Kreb's cycle is the second metabolic pathway involved in the catabolism of carbohydrates into energy. First substrate of Kreb's cycle is:

Pyruvate enters the Krebs cycle (Citric acid cycle or Tricarboxylic acid cycle) following its conversion to acetyl-CoA; the reaction is catalyzed by pyruvate dehydrogenase that links glycolysis to the citric acid cycle and produces one molecule of NADH. Ref: Janson L.W., Tischler M.E. (2012). Chapter 6. Carbohydrate Metabolism. In L.W. Janson, M.E. Tischler (Eds), The Big Picture: Medical Biochemistry.