First substrate of Kreb's cycle is

Ans. is 'a' i.e. Pyruvate The Kreb's cycle or citric acid cycle or tricarboxylic acid cycle is the final common pathway for the aerobic oxidation of carbohydrate, lipid and protein because glucose, fatty acids and most amino acids are metabolised to acetyl-CoA or intermediates of the cycle.The first reaction of the citric acid cycle is the condensation of acetyl-CoA with oxaloacetate to form citrate, catalyzed by citrate synthase. Oxalo-acetate is the first substrate of citrate synthase, it binds to the enzyme and induces a large conformational change in the flexible domain, creating a binding site for the second substrate, acetyl CoA.Thus the answer should be oxalo-acetate. Since it was not amongst the option, the best option is Pyruvate. Pyruvate formed at the end of glycolysis is taken up into mitochondria where it is converted to acetyl-CoA.