Which molecule acts as the acetyl donor entering the Krebs cycle?

• A. Pyruvate
• B. Acetyl-CoA
• C. Oxaloacetate
• D. Citrate

Answer: (B)

In the Krebs cycle, the first step is a condensation that brings in the acetyl unit from a carrier molecule. The acetyl group is donated by acetyl-CoA, which combines with oxaloacetate to form citrate. This transfer restores CoA, which is recycled for other reactions. Pyruvate is a precursor that must be converted to acetyl-CoA before it can contribute the acetyl group; it does not enter the cycle directly as the donor. Oxaloacetate is the six-carbon acceptor that combines with the acetyl group, and citrate is the product formed after this condensation.