Peptide Sequences & One-Letter Codes Explained

A peptide’s sequence is the order of its amino acids, and it’s usually written as a string of letters. Knowing how to read those codes makes spec sheets far less intimidating. Here’s how it works.

• Sequence: the exact order of amino acids in a peptide.
• One-letter codes: each amino acid has a single-letter shorthand.
• Direction: sequences are read from the N-terminus to the C-terminus.

How the codes work

Each of the 20 amino acids has both a three-letter code (like Gly, Ala, Lys) and a one-letter code (G, A, K). A sequence is simply those letters strung together in order — for example, GAK means glycine–alanine–lysine.

Reading direction

By convention, sequences are written and read from the N-terminus (the start) to the C-terminus (the end). That direction matters because a peptide read backwards would be a different molecule — see N-terminus & C-terminus.

For more terminology, see the research glossary.