Modulation of 2-Oxoglutarate Dehydrogenase Complex by Inorganic Phosphate, Mg2+, and Other Effectors

The interplay of inorganic phosphate (Pi) with other ligands such as Mg2+, ADP, ATP, and Ca2+ on the activation of 2-oxoglutarate dehydrogenase complex (2-OGDH) in both isolated enzyme complex and mitochondrial extracts was examined. Pi alone activated the enzyme, following biphasic kinetics with high (K0.5 = 1.96 ± 0.42 mM) and low (K0.5 = 9.8 ± 0.4 mM) affinity components for Pi. The activation by Pi was highly pH-dependent; it increased when the pH raised from 7.1 to 7.6, but it was negligible at pH values below 7.1. Mg-Pi and Mg-ADP, but not Mg-ATP, were more potent activators of 2-OGDH than free Pi and free ADP. ATP inhibited the 2-OGDH activity by chelating the free Mg2+ and also as a Mg-ATP complex. With or without Mg2+, ADP, and Pi activated the 2-OGDH by increasing the affinity for 2-OG and the Vm of the reaction; ATP diminished the Vm, but it increased the affinity for 2-OG in the mitochondrial extract. Pi did not modify the 2-OGDH activation by Ca2+. The results above mentioned were similar for both preparations, except for hyperbolic kinetics in the isolated enzyme and sigmoidal kinetics in the mitochondrial extracts when 2-oxoglutarate was varied. The data of this study indicated that physiological concentrations of Pi may exert a significant activation of 2-OGDH, which was potentiated by Mg2+ and high pH, but surpassed by ADP.