Mother Nature builds proteins from a set of "blocks" consisting of 20 amino acids, shown in the Table below.

• All have the same a-amino acid structure, with the same arrangement of ligands around a stereogenic center (except glycine, which is achiral).

• In the outmoded system used by biologists, this configuration is referred to as "L".
  • Amino acids of the opposite, "D", configuration are found in some bacterial peptides and proteins, such as the antiviral proteins valinomycin and gramicidin.

• The Table shows configurations in the modern IUPAC system, developed by Cahn, Ingold, and Prelog.

• Note that two amino acids, isoleucine and threonine, have a second stereogenic center in their side chain
  • Thus, four stereoisomers (two pairs of enantiomers, which are diastereomers of each other) are possible, only one of which is naturally occurring.

The line below each structure contains the three- and one-letter abbreviations widely used for the acid, the approximate natural abundance in typical proteins, and the pK_a values.

• Although amino acids under physiological conditions exist as zwitterions, in which the carboxyl proton is transferred to the amino group, they are shown here as neutrals.

• The background color groups the acids by side-chain category as: (1) nonpolar or hydrophobic (red); (2) polar, uncharged (blue); (3) negatively charged (green); and (4) positively charged (lavender).

Alanine	Valine	Leucine	Isoleucine
Ala, A; 7.8%; 2.35, 9.87	Val, V; 6.6%; 2.29, 9.74	Leu, L; 9.1%; 2.33, 9.74	Ile, I; 5.3%; 2.32, 9.76
Proline	Methionine	Phenylalanine	Tryptophan
Pro, P; 5.2%; 1.95, 10.64	Met, M; 2.2%; 2.13, 9.28	Phe, F; 3.9%; 2.16, 9.18	Trp, W; 1.4%; 2.43, 9.44
Glycine	Serine	Threonine	Cysteine
Gly, G; 7.2%; 2.35, 9.78	Ser, S; 6.8%; 2.19, 9.21	Thr, T; 5.9%; 2.09, 9.11	Cys, C; ?; 1.92, 8.35, 10.46
Tyrosine	Asparagine	Glutamine	Aspartic Acid
Tyr, Y; 3,2%; 2.20, 9.11, 10.13	Asn, N; 4.3%; 2.1, 8.84	Gln, Q; 4.3%; 2.17, 9.13	Asp, D; 5.3%; 1.99, 3.90, 9.90
Glutamic Acid	Histidine	Lysine	Arginine
Glu, E; 6.3%; 2.10, 4.07, 9.47	His, H; 2.3%; 1.80, 6.04, 9.33	Lys, K; 5.9%; 2.16, 9.18, 10.79	Arg, R; 5.1%; 1.82, 8.99, 12.48