Which molecule combines with oxaloacetetic acid to start the Krebs cycle?

Which molecule combines with oxaloacetetic acid to start the Krebs cycle?

• A. Acetyl Coenzyme A
• B. Oxaloacetic acid
• C. Citric acid
• D. Pyruvate

Answer: (A)

The key idea is how the Krebs cycle is triggered: a two-carbon acetyl group from acetyl-CoA combines with the four-carbon oxaloacetate to form citrate, the first molecule of the cycle. Acetyl-CoA provides that acetyl unit and, in the presence of the enzyme citrate synthase in the mitochondria, condenses with oxaloacetate to make citrate while CoA is released. Acetyl-CoA itself comes from pyruvate via the pyruvate dehydrogenase complex during aerobic respiration, producing NADH and releasing CO2 in the process. The other options don’t fit this initiating step: oxaloacetate is the acceptor, not the donor; pyruvate is upstream and must be converted to acetyl-CoA; citrate (the product) is formed after the reaction. So the molecule that starts the Krebs cycle by combining with oxaloacetate is Acetyl-CoA.

The key idea is how the Krebs cycle is triggered: a two-carbon acetyl group from acetyl-CoA combines with the four-carbon oxaloacetate to form citrate, the first molecule of the cycle. Acetyl-CoA provides that acetyl unit and, in the presence of the enzyme citrate synthase in the mitochondria, condenses with oxaloacetate to make citrate while CoA is released. Acetyl-CoA itself comes from pyruvate via the pyruvate dehydrogenase complex during aerobic respiration, producing NADH and releasing CO2 in the process. The other options don’t fit this initiating step: oxaloacetate is the acceptor, not the donor; pyruvate is upstream and must be converted to acetyl-CoA; citrate (the product) is formed after the reaction. So the molecule that starts the Krebs cycle by combining with oxaloacetate is Acetyl-CoA.