Protein sequence

A protein sequence records the order of amino acid residues in a polypeptide chain. The sequence alone does not specify every modification, covalent crosslink, assembly state or conformation present in a purified sample.

For fixed length $n$ and the standard twenty-amino-acid alphabet, there are $20^n$ sequences: each position has twenty choices. This counts possible sequences, not stable protein folds; the sequence-to-structure relationship is many-to-one and some sequences do not form a stable folded protein.