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Anatomy_Gray_3000	Anatomy_Gray	The tuberal region in the medial zone contains three nuclei: ventromedial, dorsomedial, and arcuate. The largest and best defined is the ventromedial nucleus, which functions as a satiety center to decrease feeding behavior. Posterior to the ventromedial nucleus is the dorsomedial nucleus, which functions in the behavioral expression of rage or aggressive behavior. Finally, the arcuate nucleus serves as a center for releasing hormones, which are transmitted by the tuberoinfundibular tract and hypophysial portal system to the anterior pituitary (eFig. 9.89).	Anatomy_Gray. The tuberal region in the medial zone contains three nuclei: ventromedial, dorsomedial, and arcuate. The largest and best defined is the ventromedial nucleus, which functions as a satiety center to decrease feeding behavior. Posterior to the ventromedial nucleus is the dorsomedial nucleus, which functions in the behavioral expression of rage or aggressive behavior. Finally, the arcuate nucleus serves as a center for releasing hormones, which are transmitted by the tuberoinfundibular tract and hypophysial portal system to the anterior pituitary (eFig. 9.89).
Anatomy_Gray_3001	Anatomy_Gray	The mammillary region or mammillary body is the final group of nuclei in the medial zone. Four nuclei comprise this region: medial, intermediate, lateral mammillary, and posterior hypothalamic. The best defined is the medial mammillary nucleus, as it is the primary site for the termination of axons from the postcommissural fornix. This pathway originates from the subiculum of the hippocampal complex and plays a key role in memory. The medial mammillary nuclei also connects to structures of the limbic system. The periventricular zone resides medial to the medial zone and adjacent to the ependymal cells of the third ventricle. Neurons from this zone predominantly synthesize releasing hormones. Axons from these neuronal cells project through the tuberoinfundibular tract to the hypophysial portal system to influence release of hormones from the anterior pituitary.	Anatomy_Gray. The mammillary region or mammillary body is the final group of nuclei in the medial zone. Four nuclei comprise this region: medial, intermediate, lateral mammillary, and posterior hypothalamic. The best defined is the medial mammillary nucleus, as it is the primary site for the termination of axons from the postcommissural fornix. This pathway originates from the subiculum of the hippocampal complex and plays a key role in memory. The medial mammillary nuclei also connects to structures of the limbic system. The periventricular zone resides medial to the medial zone and adjacent to the ependymal cells of the third ventricle. Neurons from this zone predominantly synthesize releasing hormones. Axons from these neuronal cells project through the tuberoinfundibular tract to the hypophysial portal system to influence release of hormones from the anterior pituitary.
Anatomy_Gray_3002	Anatomy_Gray	Through the review of the hypothalamus thus far, it is apparent that this small, 4-gram structure has a significant role in regulating visceral, endocrine, and behavioral system functions through multiple pathways. It is important to remember the majority of the pathways mentioned represent input–output relationships between the hypothalamus and other structures. For a review of neural and non-neural inputs and outputs, refer to the summary figures (eFig. 9.92A and B). Part XI: Olfactory and	Anatomy_Gray. Through the review of the hypothalamus thus far, it is apparent that this small, 4-gram structure has a significant role in regulating visceral, endocrine, and behavioral system functions through multiple pathways. It is important to remember the majority of the pathways mentioned represent input–output relationships between the hypothalamus and other structures. For a review of neural and non-neural inputs and outputs, refer to the summary figures (eFig. 9.92A and B). Part XI: Olfactory and
Anatomy_Gray_3003	Anatomy_Gray	Part XI: Olfactory and The sense of olfaction has a role in both pleasurable experiences and survival. The same receptors that allow us to enjoy the food we consume or experience odorants in the environment also help us avoid spoiled food or potentially hazardous situations like a fire. Unlike the other special sensory system pathways, the olfactory sensory pathway is unique in that it does not have a thalamic relay before reaching the primary olfactory cortex. In this section we will review the course of the neurons in the olfactory system and their connection to the limbic system.	Anatomy_Gray. Part XI: Olfactory and The sense of olfaction has a role in both pleasurable experiences and survival. The same receptors that allow us to enjoy the food we consume or experience odorants in the environment also help us avoid spoiled food or potentially hazardous situations like a fire. Unlike the other special sensory system pathways, the olfactory sensory pathway is unique in that it does not have a thalamic relay before reaching the primary olfactory cortex. In this section we will review the course of the neurons in the olfactory system and their connection to the limbic system.
Anatomy_Gray_3004	Anatomy_Gray	Three types of olfactory receptors make up the olfactory epithelium along the lateral and septal walls of the nasal cavity. These cells allow for regeneration (basal stem cells), support (sustentacular cells), and transmission of information (olfactory receptor neurons). Each olfactory receptor neuron has an olfactory vesicle with cilia that contain receptors for odorant molecules and an unmyelinated axon that passes through the cribriform plate to terminate in the olfactory bulb (eFig. 9.93). As the olfactory receptor neurons originate embryologically from the CNS, they are considered part of the CNS and not the PNS.	Anatomy_Gray. Three types of olfactory receptors make up the olfactory epithelium along the lateral and septal walls of the nasal cavity. These cells allow for regeneration (basal stem cells), support (sustentacular cells), and transmission of information (olfactory receptor neurons). Each olfactory receptor neuron has an olfactory vesicle with cilia that contain receptors for odorant molecules and an unmyelinated axon that passes through the cribriform plate to terminate in the olfactory bulb (eFig. 9.93). As the olfactory receptor neurons originate embryologically from the CNS, they are considered part of the CNS and not the PNS.
Anatomy_Gray_3005	Anatomy_Gray	After synapsing with the mitral cells in the glomeruli of the olfactory bulb, mitral cell axons converge to form the olfactory tract. The olfactory tract then divides into medial and lateral olfactory striae to reach different synaptic targets (eFig. 9.94). Some of the axons in the medial olfactory striae travel through the diagonal band to reach the septal area, whereas others cross the midline in the anterior commissure and inhibit mitral cell activity in the contralateral olfactory bulb to enhance localization of the olfactory stimulant. Axons in the lateral olfactory stria primarily terminate in the piriform cortex/primary olfactory cortex of the uncus and in the amygdala (eFig. 9.94). The medial forebrain bundle, traveling through the lateral hypothalamus, connects the olfactory cortex with both the hypothalamus and the brainstem to regulate autonomic responses such as arousal through the reticular formation, salivation, and gastric contraction.	Anatomy_Gray. After synapsing with the mitral cells in the glomeruli of the olfactory bulb, mitral cell axons converge to form the olfactory tract. The olfactory tract then divides into medial and lateral olfactory striae to reach different synaptic targets (eFig. 9.94). Some of the axons in the medial olfactory striae travel through the diagonal band to reach the septal area, whereas others cross the midline in the anterior commissure and inhibit mitral cell activity in the contralateral olfactory bulb to enhance localization of the olfactory stimulant. Axons in the lateral olfactory stria primarily terminate in the piriform cortex/primary olfactory cortex of the uncus and in the amygdala (eFig. 9.94). The medial forebrain bundle, traveling through the lateral hypothalamus, connects the olfactory cortex with both the hypothalamus and the brainstem to regulate autonomic responses such as arousal through the reticular formation, salivation, and gastric contraction.
Anatomy_Gray_3006	Anatomy_Gray	The limbic system is composed of several cortical and subcortical structures that participate in an intricate network of connections to regulate complicated behaviors such as memory, emotions, homeostatic functions, and motivational state. In this section we will review the major structures and pathways that form the limbic system. Grossly, the limbic lobe includes a ring-shaped area of cortical structures that border the brainstem. These cortical areas include the cingulate gyrus, parahippocampal gyrus, and subcallosal area (eFig. 9.12). Laterally, the insular cortex also participates in limbic system function (eFig. 9.10). Nuclear structures of the limbic system include the amygdala, hippocampal formation, anterior and mediodorsal thalamic nuclei, septal nuclei in the forebrain, and nucleus accumbens (eFig. 9.95).	Anatomy_Gray. The limbic system is composed of several cortical and subcortical structures that participate in an intricate network of connections to regulate complicated behaviors such as memory, emotions, homeostatic functions, and motivational state. In this section we will review the major structures and pathways that form the limbic system. Grossly, the limbic lobe includes a ring-shaped area of cortical structures that border the brainstem. These cortical areas include the cingulate gyrus, parahippocampal gyrus, and subcallosal area (eFig. 9.12). Laterally, the insular cortex also participates in limbic system function (eFig. 9.10). Nuclear structures of the limbic system include the amygdala, hippocampal formation, anterior and mediodorsal thalamic nuclei, septal nuclei in the forebrain, and nucleus accumbens (eFig. 9.95).
Anatomy_Gray_3007	Anatomy_Gray	The amygdaloid nucleus is an almond-shaped structure located anterior to the inferior horn of the lateral ventricle and tail of the caudate within the temporal lobe (eFig. 9.96). Structurally, the amygdala consists of three nuclear regions: a large basolateral group and a smaller corticomedial group, which includes the central nucleus. Functionally, the amygdala is primarily associated with the emotion of fear, but it also has an important role in autonomic and neuroendocrine pathways. Connections of the amygdala are predominantly bidirectional and follow three different pathways: the uncinate fasciculus, stria terminalis, and ventral amygdalofugal pathway (eFig. 9.97). Connections to cortical areas pass through the uncinate fasciculus, which progresses anterior to the amygdala. Projections to the septal area and hypothalamus follow the stria terminalis (eFig. 9.98A–D). Fibers forming the ventral amygdalofugal pathway project to thalamic nuclei and several brainstem and forebrain	Anatomy_Gray. The amygdaloid nucleus is an almond-shaped structure located anterior to the inferior horn of the lateral ventricle and tail of the caudate within the temporal lobe (eFig. 9.96). Structurally, the amygdala consists of three nuclear regions: a large basolateral group and a smaller corticomedial group, which includes the central nucleus. Functionally, the amygdala is primarily associated with the emotion of fear, but it also has an important role in autonomic and neuroendocrine pathways. Connections of the amygdala are predominantly bidirectional and follow three different pathways: the uncinate fasciculus, stria terminalis, and ventral amygdalofugal pathway (eFig. 9.97). Connections to cortical areas pass through the uncinate fasciculus, which progresses anterior to the amygdala. Projections to the septal area and hypothalamus follow the stria terminalis (eFig. 9.98A–D). Fibers forming the ventral amygdalofugal pathway project to thalamic nuclei and several brainstem and forebrain
Anatomy_Gray_3008	Anatomy_Gray	to the septal area and hypothalamus follow the stria terminalis (eFig. 9.98A–D). Fibers forming the ventral amygdalofugal pathway project to thalamic nuclei and several brainstem and forebrain structures (eFig. 9.98A–D).	Anatomy_Gray. to the septal area and hypothalamus follow the stria terminalis (eFig. 9.98A–D). Fibers forming the ventral amygdalofugal pathway project to thalamic nuclei and several brainstem and forebrain structures (eFig. 9.98A–D).
Anatomy_Gray_3009	Anatomy_Gray	The nucleus accumbens resides with the ventral forebrain adjacent to where the putamen and head of the caudate become continuous with one another (eFig. 9.99). Afferent axons to the nucleus accumbens come from the amygdala through the amygdalofugal pathway, hippocampal formation by way of the fornix, basal forebrain area from the stria terminalis, and ventral tegmentum through the medial forebrain bundle (eFig. 9.98A–D). Efferent axons leaving the nucleus accumbens project directly to the hypothalamus and globus pallidus and reach nuclei in the brainstem through the medial forebrain bundle. Its connections to the globus pallidus represent an important connection of the limbic system to the motor system. The overall function of the nucleus accumbens is recognized as a gratification center and has been shown to play a role in behaviors related to addiction.	Anatomy_Gray. The nucleus accumbens resides with the ventral forebrain adjacent to where the putamen and head of the caudate become continuous with one another (eFig. 9.99). Afferent axons to the nucleus accumbens come from the amygdala through the amygdalofugal pathway, hippocampal formation by way of the fornix, basal forebrain area from the stria terminalis, and ventral tegmentum through the medial forebrain bundle (eFig. 9.98A–D). Efferent axons leaving the nucleus accumbens project directly to the hypothalamus and globus pallidus and reach nuclei in the brainstem through the medial forebrain bundle. Its connections to the globus pallidus represent an important connection of the limbic system to the motor system. The overall function of the nucleus accumbens is recognized as a gratification center and has been shown to play a role in behaviors related to addiction.
Anatomy_Gray_3010	Anatomy_Gray	The septal region is located rostral to the anterior commissure along the medial aspect of the cerebral hemispheres (eFig. 9.99). This region appears to play a role in pleasurable behaviors. Conversely, lesion studies indicate that damage to this area evokes behaviors of extreme displeasure or rage. Afferent axons to the septal area arise from the amygdala, hippocampus, olfactory tract, and monoaminergic nuclei in the brainstem (eFigs. 9.100 and 9.101). The septal area also connects to a collection of cholinergic neurons along the wall and roof of the third ventricle known as the habenular nuclei. Axons from the habenular nuclei project to the interpeduncular nucleus of the reticular formation, which is believed to play a role in the sleep–wake cycle (eFig. 9.100).	Anatomy_Gray. The septal region is located rostral to the anterior commissure along the medial aspect of the cerebral hemispheres (eFig. 9.99). This region appears to play a role in pleasurable behaviors. Conversely, lesion studies indicate that damage to this area evokes behaviors of extreme displeasure or rage. Afferent axons to the septal area arise from the amygdala, hippocampus, olfactory tract, and monoaminergic nuclei in the brainstem (eFigs. 9.100 and 9.101). The septal area also connects to a collection of cholinergic neurons along the wall and roof of the third ventricle known as the habenular nuclei. Axons from the habenular nuclei project to the interpeduncular nucleus of the reticular formation, which is believed to play a role in the sleep–wake cycle (eFig. 9.100).
Anatomy_Gray_3011	Anatomy_Gray	The hippocampal formation is located in the medial ventral temporal lobe (eFig. 9.102). It consists of the hippocampus, dentate gyrus, and subiculum (eFig. 9.103A and B). The hippocampal formation plays a role in memory processes such as episodic memory, short-term memory, working memory, and consolidation of memories. Input to the hippocampal formation is primarily received by the entorhinal cortex from association cortices. Because of this, it is believed that the “storage” of memories is in the association and primary cortices, not in the medial temporal lobe. Neurons from the entorhinal cortex project to the hippocampal formation by two pathways: the perforant pathway and alvear pathway. The perforant courses directly through the hippocampal sulcus to reach the dentate gyrus (eFig. 9.104B). As the hippocampus resembles the appearance of a ram’s horn, early on it was named cornu ammonis (horn of Ammon). Based on its cytoarchitecture, it was subdivided into four regions termed the	Anatomy_Gray. The hippocampal formation is located in the medial ventral temporal lobe (eFig. 9.102). It consists of the hippocampus, dentate gyrus, and subiculum (eFig. 9.103A and B). The hippocampal formation plays a role in memory processes such as episodic memory, short-term memory, working memory, and consolidation of memories. Input to the hippocampal formation is primarily received by the entorhinal cortex from association cortices. Because of this, it is believed that the “storage” of memories is in the association and primary cortices, not in the medial temporal lobe. Neurons from the entorhinal cortex project to the hippocampal formation by two pathways: the perforant pathway and alvear pathway. The perforant courses directly through the hippocampal sulcus to reach the dentate gyrus (eFig. 9.104B). As the hippocampus resembles the appearance of a ram’s horn, early on it was named cornu ammonis (horn of Ammon). Based on its cytoarchitecture, it was subdivided into four regions termed the
Anatomy_Gray_3012	Anatomy_Gray	As the hippocampus resembles the appearance of a ram’s horn, early on it was named cornu ammonis (horn of Ammon). Based on its cytoarchitecture, it was subdivided into four regions termed the cornu ammonis 1 to 4 (eFig. 9.104A). From the dentate gyrus, axons project to CA3 of the hippocampus. Axons from the hippocampus leave via the fornix or as Shaffer collaterals to reach CA1. Axons from CA1 may enter the fornix or project to the subiculum. Finally, axons from the subiculum enter the fornix or go back to the entorhinal cortex.	Anatomy_Gray. As the hippocampus resembles the appearance of a ram’s horn, early on it was named cornu ammonis (horn of Ammon). Based on its cytoarchitecture, it was subdivided into four regions termed the cornu ammonis 1 to 4 (eFig. 9.104A). From the dentate gyrus, axons project to CA3 of the hippocampus. Axons from the hippocampus leave via the fornix or as Shaffer collaterals to reach CA1. Axons from CA1 may enter the fornix or project to the subiculum. Finally, axons from the subiculum enter the fornix or go back to the entorhinal cortex.
Anatomy_Gray_3013	Anatomy_Gray	A second afferent pathway from the entorhinal cortex to the hippocampal formation is through the alvear pathway. Axons in the alvear pathway project directly on to CA1 and CA3 of the hippocampus (eFig. 9.104B). Similar to the perforant pathway, axons leaving the alvear pathway primarily originate from CA1 and CA3, which then project to the subiculum.	Anatomy_Gray. A second afferent pathway from the entorhinal cortex to the hippocampal formation is through the alvear pathway. Axons in the alvear pathway project directly on to CA1 and CA3 of the hippocampus (eFig. 9.104B). Similar to the perforant pathway, axons leaving the alvear pathway primarily originate from CA1 and CA3, which then project to the subiculum.
Anatomy_Gray_3014	Anatomy_Gray	Efferent axons leaving the hippocampal formation primarily exit from the subiculum and form the fornix (Latin for “arch”), a white matter structure that arches over the ventricular system (eFig. 9.95). The fornix begins with axons exiting the hippocampus to form the alveus along the ventricular surface of the hippocampus. As the axons come together medially, they form a bundle referred to as the fimbria of the fornix. The fornix then emerges from the hippocampal formation and curves under the corpus callosum before coursing medially to run adjacent to the midline (eFig. 9.105). At the anterior commissure, the fornix divides into a precommissural fornix and postcommissural fornix to reach the nucleus accumbens, septal nuclei, medial frontal cortex, mammillary nucleus, ventromedial nucleus of the hypothalamus, and anterior nucleus of the dorsal thalamus (eFig. 9.95).	Anatomy_Gray. Efferent axons leaving the hippocampal formation primarily exit from the subiculum and form the fornix (Latin for “arch”), a white matter structure that arches over the ventricular system (eFig. 9.95). The fornix begins with axons exiting the hippocampus to form the alveus along the ventricular surface of the hippocampus. As the axons come together medially, they form a bundle referred to as the fimbria of the fornix. The fornix then emerges from the hippocampal formation and curves under the corpus callosum before coursing medially to run adjacent to the midline (eFig. 9.105). At the anterior commissure, the fornix divides into a precommissural fornix and postcommissural fornix to reach the nucleus accumbens, septal nuclei, medial frontal cortex, mammillary nucleus, ventromedial nucleus of the hypothalamus, and anterior nucleus of the dorsal thalamus (eFig. 9.95).
Anatomy_Gray_3015	Anatomy_Gray	Through this section we have described a collection of anatomical structures and defined their connections with other areas of the brain and brainstem without exploring how these individual structures are interconnected with one another. In the 1930s James Papez, an American neurologist, described a circuit that links these structures and cortical areas together in a way that was thought to be involved in the experience and expression of emotion. This is referred to as the Papez circuit (eFig. 9.106). The circuit begins with fibers from the subiculum, which then enter the fornix to reach the mammillary nuclei. These axons then project through the mammillothalamic tract to the anterior nucleus of the thalamus. Next, the axons from the anterior nucleus of the thalamus project through the internal capsule to the cingulate gyrus. Lastly, the cingulum fibers from the cingulate cortex project to the parahippocampal gyrus and then the entorhinal cortex and hippocampal formation (eFig.	Anatomy_Gray. Through this section we have described a collection of anatomical structures and defined their connections with other areas of the brain and brainstem without exploring how these individual structures are interconnected with one another. In the 1930s James Papez, an American neurologist, described a circuit that links these structures and cortical areas together in a way that was thought to be involved in the experience and expression of emotion. This is referred to as the Papez circuit (eFig. 9.106). The circuit begins with fibers from the subiculum, which then enter the fornix to reach the mammillary nuclei. These axons then project through the mammillothalamic tract to the anterior nucleus of the thalamus. Next, the axons from the anterior nucleus of the thalamus project through the internal capsule to the cingulate gyrus. Lastly, the cingulum fibers from the cingulate cortex project to the parahippocampal gyrus and then the entorhinal cortex and hippocampal formation (eFig.
Anatomy_Gray_3016	Anatomy_Gray	the internal capsule to the cingulate gyrus. Lastly, the cingulum fibers from the cingulate cortex project to the parahippocampal gyrus and then the entorhinal cortex and hippocampal formation (eFig. 9.106). Papez’s description of this circuit is useful for reviewing the major limbic system pathways; however, the role of some of the structures in the pathway has been shown to play little or no role in the expression of emotion. In addition, many of the structures that do play a role in the expression of emotion also have a role in other functions.	Anatomy_Gray. the internal capsule to the cingulate gyrus. Lastly, the cingulum fibers from the cingulate cortex project to the parahippocampal gyrus and then the entorhinal cortex and hippocampal formation (eFig. 9.106). Papez’s description of this circuit is useful for reviewing the major limbic system pathways; however, the role of some of the structures in the pathway has been shown to play little or no role in the expression of emotion. In addition, many of the structures that do play a role in the expression of emotion also have a role in other functions.
Biochemistry_Lippincott_0	Biochemistry_Lippinco	For additional ancillary materials related to this chapter, please visit thePoint. I. OVERVIEW	Biochemistry_Lippinco. For additional ancillary materials related to this chapter, please visit thePoint. I. OVERVIEW
Biochemistry_Lippincott_1	Biochemistry_Lippinco	Proteins are the most abundant and functionally diverse molecules in living systems. Virtually every life process depends on this class of macromolecules. For example, enzymes and polypeptide hormones direct and regulate metabolism in the body, whereas contractile proteins in muscle permit movement. In bone, the protein collagen forms a framework for the deposition of calcium phosphate crystals, acting like the steel cables in reinforced concrete. In the bloodstream, proteins, such as hemoglobin and albumin, transport molecules essential to life, whereas immunoglobulins fight infectious bacteria and viruses. In short, proteins display an incredible diversity of functions, yet all share the common structural feature of being linear polymers of amino acids. This chapter describes the properties of amino acids. Chapter 2 explores how these simple building blocks are joined to form proteins that have unique three-dimensional structures, making them capable of performing specific biologic	Biochemistry_Lippinco. Proteins are the most abundant and functionally diverse molecules in living systems. Virtually every life process depends on this class of macromolecules. For example, enzymes and polypeptide hormones direct and regulate metabolism in the body, whereas contractile proteins in muscle permit movement. In bone, the protein collagen forms a framework for the deposition of calcium phosphate crystals, acting like the steel cables in reinforced concrete. In the bloodstream, proteins, such as hemoglobin and albumin, transport molecules essential to life, whereas immunoglobulins fight infectious bacteria and viruses. In short, proteins display an incredible diversity of functions, yet all share the common structural feature of being linear polymers of amino acids. This chapter describes the properties of amino acids. Chapter 2 explores how these simple building blocks are joined to form proteins that have unique three-dimensional structures, making them capable of performing specific biologic
Biochemistry_Lippincott_2	Biochemistry_Lippinco	of amino acids. Chapter 2 explores how these simple building blocks are joined to form proteins that have unique three-dimensional structures, making them capable of performing specific biologic functions.	Biochemistry_Lippinco. of amino acids. Chapter 2 explores how these simple building blocks are joined to form proteins that have unique three-dimensional structures, making them capable of performing specific biologic functions.
Biochemistry_Lippincott_3	Biochemistry_Lippinco	II. STRUCTURE	Biochemistry_Lippinco. II. STRUCTURE
Biochemistry_Lippincott_4	Biochemistry_Lippinco	Although >300 different amino acids have been described in nature, only 20 are commonly found as constituents of mammalian proteins. [Note: These standard amino acids are the only amino acids that are encoded by DNA, the genetic material in the cell (see p. 411). Nonstandard amino acids are produced by chemical modification of standard amino acids (see p. 45).] Each amino acid has a carboxyl group, a primary amino group (except for proline, which has a secondary amino group), and a distinctive side chain (R group) bonded to the α carbon atom. At physiologic pH (~7.4), the carboxyl group is dissociated, forming the negatively charged carboxylate ion (−COO−), and the amino group is protonated (−NH3+) (Fig. 1.1A). In proteins, almost all of these carboxyl and amino groups are combined through peptide linkage and, in general, are not available for chemical reaction except for hydrogen bond formation (Fig. 1.1B). Thus, it is the nature of the side chains that ultimately dictates the role	Biochemistry_Lippinco. Although >300 different amino acids have been described in nature, only 20 are commonly found as constituents of mammalian proteins. [Note: These standard amino acids are the only amino acids that are encoded by DNA, the genetic material in the cell (see p. 411). Nonstandard amino acids are produced by chemical modification of standard amino acids (see p. 45).] Each amino acid has a carboxyl group, a primary amino group (except for proline, which has a secondary amino group), and a distinctive side chain (R group) bonded to the α carbon atom. At physiologic pH (~7.4), the carboxyl group is dissociated, forming the negatively charged carboxylate ion (−COO−), and the amino group is protonated (−NH3+) (Fig. 1.1A). In proteins, almost all of these carboxyl and amino groups are combined through peptide linkage and, in general, are not available for chemical reaction except for hydrogen bond formation (Fig. 1.1B). Thus, it is the nature of the side chains that ultimately dictates the role
Biochemistry_Lippincott_5	Biochemistry_Lippinco	peptide linkage and, in general, are not available for chemical reaction except for hydrogen bond formation (Fig. 1.1B). Thus, it is the nature of the side chains that ultimately dictates the role an amino acid plays in a protein. Therefore, it is useful to classify the amino acids according to the properties of their side chains, that is, whether they are nonpolar (have an even distribution of electrons) or polar (have an uneven distribution of electrons, such as acids and bases) as shown in Figures 1.2 and 1.3.	Biochemistry_Lippinco. peptide linkage and, in general, are not available for chemical reaction except for hydrogen bond formation (Fig. 1.1B). Thus, it is the nature of the side chains that ultimately dictates the role an amino acid plays in a protein. Therefore, it is useful to classify the amino acids according to the properties of their side chains, that is, whether they are nonpolar (have an even distribution of electrons) or polar (have an uneven distribution of electrons, such as acids and bases) as shown in Figures 1.2 and 1.3.
Biochemistry_Lippincott_6	Biochemistry_Lippinco	and polarity of their side chains at acidic pH (continued from Fig. 1.2). [Note: At physiologic pH (7.35 to 7.45), the α-carboxyl groups, the acidic side chains, and the side chain of free histidine are deprotonated.] A. Amino acids with nonpolar side chains Each of these amino acids has a nonpolar side chain that does not gain or lose protons or participate in hydrogen or ionic bonds (see Fig. 1.2). The side chains of these amino acids can be thought of as “oily” or lipid-like, a property that promotes hydrophobic interactions (see Fig. 2.10, p. 19).	Biochemistry_Lippinco. and polarity of their side chains at acidic pH (continued from Fig. 1.2). [Note: At physiologic pH (7.35 to 7.45), the α-carboxyl groups, the acidic side chains, and the side chain of free histidine are deprotonated.] A. Amino acids with nonpolar side chains Each of these amino acids has a nonpolar side chain that does not gain or lose protons or participate in hydrogen or ionic bonds (see Fig. 1.2). The side chains of these amino acids can be thought of as “oily” or lipid-like, a property that promotes hydrophobic interactions (see Fig. 2.10, p. 19).
Biochemistry_Lippincott_7	Biochemistry_Lippinco	1. Location in proteins: In proteins found in aqueous solutions (a polar environment), the side chains of the nonpolar amino acids tend to cluster together in the interior of the protein (Fig. 1.4). This phenomenon, known as the hydrophobic effect, is the result of the hydrophobicity of the nonpolar R groups, which act much like droplets of oil that coalesce in an aqueous environment. By filling up the interior of the folded protein, these nonpolar R groups help give the protein its three-dimensional shape. However, for proteins that are located in a hydrophobic environment, such as a membrane, the nonpolar R groups are found on the outside surface of the protein, interacting with the lipid environment (see Fig. 1.4). The importance of these hydrophobic interactions in stabilizing protein structure is discussed on p. 19.	Biochemistry_Lippinco. 1. Location in proteins: In proteins found in aqueous solutions (a polar environment), the side chains of the nonpolar amino acids tend to cluster together in the interior of the protein (Fig. 1.4). This phenomenon, known as the hydrophobic effect, is the result of the hydrophobicity of the nonpolar R groups, which act much like droplets of oil that coalesce in an aqueous environment. By filling up the interior of the folded protein, these nonpolar R groups help give the protein its three-dimensional shape. However, for proteins that are located in a hydrophobic environment, such as a membrane, the nonpolar R groups are found on the outside surface of the protein, interacting with the lipid environment (see Fig. 1.4). The importance of these hydrophobic interactions in stabilizing protein structure is discussed on p. 19.
Biochemistry_Lippincott_8	Biochemistry_Lippinco	Sickle cell anemia, a disease of red blood cells that causes them to become sickle shaped rather than disc shaped, results from the replacement of polar glutamate with nonpolar valine at the sixth position in the β subunit of hemoglobin A (see p. 36). 2. Proline: Proline differs from other amino acids in that its side chain and α-amino nitrogen form a rigid, five-membered ring structure (Fig. 1.5). Proline, then, has a secondary (rather than a primary) amino group. It is frequently referred to as an “imino acid.” The unique geometry of proline contributes to the formation of the fibrous structure of collagen (see p. 45), but it interrupts the α-helices found in globular proteins (see p. 16). B. Amino acids with uncharged polar side chains	Biochemistry_Lippinco. Sickle cell anemia, a disease of red blood cells that causes them to become sickle shaped rather than disc shaped, results from the replacement of polar glutamate with nonpolar valine at the sixth position in the β subunit of hemoglobin A (see p. 36). 2. Proline: Proline differs from other amino acids in that its side chain and α-amino nitrogen form a rigid, five-membered ring structure (Fig. 1.5). Proline, then, has a secondary (rather than a primary) amino group. It is frequently referred to as an “imino acid.” The unique geometry of proline contributes to the formation of the fibrous structure of collagen (see p. 45), but it interrupts the α-helices found in globular proteins (see p. 16). B. Amino acids with uncharged polar side chains
Biochemistry_Lippincott_9	Biochemistry_Lippinco	B. Amino acids with uncharged polar side chains These amino acids have zero net charge at physiologic pH, although the side chains of cysteine and tyrosine can lose a proton at an alkaline pH (see Fig. 1.3). Serine, threonine, and tyrosine each contain a polar hydroxyl group that can participate in hydrogen bond formation (Fig. 1.6). The side chains of asparagine and glutamine each contain a carbonyl group and an amide group, both of which can also participate in hydrogen bonds. 1. Disulfide bond: The side chain of cysteine contains a sulfhydryl (thiol) group (−SH), which is an important component of the active site of many enzymes. In proteins, the –SH groups of two cysteines can be oxidized to form a covalent cross-link called a disulfide bond (−S–S–). Two disulfide-linked cysteines are referred to as cystine. (See p. 19 for a further discussion of disulfide bond formation.)	Biochemistry_Lippinco. B. Amino acids with uncharged polar side chains These amino acids have zero net charge at physiologic pH, although the side chains of cysteine and tyrosine can lose a proton at an alkaline pH (see Fig. 1.3). Serine, threonine, and tyrosine each contain a polar hydroxyl group that can participate in hydrogen bond formation (Fig. 1.6). The side chains of asparagine and glutamine each contain a carbonyl group and an amide group, both of which can also participate in hydrogen bonds. 1. Disulfide bond: The side chain of cysteine contains a sulfhydryl (thiol) group (−SH), which is an important component of the active site of many enzymes. In proteins, the –SH groups of two cysteines can be oxidized to form a covalent cross-link called a disulfide bond (−S–S–). Two disulfide-linked cysteines are referred to as cystine. (See p. 19 for a further discussion of disulfide bond formation.)
Biochemistry_Lippincott_10	Biochemistry_Lippinco	Many extracellular proteins are stabilized by disulfide bonds. Albumin, a blood protein that functions as a transporter for a variety of molecules, is an example. 2. Side chains as attachment sites for other compounds: The polar hydroxyl group of serine, threonine, and (rarely) tyrosine can serve as a site of attachment for structures such as a phosphate group. In addition, the amide group of asparagine, as well as the hydroxyl group of serine or threonine, can serve as a site of attachment for oligosaccharide chains in glycoproteins (see p. 165). C. Amino acids with acidic side chains The amino acids aspartic acid and glutamic acid are proton donors. At physiologic pH, the side chains of these amino acids are fully ionized, containing a negatively charged carboxylate group (−COO−). The fully ionized forms are called aspartate and glutamate. D. Amino acids with basic side chains	Biochemistry_Lippinco. Many extracellular proteins are stabilized by disulfide bonds. Albumin, a blood protein that functions as a transporter for a variety of molecules, is an example. 2. Side chains as attachment sites for other compounds: The polar hydroxyl group of serine, threonine, and (rarely) tyrosine can serve as a site of attachment for structures such as a phosphate group. In addition, the amide group of asparagine, as well as the hydroxyl group of serine or threonine, can serve as a site of attachment for oligosaccharide chains in glycoproteins (see p. 165). C. Amino acids with acidic side chains The amino acids aspartic acid and glutamic acid are proton donors. At physiologic pH, the side chains of these amino acids are fully ionized, containing a negatively charged carboxylate group (−COO−). The fully ionized forms are called aspartate and glutamate. D. Amino acids with basic side chains
Biochemistry_Lippincott_11	Biochemistry_Lippinco	D. Amino acids with basic side chains The side chains of the basic amino acids accept protons (see Fig. 1.3). At physiologic pH, the R groups of lysine and arginine are fully ionized and positively charged. In contrast, the free amino acid histidine is weakly basic and largely uncharged at physiologic pH. However, when histidine is incorporated into a protein, its R group can be either positively charged (protonated) or neutral, depending on the ionic environment provided by the protein. This important property of histidine contributes to the buffering role it plays in the functioning of such proteins as hemoglobin (see p. 30). [Note: Histidine is the only amino acid with a side chain that can ionize within the physiologic pH range.] E. Abbreviations and symbols for commonly occurring amino acids letter symbol (Fig. 1.7). The one-letter codes are determined by the following rules. 1.	Biochemistry_Lippinco. D. Amino acids with basic side chains The side chains of the basic amino acids accept protons (see Fig. 1.3). At physiologic pH, the R groups of lysine and arginine are fully ionized and positively charged. In contrast, the free amino acid histidine is weakly basic and largely uncharged at physiologic pH. However, when histidine is incorporated into a protein, its R group can be either positively charged (protonated) or neutral, depending on the ionic environment provided by the protein. This important property of histidine contributes to the buffering role it plays in the functioning of such proteins as hemoglobin (see p. 30). [Note: Histidine is the only amino acid with a side chain that can ionize within the physiologic pH range.] E. Abbreviations and symbols for commonly occurring amino acids letter symbol (Fig. 1.7). The one-letter codes are determined by the following rules. 1.
Biochemistry_Lippincott_12	Biochemistry_Lippinco	E. Abbreviations and symbols for commonly occurring amino acids letter symbol (Fig. 1.7). The one-letter codes are determined by the following rules. 1. Unique first letter: If only one amino acid begins with a given letter, then that letter is used as its symbol. For example, V = valine. 2. Most commonly occurring amino acids have priority: If more than one amino acid begins with a particular letter, the most common of these amino acids receives this letter as its symbol. For example, glycine is more common than glutamate, so G = glycine. 3. Similar sounding names: Some one-letter symbols sound like the amino acid they represent. For example, F = phenylalanine, or W = tryptophan (“twyptophan” as Elmer Fudd would say). 4.	Biochemistry_Lippinco. E. Abbreviations and symbols for commonly occurring amino acids letter symbol (Fig. 1.7). The one-letter codes are determined by the following rules. 1. Unique first letter: If only one amino acid begins with a given letter, then that letter is used as its symbol. For example, V = valine. 2. Most commonly occurring amino acids have priority: If more than one amino acid begins with a particular letter, the most common of these amino acids receives this letter as its symbol. For example, glycine is more common than glutamate, so G = glycine. 3. Similar sounding names: Some one-letter symbols sound like the amino acid they represent. For example, F = phenylalanine, or W = tryptophan (“twyptophan” as Elmer Fudd would say). 4.
Biochemistry_Lippincott_13	Biochemistry_Lippinco	3. Similar sounding names: Some one-letter symbols sound like the amino acid they represent. For example, F = phenylalanine, or W = tryptophan (“twyptophan” as Elmer Fudd would say). 4. Letter close to initial letter: For the remaining amino acids, a one-letter symbol is assigned that is as close in the alphabet as possible to the initial letter of the amino acid, for example, K = lysine. Furthermore, B is assigned to Asx, signifying either aspartic acid or asparagine; Z is assigned to Glx, signifying either glutamic acid or glutamine; and X is assigned to an unidentified amino acid. F. Amino acid isomers	Biochemistry_Lippinco. 3. Similar sounding names: Some one-letter symbols sound like the amino acid they represent. For example, F = phenylalanine, or W = tryptophan (“twyptophan” as Elmer Fudd would say). 4. Letter close to initial letter: For the remaining amino acids, a one-letter symbol is assigned that is as close in the alphabet as possible to the initial letter of the amino acid, for example, K = lysine. Furthermore, B is assigned to Asx, signifying either aspartic acid or asparagine; Z is assigned to Glx, signifying either glutamic acid or glutamine; and X is assigned to an unidentified amino acid. F. Amino acid isomers
Biochemistry_Lippincott_14	Biochemistry_Lippinco	F. Amino acid isomers Because the α-carbon of an amino acid is attached to four different chemical groups, it is an asymmetric (chiral) atom. Glycine is the exception because its α-carbon has two hydrogen substituents. Amino acids with a chiral αcarbon exist in two different isomeric forms, designated D and L, which are enantiomers, or mirror images (Fig. 1.8). [Note: Enantiomers are optically active. If an isomer, either D or L, causes the plane of polarized light to rotate clockwise, it is designated the (+) form.] All amino acids found in mammalian proteins are of the L configuration. However, D-amino acids are found in some antibiotics and in bacterial cell walls (see p. 252). [Note: Racemases enzymatically interconvert the D-and L-isomers of free amino acids.] III. ACIDIC AND BASIC PROPERTIES	Biochemistry_Lippinco. F. Amino acid isomers Because the α-carbon of an amino acid is attached to four different chemical groups, it is an asymmetric (chiral) atom. Glycine is the exception because its α-carbon has two hydrogen substituents. Amino acids with a chiral αcarbon exist in two different isomeric forms, designated D and L, which are enantiomers, or mirror images (Fig. 1.8). [Note: Enantiomers are optically active. If an isomer, either D or L, causes the plane of polarized light to rotate clockwise, it is designated the (+) form.] All amino acids found in mammalian proteins are of the L configuration. However, D-amino acids are found in some antibiotics and in bacterial cell walls (see p. 252). [Note: Racemases enzymatically interconvert the D-and L-isomers of free amino acids.] III. ACIDIC AND BASIC PROPERTIES
Biochemistry_Lippincott_15	Biochemistry_Lippinco	III. ACIDIC AND BASIC PROPERTIES Amino acids in aqueous solution contain weakly acidic α-carboxyl groups and weakly basic α-amino groups. In addition, each of the acidic and basic amino acids contains an ionizable group in its side chain. Thus, both free amino acids and some amino acids combined in peptide linkages can act as buffers. Acids may be defined as proton donors and bases as proton acceptors. Acids (or bases) described as weak ionize to only a limited extent. The concentration of protons ([H+]) in aqueous solution is expressed as pH, where pH = log 1/[H+] or –log [H+]. The quantitative relationship between the pH of the solution and concentration of a weak acid (HA) and its conjugate base (A−) is described by the Henderson-Hasselbalch equation. A. Equation derivation	Biochemistry_Lippinco. III. ACIDIC AND BASIC PROPERTIES Amino acids in aqueous solution contain weakly acidic α-carboxyl groups and weakly basic α-amino groups. In addition, each of the acidic and basic amino acids contains an ionizable group in its side chain. Thus, both free amino acids and some amino acids combined in peptide linkages can act as buffers. Acids may be defined as proton donors and bases as proton acceptors. Acids (or bases) described as weak ionize to only a limited extent. The concentration of protons ([H+]) in aqueous solution is expressed as pH, where pH = log 1/[H+] or –log [H+]. The quantitative relationship between the pH of the solution and concentration of a weak acid (HA) and its conjugate base (A−) is described by the Henderson-Hasselbalch equation. A. Equation derivation
Biochemistry_Lippincott_16	Biochemistry_Lippinco	A. Equation derivation Consider the release of a proton by a weak acid represented by HA: The salt or conjugate base, A−, is the ionized form of a weak acid. By definition, the dissociation constant of the acid, Ka, is: [Note: The larger the Ka, the stronger the acid, because most of the HA has dissociated into H+ and A−. Conversely, the smaller the Ka, the less acid has dissociated and, therefore, the weaker the acid.] By solving for the [H+] in the above equation, taking the logarithm of both sides of the equation, multiplying both sides of the equation by −1, and substituting pH = −log [H+] and pKa = −log Ka, we obtain the Henderson-Hasselbalch equation: B. Buffers	Biochemistry_Lippinco. A. Equation derivation Consider the release of a proton by a weak acid represented by HA: The salt or conjugate base, A−, is the ionized form of a weak acid. By definition, the dissociation constant of the acid, Ka, is: [Note: The larger the Ka, the stronger the acid, because most of the HA has dissociated into H+ and A−. Conversely, the smaller the Ka, the less acid has dissociated and, therefore, the weaker the acid.] By solving for the [H+] in the above equation, taking the logarithm of both sides of the equation, multiplying both sides of the equation by −1, and substituting pH = −log [H+] and pKa = −log Ka, we obtain the Henderson-Hasselbalch equation: B. Buffers
Biochemistry_Lippincott_17	Biochemistry_Lippinco	B. Buffers A buffer is a solution that resists change in pH following the addition of an acid or base. A buffer can be created by mixing a weak acid (HA) with its conjugate base (A−). If an acid such as HCl is added to a buffer, A− can neutralize it, being converted to HA in the process. If a base is added, HA can likewise neutralize it, being converted to A− in the process. Maximum buffering capacity occurs at a pH equal to the pKa, but a conjugate acid-base pair can still serve as an effective buffer when the pH of a solution is within approximately ±1 pH unit of the pKa. If the amounts of HA and A− are equal, the pH is equal to the pKa. As shown in Figure 1.9, a solution containing acetic acid (HA = CH3 – COOH) and acetate (A− = CH3 – COO−) with a pKa of 4.8 resists a change in pH from pH 3.8 to 5.8, with maximum buffering at pH 4.8. At pH values less than the pKa, the protonated acid form (CH3 – COOH) is the predominant species in solution.	Biochemistry_Lippinco. B. Buffers A buffer is a solution that resists change in pH following the addition of an acid or base. A buffer can be created by mixing a weak acid (HA) with its conjugate base (A−). If an acid such as HCl is added to a buffer, A− can neutralize it, being converted to HA in the process. If a base is added, HA can likewise neutralize it, being converted to A− in the process. Maximum buffering capacity occurs at a pH equal to the pKa, but a conjugate acid-base pair can still serve as an effective buffer when the pH of a solution is within approximately ±1 pH unit of the pKa. If the amounts of HA and A− are equal, the pH is equal to the pKa. As shown in Figure 1.9, a solution containing acetic acid (HA = CH3 – COOH) and acetate (A− = CH3 – COO−) with a pKa of 4.8 resists a change in pH from pH 3.8 to 5.8, with maximum buffering at pH 4.8. At pH values less than the pKa, the protonated acid form (CH3 – COOH) is the predominant species in solution.
Biochemistry_Lippincott_18	Biochemistry_Lippinco	At pH values greater than the pKa, the deprotonated base form (CH3 – COO−) is the predominant species. C. Amino acid titration The titration curve of an amino acid can be analyzed in the same way as described for acetic acid. 1. Carboxyl group dissociation: Consider alanine, for example, which contains an ionizable α-carboxyl and α-amino group. [Note: Its –CH3 R group is nonionizable.] At a low (acidic) pH, both of these groups are protonated (Fig. 1.10). As the pH of the solution is raised, the −COOH group of form I can dissociate by donating a H+ to the medium. The release of a H+ results in the formation of the carboxylate group, −COO−. This structure is shown as form II, which is the dipolar form of the molecule (see Fig. 1.10). This form, also called a zwitterion (from the German word for “hybrid”), is the isoelectric form of alanine, that is, it has an overall (net) charge of zero. 2.	Biochemistry_Lippinco. At pH values greater than the pKa, the deprotonated base form (CH3 – COO−) is the predominant species. C. Amino acid titration The titration curve of an amino acid can be analyzed in the same way as described for acetic acid. 1. Carboxyl group dissociation: Consider alanine, for example, which contains an ionizable α-carboxyl and α-amino group. [Note: Its –CH3 R group is nonionizable.] At a low (acidic) pH, both of these groups are protonated (Fig. 1.10). As the pH of the solution is raised, the −COOH group of form I can dissociate by donating a H+ to the medium. The release of a H+ results in the formation of the carboxylate group, −COO−. This structure is shown as form II, which is the dipolar form of the molecule (see Fig. 1.10). This form, also called a zwitterion (from the German word for “hybrid”), is the isoelectric form of alanine, that is, it has an overall (net) charge of zero. 2.
Biochemistry_Lippincott_19	Biochemistry_Lippinco	2. Application of the Henderson-Hasselbalch equation: The dissociation constant of the carboxyl group of an amino acid is called K1, rather than Ka, because the molecule contains a second titratable group. The Henderson-Hasselbalch equation can be used to analyze the dissociation of the carboxyl group of alanine in the same way as described for acetic acid: where I is the fully protonated form of alanine and II is the isoelectric form of alanine (see Fig. 1.10). This equation can be rearranged and converted to its logarithmic form to yield: 3. Amino group dissociation: The second titratable group of alanine is the amino (−NH3+) group shown in Figure 1.10. Because this is a much weaker acid than the –COOH group, it has a much smaller dissociation constant, K2. [Note: Its pKa is, therefore, larger.] Release of a H+ from the protonated amino group of form II results in the fully deprotonated form of alanine, form III (see Fig. 1.10). 4.	Biochemistry_Lippinco. 2. Application of the Henderson-Hasselbalch equation: The dissociation constant of the carboxyl group of an amino acid is called K1, rather than Ka, because the molecule contains a second titratable group. The Henderson-Hasselbalch equation can be used to analyze the dissociation of the carboxyl group of alanine in the same way as described for acetic acid: where I is the fully protonated form of alanine and II is the isoelectric form of alanine (see Fig. 1.10). This equation can be rearranged and converted to its logarithmic form to yield: 3. Amino group dissociation: The second titratable group of alanine is the amino (−NH3+) group shown in Figure 1.10. Because this is a much weaker acid than the –COOH group, it has a much smaller dissociation constant, K2. [Note: Its pKa is, therefore, larger.] Release of a H+ from the protonated amino group of form II results in the fully deprotonated form of alanine, form III (see Fig. 1.10). 4.
Biochemistry_Lippincott_20	Biochemistry_Lippinco	4. Alanine pKs: The sequential dissociation of H+ from the carboxyl and amino groups of alanine is summarized in Figure 1.10. Each titratable group has a pKa that is numerically equal to the pH at which exactly one half of the H+ have been removed from that group. The pKa for the most acidic group (−COOH) is pK1, whereas the pKa for the next most acidic group (−NH3+) is pK2. [Note: The pKa of the α-carboxyl group of amino acids is ~2, whereas that of the α-amino group is ~9.] 5. Alanine titration curve: By applying the Henderson-Hasselbalch equation to each dissociable acidic group, it is possible to calculate the complete titration curve of a weak acid. Figure 1.11 shows the change in pH that occurs during the addition of base to the fully protonated form of alanine (I) to produce the completely deprotonated form (III). Note the following: a.	Biochemistry_Lippinco. 4. Alanine pKs: The sequential dissociation of H+ from the carboxyl and amino groups of alanine is summarized in Figure 1.10. Each titratable group has a pKa that is numerically equal to the pH at which exactly one half of the H+ have been removed from that group. The pKa for the most acidic group (−COOH) is pK1, whereas the pKa for the next most acidic group (−NH3+) is pK2. [Note: The pKa of the α-carboxyl group of amino acids is ~2, whereas that of the α-amino group is ~9.] 5. Alanine titration curve: By applying the Henderson-Hasselbalch equation to each dissociable acidic group, it is possible to calculate the complete titration curve of a weak acid. Figure 1.11 shows the change in pH that occurs during the addition of base to the fully protonated form of alanine (I) to produce the completely deprotonated form (III). Note the following: a.
Biochemistry_Lippincott_21	Biochemistry_Lippinco	Buffer pairs: The –COOH/–COO− pair can serve as a buffer in the pH region around pK1, and the –NH3+/–NH2 pair can buffer in the region around pK2. b. When pH = pK: When the pH is equal to pK1 (2.3), equal amounts of forms I and II of alanine exist in solution. When the pH is equal to pK2 (9.1), equal amounts of forms II and III are present in solution. c.	Biochemistry_Lippinco. Buffer pairs: The –COOH/–COO− pair can serve as a buffer in the pH region around pK1, and the –NH3+/–NH2 pair can buffer in the region around pK2. b. When pH = pK: When the pH is equal to pK1 (2.3), equal amounts of forms I and II of alanine exist in solution. When the pH is equal to pK2 (9.1), equal amounts of forms II and III are present in solution. c.
Biochemistry_Lippincott_22	Biochemistry_Lippinco	c. Isoelectric point: At neutral pH, alanine exists predominantly as the dipolar form II in which the amino and carboxyl groups are ionized, but the net charge is zero. The isoelectric point (pI) is the pH at which an amino acid is electrically neutral, that is, in which the sum of the positive charges equals the sum of the negative charges. For an amino acid, such as alanine, that has only two dissociable hydrogens (one from the α-carboxyl and one from the α-amino group), the pI is the average of pK1 and pK2 (pI = [2.3 + 9.1]/2 = 5.7) as shown in Figure 1.11. The pI is, thus, midway between pK1 (2.3) and pK2 (9.1). pI corresponds to the pH at which the form II (with a net charge of zero) predominates and at which there are also equal amounts of forms I (net charge of +1) and III (net charge of −1).	Biochemistry_Lippinco. c. Isoelectric point: At neutral pH, alanine exists predominantly as the dipolar form II in which the amino and carboxyl groups are ionized, but the net charge is zero. The isoelectric point (pI) is the pH at which an amino acid is electrically neutral, that is, in which the sum of the positive charges equals the sum of the negative charges. For an amino acid, such as alanine, that has only two dissociable hydrogens (one from the α-carboxyl and one from the α-amino group), the pI is the average of pK1 and pK2 (pI = [2.3 + 9.1]/2 = 5.7) as shown in Figure 1.11. The pI is, thus, midway between pK1 (2.3) and pK2 (9.1). pI corresponds to the pH at which the form II (with a net charge of zero) predominates and at which there are also equal amounts of forms I (net charge of +1) and III (net charge of −1).