Amino acids

Every amino acid has the same backbone: a central alpha-carbon carrying a carboxyl group (-COOH), an amino group (-NH₂), a hydrogen atom, and a side chain, called R. Only R differs among the 20 standard amino acids. In glycine, R is just a hydrogen; in alanine, it is a methyl group (CH₃).

In every amino acid except glycine, the alpha-carbon holds four different groups, so it is chiral, with two mirror-image forms, L and D. Only L forms are found in proteins.

Amino acids are grouped by their R group:

• Polar, uncharged: serine, threonine, cysteine, asparagine, glutamine.
• Aromatic (with a ring): phenylalanine, tryptophan, tyrosine.
• Non-polar, aliphatic: glycine, alanine, valine, leucine, isoleucine, methionine, proline.
• Basic (positively charged): lysine, arginine, histidine.
• Acidic (negatively charged): aspartate, glutamate.

Carbons in the side chain are named with Greek letters outwards from the alpha-carbon: beta, gamma, delta, epsilon. At the pH of the body (pH 7), an amino acid carries a negative charge on its carboxyl group (-COO⁻) and a positive charge on its amino group (-NH₃⁺) at the same time: it is a zwitterion.

Besides the 20 standard ones, some non-standard amino acids are made by altering standard ones after they are built into a protein, such as 4-hydroxyproline and 5-hydroxylysine (in collagen). Others, non-protein amino acids such as ornithine, citrulline and GABA, are never built into proteins at all.