<prompt> Krebs cycle is the second metabolic pathway involved in the catabolism of carbohydrates into energy. First substrate of Krebs cycle is:PyruvateGlycineHQLipoprotein  <response>: 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. 