G-protein-linked Receptors in Cell Membranes

G-protein-linked receptors are protein receptors, located in the plasma membrane of a cell, that work with G-proteins to activate a cell-signaling pathway. These receptors are structured similarly in most organisms, with seven α helices and specific loops for binding sites for signal molecules and G-proteins. When a signal molecule from the extracellular fluid attaches to the signal-binding site it activates the G-protein-linked receptor by changing its shape. When this happens, the G-protein, loosely attached to the cytoplasmic side of the cellular membrane, attaches to its binding side on the receptor protein. The inactive G-protein becomes activated when GDP is displaced by GTP, a molecule similar to ATP. When the signal molecule is released, the G-protein diffuses along the cell membrane and attaches to an inactive enzyme. This newly activated enzyme triggers the cellular response. When the protein detaches itself from the enzyme, it releases a phosphate group turning GTP back into GDP, making the G-protein inactive once again.
Receptor tyrosine kinase is a cell membrane receptor system that can trigger multiple cellular responses simultaneously. It requires two receptor tyrosine kinase proteins, which are initially individual polypeptides that each have a signal-binding site, an α helix spanning the cell membrane, and a tail of multiple tyrosines. When signal molecules bind to both proteins they attach through a process called dimerization, forming a dimer. This process activates, or phosphorylates, the ends of the tyrosines, also known as tyrosine-kinase regions. Once the dimer is activated, multiple inactive relay proteins are able to bind to the tyrosine-kinase regions. Each of these proteins trigger a cellul...

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...the adrenal glands, attaches to the signal-binding site of a G-protein-linked receptor. When this happens, the G-protein, which has three subunits, is activated. One subunit, the α subunit, of the G-protein detaches, or dissociates, itself from the other two. This subunit carries a GDP molecule which is replaced by a GTP molecule. Then it attaches to a protein called adenylyl cyclase, which converts ATP into cAMP(Adenosine Monophosphate with a cyclic group). Cyclic AMP, the secondary messenger, is released by the adenylyl cyclase and binds to the regulatory portions of the protein kinase, releasing the catalytic portions. Then these catalytic portions are going to undergo the phosphorylation cascade. The newly activated protein kinases transfer their phosphate groups to a protein called phosphorylase which releases glucose from glycogen into the bloodstream.