peptide hormone

A peptide hormone is a chemical message written in the same alphabet as proteins — short strings of amino acids — but kept brief, like a text message rather than a novel. Because these chains carry many charged, water-loving groups, peptide hormones dissolve easily in blood and cannot slip through the greasy cell membrane. Familiar examples include insulin, glucagon, and the hypothalamic releasing hormones.

Cells make peptide hormones on ribosomes, package them into storage vesicles, and release them in bursts when a signal arrives. Since they are water-soluble, they travel freely in plasma without needing a carrier protein, but for the same reason they cannot enter the target cell. Instead they dock onto receptors studded on the cell surface and trigger internal signals through second messengers.

The boundary between peptide and protein hormones is one of convenience, not biology: chemists often call chains under roughly fifty amino acids peptides and longer ones proteins. Both are synthesized as larger precursors and trimmed down, and both share the same water-soluble, surface-acting logic.