Amino acids

One backbone, twenty side chains

An amino acid has a central carbon atom, the alpha carbon, bonded to four things: an amino group, a carboxyl group, a hydrogen atom and a side chain. The first three are the same in every amino acid. The side chain, written R, is what makes glycine different from tryptophan.

Apart from glycine, whose side chain is a single hydrogen atom, the alpha carbon is a chiral centre, so each amino acid exists in two mirror-image forms, L and D. Proteins made by ribosomes use the L form.

Families of side chains

Side chains are usually grouped by how they behave in water, because that drives how a chain folds and what it binds to.

• Nonpolar: glycine, alanine, valine, leucine, isoleucine, proline, methionine, phenylalanine, tryptophan.
• Polar, uncharged: serine, threonine, cysteine, tyrosine, asparagine, glutamine.
• Basic: lysine, arginine, histidine.
• Acidic: aspartic acid, glutamic acid.

Textbooks draw these borders slightly differently; tyrosine and cysteine are the usual borderline cases.

Names and codes

Each amino acid has a three-letter and a one-letter code: alanine is Ala or A, lysine is Lys or K, tryptophan is Trp or W. A peptide sequence is written with these codes, starting from the end with the free amino group and finishing at the end with the free carboxyl group.

Essential and non-essential

Nine amino acids cannot be made by the human body in sufficient amounts and must come from food: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.

Beyond the twenty

Selenocysteine and pyrrolysine are sometimes counted as the twenty-first and twenty-second, because some organisms insert them while building proteins. Chemists also build peptides with residues that ribosomes never use, such as D-amino acids or alpha-aminoisobutyric acid, typically to make the chain harder for enzymes to break down.

General chemistry, written for orientation. It does not describe how to use any substance.