Question

Please Describe AND diagram when and how six carbons in glucose are all transferred and released, and in what form (molecule), from glycolysis through the Krebs (TCA) cycle. What else happens each time carbons are released?

Answer 1

In glycolysis, one glucose (6 carbon) is converted to two pyruvate (3C) molecules. There is no loss of carbon in the glycolysis.

These pyruvate (3C) loses its first carbon upon conversion to acetyl CoA (2C) by pyruvate dehydrogenase.

This acetyl CoA (2C) joins oxaloacetate (4C) to form citrate (6C) by citrate synthase. Citrate is isomerised to isocitrate by aconitase.

Loss of second carbon occurs when isocitrate (6C) is converted to α-keto glutarate (5C) by isocitrate dehydrogenase.

Loss of third carbon occurs when α-keto glutarate (5C) to succinyl CoA (4C) by α-keto glutarate dehydrogenase.

Later, succinyl CoA is converted to oxaloacetate vi succinyl CoA-fumarate-malate-oxaloacetate steps.

So, the three carbon pyruvate loses all its carbons in three reactions catalysed by pyruvate dehydrogenase, isocitrate dehydrogenase, and α-keto glutarate dehydrogenase. All these reactions are decarboxylation reactions releasing the carbon in the form of CO₂. These 3 reactions also release energy in the form of NADH + H⁺.

Finally, 1 glucose produces 2 pyruvate in glycolysis. For the complete loss of all 6 carbons of glucose, it has to go through one glycolysis cycle and two cycles of Krebs cycle for two pyruvate molecules.