Proteinhormoner

Proteinhormoner, or protein hormones, are hydrophilic signaling molecules composed of polypeptide chains or small proteins produced by endocrine cells. They are encoded by specific genes and usually synthesized as larger precursors. Stored in secretory vesicles, they are released by regulated exocytosis and travel in the bloodstream dissolved in plasma. The term covers peptide hormones and related small protein hormones.

Synthesis and processing: translation begins as preprohormones; signal peptides direct nascent chains into the endoplasmic reticulum. After removal of the signal peptide, prohormones are further cleaved by prohormone convertases in the Golgi and secretory granules to generate active hormones. Some require disulfide bonds or glycosylation. Hormones are stored until a stimulus prompts exocytosis.

Mechanism of action: being hydrophilic, protein hormones cannot cross the plasma membrane; they bind to cell-surface receptors such as G protein-coupled receptors and receptor tyrosine kinases. Receptor activation triggers second messenger systems (cAMP, Ca2+, IP3/DAG) or kinase cascades, producing rapid changes in metabolism, growth, secretion, or gene expression in target cells.

Examples and properties: insulin and glucagon regulate glucose metabolism; growth hormone and prolactin influence growth and lactation; parathyroid hormone and calcitonin regulate calcium; vasopressin and oxytocin control water balance and reproductive processes. Protein hormones generally have shorter plasma half-lives than steroid hormones and are degraded by circulating proteases. They are central to physiology and are a major focus of therapeutic peptide development.