Peptide Sequence

A peptide sequence designates the precise order in which amino acids are strung together to form a peptide. By universal convention, it reads from the N-terminus (free amine) to the C-terminus (free carboxyl), and is written using the three-letter code (Gly-Ala-Pro) or the one-letter code (GAP). This sequence is the primary structure of the peptide: it contains all the information needed to predict, modulate, and understand its biological behavior.

The sequence role is decisive. Per Anfinsen's principle, a peptide or protein three-dimensional structure is entirely encoded in its primary sequence. The residue order governs intramolecular interactions (hydrogen bonds, hydrophobic interactions, disulfide bridges, salt bridges) that dictate folding into alpha-helix, beta-sheet, turn, or disordered conformation. A single substitution can trigger major consequences: the Val → Glu mutation at position 6 of beta-hemoglobin causes sickle-cell disease, and changing Lys → Arg on a peptide can double its receptor affinity.

In practice, three information levels can be distinguished within a sequence. The recognition motif (RGD for integrins, GxxxG for transmembrane helices) identifies critical functional sequences — this is the peptide's "pharmacophore". Structural residues (prolines to break helices, glycines for flexibility, cysteines for disulfide bridges) stabilize 3D geometry. Accessory residues modulate physicochemical properties without being directly involved in activity.

Comparing sequences across homologous peptides reveals conserved residues: these are almost systematically essential to function. For example, the GLP-1 family shares a conserved N-terminal architecture (His1-Ala2-Glu3-Gly4) indispensable for receptor binding. Synthetic agonists (semaglutide, liraglutide, tirzepatide) introduce strategic substitutions on this ancestral sequence (Aib at position 2, K26 acyl) that extend half-life without abolishing recognition.

For the analyst, sequence verification is a crucial step. Tandem mass spectrometry (MS/MS), combined with Edman N-terminal sequencing, reconstructs the actual sequence of a production batch. Any anomaly (missing amino acid, unintended substitution, Asn → Asp deamidation, D/L isomerization) is detectable by this approach and appears in the certificate of analysis (COA). A quality research peptide shows 100% sequence conformity to the theoretical design, >95% HPLC purity, and molecular mass confirmed by MS.