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Secondary Messenger Systems
Most signal-transduction pathways (pathways that involve the passing down of a signal from one cell to another) involve a set of molecules called primary messenger and secondary messenger molecules. These systems, which are usually controlled by G-protein complexes found on the membrane of the cell, are called secondary messenger systems. To demonstrate how these systems work, lets take a look at the secondary messenger system of the activation of protein kinase A (protein that catalyzes the phosphorylation of other proteins). The primary messenger (also known as first messenger) in this signal-transduction pathway is epinephrine and it attaches to a binding site on the extracellular side of a transmembrane protein called beta-adrenergic receptor. On the cytoplasmic side of the transmembrane are a group of proteins (alpha, beta and gamma subunits) which are bound together. The binding of the epinephrine induces conformational changes to the protein and causes the alpha subunit (a G protein) to dissociate from the complex and move onto another membrane protein called adenylate cyclase, which converts ATP into cyclic AMP. Cyclic AMP is the secondary messenger and it goes on to convert protein kinase A into its active form.
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