protein hormone

A protein hormone is essentially a large peptide hormone — a long chain of amino acids that may fold into a complex shape or even join with other chains. If a peptide hormone is a short text message, a protein hormone is a full letter, sometimes folded and sealed in an envelope. Growth hormone and insulin are classic examples, and some, like the pituitary gonadotropins, are stitched together from two separate subunits.

Like all amino-acid-based hormones, protein hormones are made on ribosomes as larger precursors, folded inside the cell, often decorated with sugar chains (glycosylation) that protect them and tune their lifespan, then stored in granules until release. Being water-soluble, they circulate freely and act on cell-surface receptors, never entering the cell themselves.

The distinction from peptide hormones is purely one of size and folding complexity. In practice the two terms blur together, and many textbooks simply speak of peptide hormones as a single broad class covering everything from three-amino-acid messengers to large glycoproteins.