Proteinhormone

Protein hormones, also referred to as peptide hormones, are signaling molecules composed of amino acid sequences. They are typically water-soluble and cannot readily cross the lipid bilayer, in contrast to lipid-soluble steroid hormones. They include small peptides, larger polypeptides, and glycoprotein hormones. Protein hormones act by binding to specific receptors on the surface of target cells.

Biosynthesis and processing: They are produced by endocrine or neuroendocrine cells as gene-encoded precursors, such as preprohormones or prohormones. These precursors are processed in the endoplasmic reticulum and Golgi apparatus to mature active forms. Many are stored in secretory vesicles and released by exocytosis in response to physiological stimuli. They often have relatively short circulating half-lives due to proteolysis and renal clearance.

Mechanism of action: Protein hormones exert their effects by binding to cell-surface receptors, which may be G protein-coupled receptors, receptor tyrosine kinases, or other signaling receptors. Receptor activation triggers intracellular signaling cascades, such as cAMP, IP3/DAG, Ca2+-mediated pathways, or the JAK-STAT pathway, leading to rapid cellular responses. Because they do not easily cross membranes, their effects are mediated through translated signaling at the cell surface.

Physiology and clinical relevance: Protein hormones regulate metabolism, growth, reproduction, osmoregulation, and homeostasis. Their secretion is tightly controlled by feedback mechanisms and neuronal input. Defects in production, signaling, or receptor function can cause diseases such as diabetes mellitus (insulin-related), acromegaly (growth hormone excess), or hypopituitarism. Therapeutically, many peptide hormones or their analogs are used clinically, typically via injection due to poor oral stability.

Examples include insulin, glucagon, growth hormone, prolactin, ACTH, TSH, FSH, LH, vasopressin, oxytocin, calcitonin, and parathyroid hormone.