Final answer:
Citrate synthase in the Krebs cycle binds the substrate oxaloacetate first to catalyze a condensation reaction with acetyl-CoA, forming citrate. The reaction is highly exergonic, not endergonic, and citrate synthase does not bind pyruvate.
Step-by-step explanation:
The question pertains to the function of the enzyme citrate synthase in the Krebs cycle or citric acid cycle, a fundamental metabolic pathway that occurs in the mitochondria of eukaryotic cells. Answering the multiple-choice question regarding citrate synthase, the correct response is:
- A. Binds the substrate oxaloacetate first.
Citrate synthase catalyzes a condensation reaction that combines acetyl-CoA with oxaloacetate to form citrate. This reaction is considered highly exergonic, meaning it releases energy and drives the process forward. Therefore, it is not endergonic as an endergonic reaction absorbs energy. Citrate synthase does not bind to pyruvate but to acetyl-CoA, which implies that option D is incorrect. Finally, the action of citrate synthase is vital to the continuation of the Krebs cycle, as it prepares the six-carbon citrate molecule for subsequent reactions that contribute to cellular respiration and energy production.