Beef liver glutamate dehydrogenase: a study of the oxidation of various alternative amino acid substrates retaining the correct spacing of the two carboxylate groups

Conversion of a glutamate dehydrogenase into methionine/norleucine dehydrogenase by site-directed mutagenesis

In earlier attempts to shift the substrate specificity of glutamate dehydrogenase (GDH) in favour of monocarboxylic amino-acid substrates, the active-site residues K89 and S380 were replaced by leucine and valine, respectively, which occupy corresponding positions in leucine dehydrogenase. In the GDH framework, however, the mutation S380V caused a steric clash. To avoid this, S380 has been replaced with alanine instead. The single mutant S380A and the combined double mutant K89L/S380A were satisfactorily overexpressed in soluble form and folded correctly as hexameric enzymes. Both were purified successfully by Remazol Red dye chromatography as routinely used for wild-type GDH. The S380A mutant shows much lower activity than wild-type GDH with glutamate. Activities towards monocarboxylic substrates were only marginally altered, and the pH profile of substrate specificity was not markedly altered. In the double mutant K89L/S380A, activity towards glutamate was undetectable. Activity towards L-methionine, L-norleucine and L-norvaline, however, was measurable at pH 7.0, 8.0 and 9.0, as for wild-type GDH. Ala163 is one of the residues that lines the binding pocket for the side chain of the amino-acid substrate. To explore its importance, the three mutants A163G, K89L/A163G and K89L/S380A/A163G were constructed. All three were abundantly overexpressed and showed chromatographic behaviour identical with that of wild-type GDH. With A163G, glutamate activity was lower at pH 7.0 and 8.0, but by contrast higher at pH 9.0 than with wild-type GDH. Activities towards five aliphatic amino acids were remarkably higher than those for the wild-type enzyme at pH 8.0 and 9.0. In addition, the mutant A163G used L-aspartate and L-leucine as substrates, neither of which gave any detectable activity with wild-type GDH. Compared with wild-type GDH, the A163 mutant showed lower catalytic efficiencies and higher K(m ) values for glutamate/2-oxoglutarate at pH 7.0, but a similar k(cat)/K(m) value and lower K(m) at pH 8.0, and a nearly 22-fold lower S(0.5) (substrate concentration giving half-saturation under conditions where Michaelis-Menten kinetics does not apply) at pH 9.0. Coupling the A163G mutation with the K89L mutation markedly enhanced activity (100-1000-fold) over that of the single mutant K89L towards monocarboxylic amino acids, especially L-norleucine and L-methionine. The triple mutant K89L/S380A/A163G retained a level of activity towards monocarboxylic amino acids similar to that of the double mutant K89L/A163G, but could no longer use glutamate as substrate. In terms of natural amino-acid substrates, the triple mutant represents effective conversion of a glutamate dehydrogenase into a methionine dehydrogenase. Kinetic parameters for the reductive amination reaction are also reported. At pH 7 the triple mutant and K89L/A163G show 5 to 10-fold increased catalytic efficiency, compared with K89L, towards the novel substrates. In the oxidative deamination reaction, it is not possible to estimate k(cat) and K(m) separately, but for reductive amination the additional mutations have no significant effect on k(cat) at pH 7, and the increase in catalytic efficiency is entirely attributable to the measured decrease in K(m). At pH 8 the enhancement of catalytic efficiency with the novel substrates was much more striking (e.g. for norleucine approximately 2000-fold compared with wild-type or the K89L mutant), but it was not established whether this is also exclusively due to more favourable Michaelis constants.

Negative co-operativity in glutamate dehydrogenase. Involvement of the 2-position in glutamate in the induction of conformational changes

The 2-position substituent on substrates or substrate analogues for glutamate dehydrogenase is shown to be intimately involved in the induction of conformational changes between subunits in the hexamer by coenzyme. These conformational changes are associated with the negative co-operativity exhibited by this enzyme. 2-Oxoglutarate and L-2-hydroxyglutarate induce indications of co-operativity similar to those induced by the substrate of oxidative deamination, glutamate, in kinetic studies. Glutarate (2-position CH2) does not. A comparison of the effects of L-2-hydroxyglutarate and D-2-hydroxyglutarate or D-glutamate indicates that the 2-position substituent must be in the L-configuration for these conformational changes to be triggered. In addition, glutarate and L-glutamate in ternary enzyme-NAD(P)H-substrate complexes induce very different coenzyme fluorescence properties, showing that glutamate induces a different conformation of the enzyme-coenzyme complex from that induced by glutarate. Although glutamate and glutarate both tighten the binding of reduced coenzyme to the active site, the effect is much greater with glutamate, and the binding is described by two dissociation constants when glutamate is present. The data suggest that the two carboxy groups on the substrate are required to allow synergistic binding of coenzyme and substrate to the active site, but that interactions between the 2-position on the substrate and the enzyme trigger the conformational changes that result in subunit-subunit interactions and in the catalytic co-operativity exhibited by this enzyme.

Kinetic studies of ox-liver glutamate dehydrogenase oxidative deamination of two glutamate analogues, L-threo-gamma-methylglutamate and L-alpha-amino-gamma-nitraminobutyrate, in the presence of the allosteric effector ADP

Ox-liver glutamate dehydrogenase is known to utilise a wide range of amino acid substrates. Kinetic studies are presented here for L-threo-gamma-methylglutamate and L-alpha-amino-gamma-nitraminobutyrate in the presence of the allosteric effector ADP. The results presented are considered in the light of similar studies presented elsewhere in which the cofactor was systematically replaced by a variety of analogues. These amino acid analogues share the same pH optimum as glutamate, unlike the monocarboxylic amino acids including alanine and norvaline, and give linear double-reciprocal plots under the conditions used here. Studies with the alternative coenzymes have suggested an ordered addition of glutamate before coenzyme in the presence of ADP. The present results obtained under identical conditions support this conclusion.

The kinetic mechanism of ox liver glutamate dehydrogenase in the presence of the allosteric effector ADP. The oxidative deamination of L-glutamate

In steady-state kinetic studies of ox liver glutamate dehydrogenase in 0.11 M-potassium phosphate buffer, pH7, at 25 degrees C, the concentration of ADP was varied from 0.5 to 1000 microM. Inhibition was observed except when the concentrations of both glutamate and coenzyme were high, when activation was seen. With NAD+ or NADP+ as coenzyme, 200 microM-ADP was sufficient to saturate the enzyme with respect to the major effect of this nucleotide. In the presence of 210 microM-ADP, widely varied concentrations of coenzyme give linear Lineweaver-Burk plots, in marked contrast with results obtained previously for kinetics without ADP. This has allowed evaluation for the reaction with NAD+, NADP+ and acetylpyridine-adenine dinucleotide (315 microM-ADP in the last case) of all four initial rate parameters, i.e. the phi coefficients in the equation: (Formula: see text) where A is coenzyme and B is glutamate. The relative constancy of phi B and of phi AB/phi A with the different coenzymes point to a compulsory-order mechanism with glutamate as the leading substrate. This conclusion, though unexpected, agrees well with various previous observations on the binding of oxidized coenzyme.