NAD-glutamate dehydrogenase from Halobacterium halobium: inhibition and activation by TCA intermediates and amino acids

Abstract

A variety of metabolites have been found to elicit a form of inhibition or activation on an NAD-specific glutamate dehydrogenase (NAD-GDH, EC 1.4.1.2) from Halobacterium halobium. The purified haloppilic enzyme was tested with several compounds known to be allosteric modifiers of mammalian glutamate dehydrogenases to determine their effects on enzyme activity. GTP, ATP, ADP and AMP did not affect the enzyme, so these effectors of bovine glutamate dehydrogenase do not play a role in the regulation of the halophilic enzyme. However, the halophilic enzyme was subject to strong inhibition by TCA intermediates. When measuring the initial rate of the reaction, the oxidative deamination ofl-glutamate was inhibited by TCA metabolites such as: fumarate, oxalacetate, succinate and malate; by substrate analogues such as: NADP+,d-glutamate and glutarate; and by dicarboxylic compounds such as adipate. On the other hand, all the amino acids tested were activators of this enzyme, except thed-isomer of the substratel-glutamate that acted as an inhibitor. The relative effectiveness of each inhibitor or activator (KiorKa values) was correlated with the dipole moment (μ), HOMO and LUMO molecular orbital energies, optimal distance between two carboxyl groups, and hydrophobicity. Compounds with high dipole moment acted as good activators while compounds with low dipole moment were inhibitors. We have also found that the best activators were amino acids with no polar lateral chain.