicrons. Fatty Acid, Triacylglycerol, and Ketone Body Metabolism 16 For additional ancillary materials related to this chapter, please visit thePoint. I. OVERVIEW	Biochemistry_Lippinco. D. Deficiencies in the ability to absorb fat result in excessive amounts of chylomicrons in the blood. Correct answer = B. The triacylglycerols (TAG) in chylomicrons are degraded to fatty acids (FA) and glycerol by lipoprotein lipase on capillary endothelial surfaces in muscle and adipose tissue, thus providing a source of FA to these tissues for degradation or storage and providing glycerol for hepatic metabolism. In the duodenum, TAG are degraded to one 2-monoacylglycerol + two free FA that get absorbed. Medium-and short-chain FA enter directly into blood (not lymph), and they neither require micelles nor get packaged into chylomicrons. Because chylomicrons contain dietary lipids that were digested and absorbed, a defect in fat absorption would result in decreased production of chylomicrons. Fatty Acid, Triacylglycerol, and Ketone Body Metabolism 16 For additional ancillary materials related to this chapter, please visit thePoint. I. OVERVIEW