Classification

In addition to the amino and carboxyl groups, amino acids have a side chain or R group attached to the α-carbon. Each amino acid has unique characteristics arising from the size, shape, solubility, and ionization properties of its R group. As a result, the side chains of amino acids exert a profound effect on the structure and biological activity of proteins. Although amino acids can be classified in various ways, one common approach is to classify them according to whether the functional group on the side chain at neutral pH is nonpolar, polar but uncharged, negatively charged, or positively charged. The structures and names of the 20 amino acids, their one- and three-letter abbreviations, and some of their distinctive features are given in Table (PageIndex{1}).

Table (PageIndex{1}): Common Amino Acids Found in Proteins Common Name Abbreviation Structural Formula (at pH 6) Molar Mass Distinctive Feature Amino acids with a nonpolar R group glycine gly (G) 75 the only amino acid lacking a chiral carbon alanine ala (A) 89 — valine val (V) 117 a branched-chain amino acid leucine leu (L) 131 a branched-chain amino acid isoleucine ile (I) 131 an essential amino acid because most animals cannot synthesize branched-chain amino acids phenylalanine phe (F) 165 also classified as an aromatic amino acid tryptophan trp (W) 204 also classified as an aromatic amino acid methionine met (M) 149 side chain functions as a methyl group donor proline pro (P) 115 contains a secondary amine group; referred to as an α-imino acid Amino acids with a polar but neutral R group serine ser (S) 105 found at the active site of many enzymes threonine thr (T) 119 named for its similarity to the sugar threose cysteine cys (C) 121 oxidation of two cysteine molecules yields cystine tyrosine tyr (Y) 181 also classified as an aromatic amino acid asparagine asn (N) 132 the amide of aspartic acid glutamine gln (Q) 146 the amide of glutamic acid Amino acids with a negatively charged R group aspartic acid asp (D) 132 carboxyl groups are ionized at physiological pH; also known as aspartate glutamic acid glu (E) 146 carboxyl groups are ionized at physiological pH; also known as glutamate Amino acids with a positively charged R group histidine his (H) 155 the only amino acid whose R group has a pKa (6.0) near physiological pH lysine lys (K) 147 — arginine arg (R) 175 almost as strong a base as sodium hydroxide

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The first amino acid to be isolated was asparagine in 1806. It was obtained from protein found in asparagus juice (hence the name). Glycine, the major amino acid found in gelatin, was named for its sweet taste (Greek glykys, meaning “sweet”). In some cases an amino acid found in a protein is actually a derivative of one of the common 20 amino acids (one such derivative is hydroxyproline). The modification occurs after the amino acid has been assembled into a protein.

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