NAD-GLUTAMATE DEHYDROGENASE FROM HALOBACTERIUM-HALOBIUM - INHIBITION AND ACTIVATION BY TCA INTERMEDIATES AND AMINO-ACIDS

A variety of metabolites have been found to elicit a form of inhibitio n or activation on an NAD-specific glutamate dehydrogenase (NAD-GDH, E C 1.4.1.2) from Halobacterium halobium,. The purified halophilic enzym e was tested with several compounds known to be allosteric modifiers o f mammalian glutamate dehydrogenases to determine their effects on enz yme activity. GTP, ATP, ADP and AMP did not affect the enzyme, so thes e effecters of bovine glutamate dehydrogenase do not play a role in th e 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 of L-gluta mate was inhibited by TCA metabolites such as: fumarate, oxalacetate, succinate and malate; by substrate analogues such as: NADP(+), D-gluta mate and glutarate; and by dicarboxylic compounds such as adipate. On the other hand, all the amino acids tested were activators of this enz yme, except the D-isomer of the substrate L-glutamate that acted as an inhibitor. The relative effectiveness of each inhibitor or activator (K-1 or K-2 values) was correlated with the dipole moment (mu), HOMO a nd LUMO molecular orbital energies, optimal distance between two carbo xyl groups, and hydrophobicity. Compounds with high dipole moment acte d as good activators while compounds with low dipole moment were inhib itors. We have also found that the best activators were amino acids wi th no polar lateral chain.