Hormones (Greek meaning to excite) are chemical substance that has their site of synthesis and action at different places in an organism. They are secreted and synthesized by specialized endocrine glands. Mostly, fall under they four categories;
- Amino acid derivatives
Insulin is one of the most important polypeptide hormones. It was discovered by Banting and Best is 1922 and sequenced in 1955 by Fred Sanger. About 20 years later, Stainer discovered that the two polypeptide chains of insulin are synthesized as a single polypeptide, proinsulin, which folds and is crosslinked by disulfide bonds. The internal peptide is then removed by the concerted action of specific proteases. All these events occur in pancreatic beta cells that synthesize insulin.
All polypeptide hormones are synthesized as large precursors. Some examples of polypeptide hormones are oxytocin, vasopressin, glucagon, Thyrotropin-releasing hormone (TRH) etc.
Amino acid derivatives:
Thyroid hormones and epinephrine are amino acid derivatives. Thyroxine (T4) and the more potent triiodothyroxin (T3) are cleaved from a large precursor protein called thyroglobulin. Thyroglobulin exists as a dimer of two identical polypeptides. It is a strange protein for iodine and can be considered a prehormone of the circulating thyroid hormone.
Thyroglobulin is excreted into the lumen of the thyroid gland, where specific residues are iodinated in one or two positions by special peroxidases. Then two iodinated residues condense and ultimately lysed from the protein in form of hormone. The whole processing takes place on the protein itself.
Sometime epinephrine also called as adrenalin. Its precursors-norepinephrin are synthezed from tyrosine in cells of adrenal medulla.
Steroid hormones are derived from cholesterol: Steroid hormones are derived from cholesterol by stepwise removal of carbon atoms and hydroxylation. They are synthesized by cells of the adrenal cortex and the gondas (e.g. estrogen progestrons, androgens)
Mode of action
Many hormones, especially water soluble ones, do not enter the cells and exert their influence by binding to the receptor proteins and communicate through small molecules. The best-understood messenger is cAMP. Some hydrophilic hormones, like thyroxine and steroid, enter the cells and bind to specific receptors within the cytoplasm of target tissues. These together may directly bind the DNA and influence the cell metabolism.