What is Amino acid? — Biology

Definition 12.1 University Biology — Year 1 · Chapter 12 — Amino Acids and Proteins

An amino acid has a central $\alpha$ carbon bearing an amino group ( $-\mathrm{NH_2}$ ), a carboxyl group ( $-\mathrm{COOH}$ ), a hydrogen and a side chain $R$ that distinguishes the twenty standard amino acids of proteins. At the pH of the cell the amino group is protonated and the carboxyl ionised: the molecule is a zwitterion, $\mathrm{^+H_3N{-}CHR{-}COO^-}$ , with no net charge. The $\alpha$ carbon is chiral in all but glycine, and living things use the L enantiomer only. Side chains group by their chemistry:

class	amino acids (three- and one-letter codes)	side chain
non-polar, aliphatic	Gly G, Ala A, Val V, Leu L, Ile I, Met M, Pro P	hydrocarbon; Pro a ring
aromatic	Phe F, Tyr Y, Trp W	rings; Tyr and Trp absorb UV
polar, uncharged	Ser S, Thr T, Cys C, Asn N, Gln Q	$-$ OH, $-$ SH, amide
positively charged	Lys K, Arg R, His H	bases; His p $K_a$ 6.0
negatively charged	Asp D, Glu E	carboxylates, p $K_a$ 4

Nine of them (His, Ile, Leu, Lys, Met, Phe, Thr, Trp, Val) are essential for humans: we cannot make them and must eat them.

Titration of glycine. Two buffering plateaus, at the pK_a of the carboxyl and of the amino group; between them, at the isoelectric point, the zwitterion carries no net charge. — Titration of glycine. Two buffering plateaus, at the p $K_a$ of the carboxyl and of the amino group; between them, at the isoelectric point, the zwitterion carries no net charge.

Examples

Example 12.10 (Collagen)

A quarter of a mammal’s protein is collagen: three chains, each a left-handed helix of a thousand residues with glycine at every third position (Gly–X–Y, Y often hydroxyproline), wound together into a right-handed triple helix $300\,\mathrm{nm}$ long, then packed side by side into fibrils cross-linked by covalent bonds. Glycine’s absence of a side chain is what lets the three chains pack at the axis; a single substitution of glycine by any other residue kinks the helix and gives brittle bones. Vitamin C is needed to hydroxylate the prolines; without it the helix is unstable and the fibrils fail: scurvy.

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Examples

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