Question
Does pyruvic acid enter the 'Krebs' cycle directly?
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Answer
No, before pyruvic acid as such does not enter 'Krebs' cycle, pyruvate ie first decarboxylated, and then oxidized by the enzyme pyruvate dehydrogenase to form acetyl Co-A. This molecule enters the mitochondria for further oxiation reaction. This is the connecting link between glycolysis and Krebs' cycle. During the process, $\mathrm{NAD}^{+}$is reduced to $\mathrm{NADH}+\mathrm{H}^{+}$. The summary of the reaction is given below: $\text{Pyruvic acid}+\text{Co}-\text{A}+\text{NAD}^+\xrightarrow[\text{Mg}^{2+}]{\text{Pyruvate dehydrogenase}}\text{Acetyl}$ $\text{Co}-\text{A}+\text{Co}_2+\text{NADH}+\text{H}^+$ During this process, two molecules of NADH are produced (from the metabolism of two molecules of pyruvic acid produced during glycolysis), and thus, it results in a net gain of 6 ATP molecules (2 NADH × 3 = 6 ATP).
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