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25.1: Types of Biologically Important Amino Acids

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    Protein Amino Acids

    The amino acids that occur naturally as constituents of proteins have an amino group \(\left( \ce{NH_2} \right)\) and a carboxylic acid group \(\left( \ce{CO_2H} \right)\) attached to the same carbon. They are called \(\alpha\)-amino acids and have the general formula

    Roberts and Caserio Screenshot 25-1-1.png

    They differ only in the nature of the \(\ce{R}\) group on the \(\alpha\) carbon and, with few exceptions, they are chiral molecules with the \(L\) configuration at the chiral \(\alpha\) carbon:\(^1\)

    Roberts and Caserio Screenshot 25-1-2.png

    The structures and names of some particularly important \(\alpha\)-amino acids are shown in Table 25-1. You will notice that the names in common use for amino acids are not descriptive of their structural formulas; but at least they have the advantage of being shorter than the systematic names. The abbreviations Gly, Glu, and so on, that are listed in Table 25-1 are particularly useful in designating the sequences of amino acids in proteins and peptides, as will become evident later in the chapter.

    Table 25-1: Amino Acids Important as Constituents of Proteins

    Roberts and Caserio Screenshot 25-1-3.png

    Roberts and Caserio Screenshot 25-1-4.png

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    The nature of the substituent \(\ce{R}\) varies considerably. In some amino acids, \(\ce{R}\) is a hydrocarbon group, whereas in others it possesses functional groups such as \(\ce{OH}\), \(\ce{SH}\), \(\ce{SCH_3}\), \(\ce{CO_2H}\), or \(\ce{NH_2}\). Amino acids that have amine or other basic functions in the \(\ce{R}\) group are called basic amino acids (lysine and arginine), whereas those with acidic groups are called acidic amino acids (aspartic and glutamic acids). Three of the amino acids listed in Table 25-1 (cysteine, cystine, and methionine) contain sulfur in \(\ce{-SH}\), \(\ce{-S-S}-\), and \(\ce{-SCH_3}\) groups. Cysteine and cystine can be interconverted readily with a wide variety of oxidizing and reducing agents according to the general reaction \(2 \ce{RSH} \underset{\left[ \ce{H} \right]}{\overset{\left[ \ce{O} \right]}{\rightleftharpoons}} \ce{RSSR}\). This is an important process in the biochemistry of sulfur-containing peptides and proteins (Section 25-8A).

    The \(\alpha\)-amino function of the common amino acids is primary \(\ce{-NH_2}\) in all except proline and hydroxyproline. Several of the amino acids have aromatic \(\ce{R}\) groups (phenylalanine, tyrosine, tryptophan), while histidine and tryptophan have azarene \(\ce{R}\) groups.

    Nonprotein Amino Acids

    The most abundant amino acids are those that are protein constituents and these are always \(\alpha\)-amino acids. However, there are many other amino acids that occur naturally in living systems that are not constituents of proteins, and are not \(\alpha\)-amino acids. Many of these are rare, but others are common and play important roles in cellular metabolism. For example, 3-aminopropanoic acid is a precursor in the biosynthesis of the vitamin, pantothenic acid,\(^2\)

    Roberts and Caserio Screenshot 25-1-6.png

    and 4-aminobutanoic acid is involved in the transmission of nerve impulses.

    Roberts and Caserio Screenshot 25-1-7.jpg

    Homocysteine\(^3\) and homoserine are among the important \(\alpha\)-amino acids that are not constituents of proteins. These substances are precursors in the biosynthesis of methionine.

    Roberts and Caserio Screenshot 25-1-8.png

    \(^1\)A number of \(D\)-amino acids have been found to be constituents of peptides in the cell walls of bacteria.

    \(^2\)Pantothenic acid is in turn a precursor for the synthesis of coenzyme A, which is essential for the biosynthesis of fats and lipids (Sections 18-8F and 30-5A).

    \(^3\)The prefix homo implies an additional carbon in the longest chain.

    Contributors and Attributions

    John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."

    This page titled 25.1: Types of Biologically Important Amino Acids is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by John D. Roberts and Marjorie C. Caserio.