Answer :
citrate synthase, aconitase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase is the correct order.
What are the steps of citric acid cycle?
Step 1:
The two-carbon acetyl group (from acetyl CoA) is combined with a four-carbon oxaloacetate molecule in the first step to create a six-carbon citrate molecule. The sulfhydryl group (-SH) that CoA is linked to diffuses away and finally joins with another acetyl group. Due to its strong exergonic nature, this action cannot be undone. The availability of ATP and negative feedback both affect how quickly this reaction proceeds. The pace of this process reduces as ATP levels rise. The rate increases when ATP is scarce.
Step 2:
As citrate is transformed into its isomer, isocitrate, it loses one water molecule and gets another.
Step 3 and 4:
Isocitrate is oxidized in step three, resulting in the formation of the five-carbon molecule -ketoglutarate, a molecule of CO2, and two electrons that decrease NAD+ to NADH. ADP's beneficial effect and the negative feedback from ATP and NADH both influence this process. By releasing electrons that convert NAD+ to NADH and carboxyl groups that release CO2 molecules, steps three and four are both oxidation and decarboxylation processes. The results of steps three and four are -ketoglutarate and a succinyl group, respectively. To create succinyl CoA, CoA binds to the succinyl group. ATP, succinyl CoA, and NADH all function as feedback inhibitors, which control the enzyme that catalyses step four.
Step 5:
Coenzyme A is replaced with a phosphate group, and a high-energy link is created. When the succinyl group is changed into succinate, this energy is utilized in substrate-level phosphorylation to create either guanine triphosphate (GTP) or ATP. Depending on the kind of animal tissue they are present in, there are two isoenzyme versions of the enzyme for this step. One type is present in organs like the heart and skeletal muscle that require a lot of ATP. This kind generates ATP. The liver is one of the tissues that contains a lot of anabolic pathways and hence contains the second version of the enzyme. This kind generates GTP. Energy-wise, GTP is equal to ATP, but its application is more constrained. GTP is particularly important for protein synthesis.
Step 6:
Dehydration is the sixth step, which changes succinate into fumarate. Transferring two hydrogen atoms to FAD results in FADH2. Although these atoms' electrons have enough energy to reduce FAD, they lack the necessary energy to reduce NAD+. This carrier, as opposed to NADH, stays affixed to the enzyme and directly delivers the electrons to the electron transport chain. The enzyme that catalyses this step is situated inside the inner membrane of the mitochondrion, which enables this process.
Step 7:
In step seven, fumarate is mixed with water to create malate. Oxidizing malate is the final stage in the citric acid cycle that regenerates oxaloacetate. Another NADH molecule is created.
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