The first step of the citric acid cycle involves the combination of acetyl-CoA with oxaloacetate, producing citrate. Next, aconitase catalyzes the isomerization of citrate to isocitrate, via the intermediate known as cis-aconitate. The conversion of isocitrate to alpha-ketoglutarate is catalyzed by isocitrate dehydrogenase. The citric acid cycle involves the conversion of fumarate to malate, not the reverse, although some bacteria do the reverse citric acid cycle to produce complex organic compounds. Succinyl-CoA is converted to succinate via succinyl-CoA synthetase (also known as succinate thiokinase).

Succinate dehydrogenase (also known as succinate-coenzyme Q reductase or complex II) is bound to the inner mitochondrial membrane. It participates in both the citric acid cycle and the electron transport chain. Aconitase is an enzyme involved in glycolysis, not the citric acid cycle (Krebs cycle).

All enzymes that have NADH as a product would be inhibited by the addition of NADH. The correct answers are citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase. Pyruvate dehydrogenase, which synthesizes acetyl-CoA from pyruvate, is also inhibited by NADH.

Each enzyme listed in the answer choices catalyzes a reaction in glycolysis or the citric acid cycle, but pyruvate dehydrogenase is the only one responsible for converting pyruvate to acetyl-CoA. Pyruvate is decarboxylated (removal of a ) and this is replaced with an S-CoA group, forming the necessary acetyl-CoA molecule to feed into the citric acid cycle. 

Succinate dehydrogenase is the only enzyme in the citric acid cycle to use . The other dehydrogenases use  while citrate synthase performs an unrelated reaction using acetyl-CoA.

Malate dehydrogenase catalyzes the conversion of malate to oxaloacetate and vice versa, while the other enzymes are the three irreversible steps in the citric acid cycle.

Succinate dehydrogenase catalyzes the reaction from succinate to fumarate.  A byproduct of this reaction is the formation of  from .  

The only enzyme listed in the answer choices that does not catalyze a reaction within the citric acid cycle is pyruvate kinase. Pyruvate kinase is an important enzyme in the final step of glycolysis, as it catalyzes the conversion of phosphoenolpyruvate and ADP to pyruvate and ATP. This reaction is not contained within the citric acid cycle though, and therefore pyruvate kinase does not catalyze any reactions in the citric acid cycle.

Each of the other enzymes listed catalyzes reactions within the citric acid cycle, as follows:

Isocitrate dehydrogenase catalyzes the conversion of isocitrate to alpha-ketoglutarate. 

Alpha-ketoglutarate dehydrogenase catalyzes the conversion of alpha-ketoglutarate to succinyl-CoA. 

Succinyl-CoA synthetase catalyzes the conversion of succinyl-CoA to succinate. 

Fumarase catalyzes the conversion of fumarate to malate. 

The only enzyme listed in the answer choices that catalyzes a reaction within the citric acid cycle is isocitrate dehydrogenase, as it catalyzes the formation of alpha-ketoglutarate, carbon dioxide, NADH, and a proton, from isocitrate and .

Each of the other enzymes listed do not catalyze reactions within the citric acid cycle, but rather they catalyze reactions within glycolysis, as follows:

Pyruvate kinase catalyzes the conversion of phosphoenolpyruvate and ADP to pyruvate and ATP.

Glyceraldehyde-3-phosphate dehydrogenase catalyzes the conversion of glyceraldehyde-3-phosphate, , and inorganic phosphate to 1,3-bisphosphoglycerate, NADH, and a proton.

Phosphoglycerate kinase catalyzes the conversion of 1,3-bisphosphoglycerate and ADP to 3-phosphoglycerate and ATP.

Enolase catalyzes the conversion of 2-phosphoglycerate to phosphoenolpyruvate.

The only enzyme listed that participates in the citric acid cycle and catalyzes a step producing  is succinate dehydrogenase. Succinate dehydrogenase catalyzes the conversion of succinate and FAD to fumarate and . 

Pyruvate kinase is incorrect as it neither participates in the citric acid cycle (it is part of glycolysis), nor catalyzes a reaction that produces . 

Isocitrate dehydrogenase catalyzes the conversion of isocitrate and  to alpha-ketoglutarate, NADH, , and CO2, but not .

Fumarase catalyzes the conversion of fumarate and  to malate, but not .

Aconitase catalyzes the conversion of cis-aconitate and  to isocitrate, but not .