Pyruvate Dehydrogenase Kinase Isoform 2 Activity Stimulated by Speeding Up the Rate of Dissociation of ADP

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Abstract
Pyruvate dehydrogenase kinase 2 (PDK2) activity is stimulated by NADH and NADH plus acetyl-CoA via the reduction and reductive acetylation of the lipoyl groups of the dihydrolipoyl acetyltransferase (E2) component. Elevated K+ and Cl- were needed for significant stimulation. Stimulation substantially increased both kcat and the Km for ATP; the fractional stimulation increased with the level of ATP. With an E2 structure lacking the pyruvate dehydrogenase (E1) binding domain, stimulation of PDK2 was retained, the Km for E1 decreased, and the equilibrium dissociation constant for ATP increased but remained much lower than the Km for ATP. Stimulation of PDK2 activity greatly reduced the fraction of bound ADP. These results fit an ordered reaction mechanism with ATP binding before E1 and stimulation increasing the rate of dissociation of ADP. Conversion of all of the lipoyl groups in the E2 60mer to the oxidized form (E2ox) greatly reduced kcat and the Km of PDK2 for ATP. Retention over an extended period of time of a low portion of reduced lipoyl groups maintains E2 in a state that supported much higher PDK2 activity than short-term (5 min) reduction of a large portion of lipoyl groups of E2ox, but reduction of E2ox produced a larger fold stimulation. Reduction and to a greater extent reductive acetylation increased PDK2 binding to E2; conversion to E2ox did not significantly hinder binding. We suggest that passing even limited reducing equivalents among lipoyl groups maintains E2 lipoyl domains in a conformation that aids kinase function.