Both single-celled organisms and cells within tissues can respond to changes within their extracellular environment, changes which trigger a biochemical series of events within the cell. The general mechanism involves the binding of extracellular molecules (signaling ligands) to specific receptors. The receptors then communicate the event of binding to the cell's interior through a signal transduction mechanism initiated by changes in the receptor, such as conformational changes or receptor dimerization.
Many different specific receptors have been identified that operate on the surface of human cells within various tissues. Signaling receptors fall within four general classes. The first class are the enzyme linked receptors which include the cytokine receptor family and growth factor receptor family. G protein-coupled receptors constitute the second class. For future doctors it is relevant to note that G protein-coupled receptors are the most important targets in modern pharmacology. The third group are receptors found intracellularly, which, upon ligand binding, proceed to alter gene transcription as transcription factors. The fourth class are ligand gated ion channels.
Conceptual Vocabulary for Signal Transduction