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The cytosolic acetyl-CoA can also condense with acetoacetyl-CoA to form 3-hydroxy-3-methylglutaryl-CoA which is the rate-limiting step controlling the synthesis of cholesterol. [16] Cholesterol can be used as is, as a structural component of cellular membranes, or it can be used to synthesize steroid hormones , bile salts , and vitamin D .
In this system, the substances being transported are malate and citrate. The starting material is acetyl-CoA. It is a molecule that is involved in ATP synthesis, protein metabolism, and lipid metabolism. [6] As the inner membrane is not permeable to this molecule, acetyl-CoA needs to be converted into other products for effective transport. [7]
Acetyl Co-A can also be used in fatty acid synthesis, and a common function of the synthetase is to produce acetyl Co-A for this purpose. [3] The reaction catalyzed by acetyl-CoA synthetase takes place in two steps. First, AMP must be bound by the enzyme to cause a conformational change in the active site, which allows the reaction to take place.
The regioselectivity of this step is essential for the subsequent hydration and oxidation reactions. acyl CoA dehydrogenase: trans-Δ 2-enoyl-CoA Hydration: The next step is the hydration of the bond between C-2 and C-3. The reaction is stereospecific, forming only the L isomer. Hydroxyl group is positioned suitable for the subsequent oxidation ...
During this process, acetyl-CoA and water are used as substrates. As a result, the cell does not lose 2 molecules of carbon dioxide as it does in the Krebs cycle. The glyoxylate cycle, facilitated by malate synthase and isocitrate lyase, allows plants and bacteria to subsist on acetyl-CoA or other two carbon compounds.
Glyoxylate condenses with acetyl-CoA (a step catalyzed by malate synthase), yielding malate. Both malate and oxaloacetate can be converted into phosphoenolpyruvate , which is the product of phosphoenolpyruvate carboxykinase , the first enzyme in gluconeogenesis .
Two specific enzymes participate on the carbon monoxide side of the pathway: CO dehydrogenase and acetyl-CoA synthase. The former catalyzes the reduction of the CO 2 and the latter combines the resulting CO with a methyl group to give acetyl-CoA. [2] [1] [3] Some anaerobic bacteria use the Wood–Ljungdahl pathway in reverse to break down acetate.
Acetyl-CoA is formed into malonyl-CoA by acetyl-CoA carboxylase, at which point malonyl-CoA is destined to feed into the fatty acid synthesis pathway. Acetyl-CoA carboxylase is the point of regulation in saturated straight-chain fatty acid synthesis, and is subject to both phosphorylation and allosteric regulation. Regulation by phosphorylation ...