These are transmembrane receptors linked with intracellular enzymes; physiological receptors consisting of an extracellular ligand-binding domain and an intracellular effector domain (intrinsic enzymatic activity) which activates the signal transduction pathway that leads to amplification and propagation of signals within the cell to alter cellular process via secondary messenger or transducers. fig: Enzyme-linked receptors. The receptor molecules consist of single polypeptide chains with large, cysteine-rich extracellular domains, a short hydrophobic transmembrane domain (single α-helix), and an intracellular region containing one or more protein kinase domains. Signal Transduction Pathway: 1. Ligand Binding The inactive state of receptors is monomeric; when activated, these receptors undergo conformational changes resulting in increased cytosolic enzyme activity. This response lasts from minutes to an hour. 2. Conformational Change When the ligand binds to two
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