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Steps 5-8 of Citric Acid Cycle
Step 5 of the citric acid cycle involves the conversion of succinyl CoA into succinate by the action of succinyl CoA synthetase. In this reaction, the breaking of the thioester bond releases free energy that can be used to drive the endergonic phosphorylation of GDP into GTP. This is the only step of the citric acid cycle that produces a high-energy purine nucleoside triphosphate. The GTP can now be used to power a G-protein or it can be used to create an ATP molecule. In the remaining three steps of the citric acid cycle, the succinate is transformed into oxaloacetate. In step 6, the succinate is first transformed into fumarate via an oxidation-reduction reaction by the action of succinyl dehydrogenase. In step 7, the fumarate is converted into the L-isomer of malate via a hydration reaction by the enzyme fumarase. In the final step, the malate is transformed into oxaloacetate via an oxidation reduction reaction and the cycle can begin all over again. The final step is catalyzed by malate dehydrogenase. Note that in step 6, we reduce an FAD into an FADH2 and in step 8, we reduce an NAD+ into an NADH.
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