Create a list of reactions and corresponding rate equations representing the sig

Question

Create a list of reactions and corresponding rate equations representing the signaling pathway activated by odorantsin olfactory neurons. Your model should include the following species, as well as any complexes formed between them:Odorant, Olfactory Receptor (OR), Golf, G (you do not need to worry about the association/dissociation of beta and gamma with each other; you can assume they stay associated at all times and treat them as one species), GTP, GDP,Adenylyl cyclase, ATP, cAMP, CNGopen, CNGclosed. For this part of the problem, you are just responsible for the steps shown in the figure (and any intermediate steps which are implied but not shown), beginning with the process of odorant binding to an olfactory receptor and ending with the opening of the CNG cation channel. You do not need to include Ca2+ or Na+  or their transport. Please include dissociation reactions for any non-covalently associated complexes, but you do not need to add any reactions that act as negative. Then construct a system of differential equations corresponding to the rate equations.

Explanation / Answer

Many G-protein-coupled receptors have a large extracellular ligand-binding domain. When an appropriate protein ligand binds to this domain, the receptor undergoes a conformational change that is transmitted to its cytosolic regions, which now activate a trimeric GTP-binding protein (or G protein for short). As the name implies, a trimeric G protein is composed of three protein subunits called alpha, beta, and gamma. Both the alpha and gamma subunits have covalently attached lipid tails that help anchor the G protein in the plasma membrane. In the absence of a signal, the alpha subunit has a GDP bound, and the G protein is inactive. In some cases, the inactive G protein is associated with the inactive receptor, while, in other cases, as shown here, it only binds after the receptor is activated. In either case, an activated receptor induces a conformational change in the alpha subunit, causing the GDP to dissociate. GTP, which is abundant in the cytosol, can now readily bind in place of the GDP. GTP binding causes a further conformational change in the G protein, activating both the alpha subunit and beta-gamma complex. In some cases, as shown here, the activated alpha subunit dissociates from the activated beta-gamma complex, whereas in other cases the two activated components stay together. In either case, both of the activated components can now regulate the activity of target proteins in the plasma membrane, as shown here for a GTP-boundalpha subunit. The activated target proteins then relay the signal to other components in the signaling cascade. Eventually, the alpha subunit hydrolyses its bound GTP to GDP, which inactivates the subunit. This step is often accelerated by the binding of another protein, called a regulator of G-protein signaling (or RGS). The inactivated, GDP-bound alpha subunit now reforms an inactive G protein with a beta-gamma complex, turning off other downstream events. As long as the signaling receptor remains stimulated, it can continue to activate G-proteins. Upon prolonged stimulation, however, the receptors eventually inactivate, even if their activating ligands remain bound. In this case, a receptor kinase phosphorylates the cytosolic portions of the activated receptor. Once a receptor has been phosphorylated in this way, it binds with high affinity to an arrestin protein, which inactivates the receptor by preventing its interaction with G proteins. Arrestins also act as adaptor proteins, and recruit the phosphorylated receptors to clathrin-coated pits, from where the receptors are endocytosed, and afterwards they can either be degraded in lysosomes or activate new signaling pathways.

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