Voet Chapter 4

Voet Chapter 4 - C hapter 4 Amino Acids 1 T he A mino Acids...

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Chapter 4 Amino Acids 1 • The Amino Acids of Proteins A. General Properties B. Peptide Bonds C. C'lilssiJjcation and Characteristics D. Acid-I3a&e Properties E. A Few Words on Nomt:nclature 2 • Optical Activit), A. An Operational Classification B. The Fischer Convention C. The Cahn-Ingold-Prelog. System D. Chirality and Biochemistry 3. "Nonstandard" Amino Acids A. Amino Acid D<:rivatives in Proteins Specialized Roles of Amino Acids It is hardly surprising that much of the early hiochemieal research was concerned wi Lh lhe study of proteins. Proteins fO/1l1 the class of hiological macromoleeu!l;s that have the most well-ddined physicochemical properties. and conse- quently Lhey were generally easier to isolate and charac- terize than nucleic acids, polysaccharides, or lipids. Furthermore, proteins, particularly in the form of enzymes. have ohvious hioehemical functions. The central role that proLeins play in biological processes has therefore been recognized since the earliest days of biochemistry. In con- Lrast. the task of nucleic acids in the transmission and ex- pression of genetic information was not realized until the laLc 1940s and their catalytic function only began to come Lo light in the 1980s, the role of lipids in biological membranes was noL appreciated until the 1960s, and the biological functions of polysaccharides are still somewhat mysterious. In this chapter we study the structures and properties of the monomeric units of proteins. the amino acids. is frol11 FIGURE 4-1 Ceneral structural rormula ror a-amino acids. There are 20 diJferent R groups in the cOillmonly occurring aminu acids (Table 4-1). these substances that proteins are synthesized through processes that we discuss in Chapter 32. Amino acids are also energy metabolites and, ill animals. many of them are essentjalnutrients (Chapter 26). In addition. as we shall see, many amino acids and their derivatives are of biochemical importance in their own right (Section 4-3.8). 1 • THE AMINO ACIDS OF PROTEINS Tile analyses of a vast number of proteins from almost every conceivable source have shown that all proteins are composed of the 20 "standard" amino acids listed in Table 4-1. These substances are known as (X-amino acids because. with the exception of proline, they bave a primary amino group and a carboxylic acid group substituent on the same carbon atom (Fig. 4-1.; proline has a secondary amino group). General Properties The pK values of the 20 "standard" a-amino acids or rro- teins are tabulated in Table 4-1. Here pK] am) pK b re- spectively. refer to tbe a-carboxylic aciel and a-amino groups, and R refers to the side groups with aciJ-hase properties. Table 4-1 indicates that the pK values or lhe a-carboxylic acid groups lie in a slllall range around 2.2 so that above pH 3.5 these groups are almost entirely in their carboxylate forms. The <X-amino groups alllwve pK values near 9.4 and are therefore almost entirely in their ammo- nium ion forms below pH 8.0. This leads to an important structural point: In the physiological pH range, bUlh the car-
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Voet Chapter 4 - C hapter 4 Amino Acids 1 T he A mino Acids...

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