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 . 

The citric acid cycle enzyme that catalyzes the reaction directly responsible for the production of succinyl-CoA is alpha-ketoglutarate dehydrogenase. Alpha-ketoglutarate dehydrogenase catalyzes the conversion of alpha-ketoglutarate, , and CoA-SH to succinyl-CoA, NADH, , and . 

Each of the other enzymes listed are enzymes that participate in the citric acid cycle, but not in the step the directly produces succinyl-CoA. Their general roles are as follows:

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

Succinate dehydrogenase catalyzes the conversion of succinate to fumarate.

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

Fumarase catalyzes the conversion of fumarate to malate.

The only correct statement within the answer choices is that succinyl-CoA synthetase catalyzes the reaction responsible for the formation of succinate. In this reaction, the citric acid cycle enzyme, succinyl-CoA synthetase, catalyzes the conversion of succinyl-CoA, GDP (or ADP) and inorganic phosphate to succinate, CoA-SH, and GTP (or ATP). 

The incorrect answer choices are explained below:

"Succinyl-CoA synthetase is produced by the reaction catalyzed by alpha-ketoglutarate dehydrogenase."

This is incorrect because it is succinyl-CoA that is produced by the reaction catalyzed by alpha-ketoglutarate dehydrogenase. Succinyl-CoA synthetase, the enzyme, is not produced by this reaction.

"Succinyl-CoA synthetase catalyzes the reaction responsible for the rate-limiting step of glycolysis."

Succinyl-CoA synthetase does not participate in glycolysis; it participates in the citric acid cycle. Furthermore, even in the citric acid cycle, it does not catalyze the reaction responsible for the rate-limiting step of the citric acid cycle.

"Succinyl-CoA synthetase catalyzes the reaction responsible for the formation of succinyl-CoA."

Alpha-ketoglutarate dehydrogenase catalyzes the reaction responsible for the formation of succinyl-CoA. Succinyl-CoA synthetase catalyzes the reaction responsible for the conversion of succinyl-CoA to succinate.

"Succinyl-CoA synthetase directly catalyzes a citric acid cycle reaction that produces ."

Succinate dehydrogenase directly catalyzes a citric acid cycle reaction that produces , not succinyl-CoA synthetase.

The rate-limiting step of the citric acid cycle is catalyzed by the enzyme, isocitrate dehydrogenase. Isocitrate dehydrogenase catalyzes the conversion of isocitrate and  to alpha-ketoglutarate, NADH, a proton, and a molecule of carbon dioxide. 

Phosphofructokinase-1 (PFK-1) is incorrect, as it catalyzes the rate-limiting step of glycolysis, not the citric acid cycle.

Fructose 1,6-bisphosphatase is incorrect, as it catalyzes the rate-limiting step of gluconeogenesis, not the citric acid cycle.

Succinyl-CoA synthetase is incorrect, as it catalyzes a reaction within the citric acid cycle that is not the rate-limiting step.

Pyruvate kinase is incorrect as the reaction that it catalyzes is neither within the citric acid cycle, nor a rate-limiting step.

Phosphoenolpyruvate (PEP) is an intermediate in glycolysis, not the citric acid cycle. PEP is the product of the ninth reaction in glycolysis, which involves the enolase-catalyzed conversion of 2-phosphoglycerate into PEP. All other molecules are indeed intermediates in the citric acid cycle.

The three-carbon molecule pyruvate produced from glycolysis is converted to the two-carbon molecule acetyl-coenzyme A (acetyl-CoA). This is carried out by a combination of three enzymes collectively known as the pyruvate dehydrogenase complex. The conversion of pyruvate to acetyl-CoA produces one . Acetyl-CoA has one less carbon than pyruvate; this third carbon from pyruvate was lost as carbon dioxide during its conversion to acetyl-CoA via the pyruvate dehydrogenase complex.