Residues Are the Letters Peptides Are Spelled With

A chain is a sentence

When biochemists describe a peptide they do not draw every atom. They write a string of letters, one for each residue, which is what an amino acid is called once it has been built into a chain. Glycine is G, alanine is A, tryptophan is W. The string reads from the N-terminus (the free amine end) to the C-terminus (the free acid end).

A tiny example: GHK is glycine, histidine, lysine. Three residues, three letters.

Why order matters

The same three residues in a different order are a different molecule, with different shape, charge distribution and behaviour. KHG is not GHK. Biology is picky about sequence in the same way a lock is picky about a key.

Four families of residues

Beyond the twenty

Real peptide drugs often step outside the standard alphabet. Chemists add D-forms (mirror images), N-methyl groups or entirely synthetic residues to slow down the enzymes that cut chains apart. When a label lists a sequence with odd symbols, that is usually the reason.

What to take away

• A peptide's identity is its sequence, written N to C.
• Residue properties explain more behaviour than the length does.
• If a product page never states the sequence, you cannot know what it is.