Pyruvic acid combines with coenzyme A to form which molecule that enters the Krebs cycle?

• A. Coenzyme A
• B. Oxaloacetate
• C. Acetyl-CoA
• D. CoA-SH

Answer: (C)

The key idea is that pyruvate must be activated before entering the Krebs cycle. The pyruvate dehydrogenase complex converts pyruvate into acetyl-CoA, releasing CO2 and generating NADH in the process. The acetyl-CoA then donates its acetyl group to oxaloacetate to form citrate, kickstarting the Krebs cycle. Among the options, the molecule formed from pyruvate that actually enters the cycle is acetyl-CoA. The other choices are not the carrier of the two-carbon unit into the cycle: free CoA or CoA-SH are just carriers without the acetyl group; oxaloacetate is a downstream Krebs cycle intermediate; pyruvate itself is not directly in the cycle.