The first enzyme of the complex is PDH itself which oxidatively decarboxylates
pyruvate. During the course of the reaction
the hydroxyethyl group derived from decarboxylation of
pyruvate is bound to TPP. The hydroxyethyl group is oxidated to acetyl group during the transfer to lipoic acid, the covalently bound coenzyme of
lipoyl transacetylase. The transfer of the acetyl group from acyl-lipoamide to CoA results in the formation of 2 sulfhydryl (SH) groups in lipoate requiring reoxidation to the disulfide (S-S) form to regenerate lipoate as a competent acyl acceptor. The enzyme
dihydrolipoyl dehydrogenase, with FAD+ as a cofactor, catalyzes that oxidation reaction. The final
activity of the PDH complex is the transfer of reducing equivalents from the FADH2 of dihydrolipoyl dehydrogenase to NAD+. The fate of the NADH is oxidation via mitochondrial electron transport, to produce 3 equivalents of ATP:
The net result of the reactions of the PDH complex are:
Pyruvate + CoA + NAD+ ------> CO2 + acetyl-CoA + NADH + H+ Regulation of the PDH Complex The reactions of the PDH complex serves to interconnect the metabolic pathways of glycolysis, gluconeogenesis and fatty acid synthesis to the TCA cycle. As a consequence, the activity of the PDH complex is highly regulated by a variety of allosteric effectors and by covalent modification. The importance of the PDH complex to the maintenance of homeostasis is evident from the fact that although diseases associated with deficiencies of the PDH complex have been observed, affected individuals often do not survive to maturity. Since the energy metabolism of highly aerobic tissues such as the brain is dependent on normal conversion of pyruvate to acetyl-CoA, aerobic tissues are most sensitive to deficiencies in components of the PDH complex. Most genetic diseases associated with PDH complex deficiency are due to mutations in PDH. The main pathologic result of such mutations is moderate to severe cerebral lactic acidosis and encephalopathies.
The main regulatory features of the PDH complex are diagrammed below. http://www.med.unibs.it/~marchesi/pdhreg.gif
Two products of the complex, NADH and acetyl-CoA, are negative allosteric effectors on
PDH-a, the non-phosphorylated, active form of PDH. These effectors reduce the affinity of the enzyme for pyruvate, thus limiting the flow of carbon through the PDH complex. In addition, NADH and acetyl-CoA are powerful positive effectors on
PDH kinase, the enzyme that inactivates PDH by converting it to the phosphorylated
PDH-b form. Since NADH and acetyl-CoA accumulate when the cell energy charge is high, it is not surprising that high ATP levels also up-regulate
PDH kinase activity, reinforcing down-regulation of PDH activity in energy-rich cells. Note, however, that pyruvate is a potent negative effector on
PDH kinase, with the result that when pyruvate levels rise,
PDH-a will be favored even with high levels of NADH and acetyl-CoA.
Concentrations of pyruvate which maintain PDH in the active form (
PDH-a) are sufficiently high so that, in energy-rich cells, the allosterically down-regulated, high Km form of PDH is nonetheless capable of converting pyruvate to acetyl-CoA. With large amounts of pyruvate in cells having high energy charge and high NADH, pyruvate carbon will be directed to the 2 main storage forms of carbon---glycogen via gluconeogenesis and fat production via fatty acid synthesis---where acetyl-CoA is the principal carbon donor.
Although the regulation of
PDH-b phosphatase is not well understood, it is quite likely regulated to maximize pyruvate oxidation under energy-poor conditions and to minimize PDH activity under energy-rich conditions.