What is a peptide?

A peptide is a molecule made of two or more amino acids connected by peptide bonds. The distinction between 'peptide' and 'protein' is a matter of length rather than chemistry: conventionally, chains up to about 50 amino acids are called peptides, and chains longer than that are called proteins. Both are polymers of the same 20 standard amino acids, joined by the same amide-forming reaction between one residue's carboxyl group and the next residue's amine group.

Biologically, peptides act as hormones, neurotransmitters, growth factors, immune signals, and structural components. Insulin (51 amino acids) is a peptide hormone; glucagon (29 amino acids) is a peptide hormone; GLP-1 (30 amino acids) is a peptide hormone; oxytocin (9 amino acids) is a peptide hormone. The therapeutic peptides in this catalog — semaglutide, tirzepatide, BPC-157, PT-141 — are all synthetic peptides designed to engage the same receptors that native peptide hormones engage.

The reason peptides are pharmacologically interesting is that they are potent (native peptide hormones act at nanomolar or picomolar concentrations), specific (they engage receptors with high selectivity), and biologically native (they use signalling pathways the body already runs). The reason they are pharmaceutically challenging is that they are generally not stable in the gastrointestinal tract, so most peptide drugs are injected rather than swallowed.