Regulation of the Mammalian Pyruvate Dehydrogenase Multienzyme Complex by Mg2+ and the Adenine Nucleotide Pool

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Abstract
The activity of the mammalian pyruvate dehydrogenase multienzyme complex in vitro depends on the composition of the adenine nucleotide pool (energy charge). An increase of the energy charge from 0.5 to 0.8 can cause a decrease of the activity of the complex by 70%.At high energy charge, i.e. when the amount of AMP and ADP in the adenine nucleotide pool is low, ATP inhibits the pyruvate‐dehydrogenase phosphatase, the enzyme which reactivates the pyruvate dehydrogenase complex by dephosphorylation. ATP causes this effect by binding Mg2+ essential for phosphatase activity, because it binds Mg2+ much more strongly than ADP or AMP.From the three enzymes of the multienzyme complex requiring Mg2+ as cofactor, pyruvate dehydrogenase, pyruvate‐dehydrogenase kinase, and pyruvate‐dehydrogenase phosphatase, it is shown that only the latter has a Km for Mg2+ higher than the physiological magnesium concentration. Its Km is 20 mM in phosphate buffer. Therefore the variation of the concentration of unbound Mg2+ caused by a variation of the composition of the adenine nucleotide pool (energy charge) in vivo can be meaningful only for the pyruvate‐dehydrogenase phosphatase activity.The phosphorylation‐dephosphorylation cycle of the pyruvate dehydrogenase complex results in a net consumption of ATP unless this futil cycle is inhibited. At high ATP concentrations (energy charge > 0.5) this inhibition takes place by binding Mg2+ ions.Adenosine 3′: 5′‐cyclic monophosphate has no effect on the activity of the pyruvate dehydrogenase complex. Controversial reports on this matter by other authors are discussed.