Pyruvate dehydrogenase activity during aerobic respiration...

Link reaction: If sufficient oxygen is available, each 3-carbon pyruvate molecule enters the mitochondrial matrix where its oxidation is completed by aerobic means. It is called a gateway step or link reaction between glycolysis and Krebs' cycle. Acetyl CoA is a common intermediate of fat and carbohydrate metabolism. Later, this acetyl CoA from both sources enters the TCA cycle. Link reaction also called oxidative decarboxylation reaction is not a part of Krebs' cycle.

Pyruvate + CoA + {NAD}^{+}\mathop{\to}_{{Mg}^{+ 2}, TPP, Lipoicacid}^{Pyruvatedehydrogenase}AcetylCoA + NADH . {H}^{+} + {CO}_{2}