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Steps 2-4 of Citric Acid Cycle
Once citrate has formed, the citrate then goes on to step 2 in which it is transformed into an isomer molecule called isocitrate. This step is carried out in two different reactions; a dehydrolysis reaction followed by a hydrolysis reaction. This step is catalyzed by an enzyme called aconitase, which is given this name because this step goes through an intermediate molecule called cis-aconitate. The isocitrate differs from the citrate it the positioning of the hydroxyl group. The location of the hydrolysis on isocitrate allows it to undergo step 3, the first oxidative decarboxylation reaction in the citric acid cycle. Step 3 is also a two-step process that is catalyzed by an enzyme called isocitrate dehydrogenase. The first step is an oxidation-reduction reaction that abstracts high energy electrons from the isocitrate to from an NADH and oxalosuccinate. The oxalosuccinate then undergoes a decarboxylation step to release carbon dioxide and produce alpha-ketoglutarate. The alpha-ketoglutarate then moves on to step 4, which is the second oxidative decarboxylation reaction of the citric acid cycle. This step releases a CO2 molecule, abstracts electrons to produce an NADH molecule and produces a product molecule called succinyl-CoA. The enzyme that catalyzes step 4 is called the alpha-ketoglutarate dehydrogenase complex. This enzyme is very similar to the enzyme complex used by step 1 of the citric acid cycle in that it also consists of three enzymes and uses several different coenzymes such as thiamine pyrophosphate, lipoic acid and flavin adenine dinucleotide.
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