Interaction of difluoro-oxaloacetate with aspartate transaminase

Oxaloacetate hydrolase, the C-C bond lyase of oxalate secreting fungi

Oxalate secretion by fungi is known to be associated with fungal pathogenesis. In addition, oxalate toxicity is a concern for the commercial application of fungi in the food and drug industries. Although oxalate is generated through several different biochemical pathways, oxaloacetate acetylhydrolase (OAH)-catalyzed hydrolytic cleavage of oxaloacetate appears to be an especially important route. Below, we report the cloning of the Botrytis cinerea oahA gene and the demonstration that the disruption of this gene results in the loss of oxalate formation. In addition, through complementation we have shown that the intact B. cinerea oahA gene restores oxalate production in an Aspergillus niger mutant strain, lacking a functional oahA gene. These observations clearly indicate that oxalate production in A. niger and B. cinerea is solely dependent on the hydrolytic cleavage of oxaloacetate catalyzed by OAH. In addition, the B. cinera oahA gene was overexpressed in Escherichia coli and the purified OAH was used to define catalytic efficiency, substrate specificity, and metal ion activation. These results are reported along with the discovery of the mechanism-based, tight binding OAH inhibitor 3,3-difluorooxaloacetate (K(i) = 68 nM). Finally, we propose that cellular uptake of this inhibitor could reduce oxalate production.

Transient-state kinetic analysis of Synechococcus glutamate 1-semialdehyde aminotransferase

We report a transient-state kinetic analysis relating to the mechanism of glutamate 1-semialdehyde aminotransferase (GSAT). Multiple-wavelength spectral kinetic data were collected by micro-stopped-flow spectrophotometry. Time resolved spectral sketches resulting from reactions with glutamate 1-semialdehyde (GSA), 4,5-diaminovalerate (DAVA), and 5-aminolevulinate (ALA) indicated various transient chromophoric intermediates. On the basis of the generally accepted mechanism of other aminotransferases and absorbance characteristics of associated intermediates, these transient chromophores are likely associated with Schiff base formation, ketimine/aldimine tautomerization, and transimidation etc. Spectral kinetic changes associated with these putative intermediates were, in general, concentration dependent. Various experimental evidence, including reactions with the GSAT lys272ile mutant, suggested rapid equilibrium of isomeric aldimines and geminal diamines. With this and related simplifying assumptions, a minimal mechanism was derived which provided a means for transient-state spectral kinetic analysis of reactions with GSA, DAVA, and ALA, all of which lead to the formation of the same putative central enzyme complex. Resulting kinetic constants were internally consistent, in general agreement with steady-state and equilibrium data (KM, kcat, and Keq), and provided the basis for a reasonable computer simulation of the original data set (variance approximately 4 x 10(-5)). Reequilibration of enzyme intermediates following an apparent pseudoequilibrium indicated thermodynamically driven dissociation of the central aldiminic enzyme complex. This is consistent with previous observations and the minimal mechanism used in this kinetic analysis and suggests a plausible regulatory mechanism of GSAT.

Determination of half-reaction equilibrium in a ping-pong enzyme mechanism

Substituted enzyme (or ping-pong) mechanisms usually involve enzymes that exist in two forms that alternate during the catalytic reaction. A method is described here for determining the position of the equilibrium of a half reaction in a ping-pong enzyme mechanism that is based on the kinetics of the burst reaction which occurs upon addition of reactants that recycle the enzyme from one form to another. The theoretical basis for the analysis is developed, and the method is applied to the half reaction of the aldimine form of aspartate transaminase with difluoro-oxaloacetate.

Reaction of the aminic form of aspartate transaminase with difluoro-oxaloacetate

Addition of difluoro-oxaloacetate to the aminic form of aspartate transaminase causes a rapid shift of absorbance maximum of the enzyme from 332 nm to 328 nm, followed by a much slower shift to 360 nm corresponding to complete conversion of the aminic form of the enzyme into the aldimine form or a species with similar spectral parameters in rapid equilibrium with it. Kinetic analysis of both the initial fast reaction and the overall slow reaction by using repeated spectral scanning and stopped-flow techniques allows formulation of a basic reaction mechanism involving at least two intermediate enzyme complexes. Computer simulation of the progress curves of the initial fast reaction based on the suggested reaction mechanism gives kinetic parameters that are consistent with all the data obtained by other methods. A molecular reaction scheme involving a ketimine Schiff-base intermediate is proposed.

[19F]fluorine nuclear-magnetic-resonance study of the interaction of difluoro-oxaloacetate with aspartate transaminase

Difluoro-oxaloacetate interacts with the aldimine form of aspartate transaminase to give a complex, the dissociation constant of which has been determined spectrophotometrically and by 19F n.m.r. (nuclear magnetic resonance). The 19F n.m.r. line-width-pH and chemical-shift-pH profiles of difluoro-oxaloacetate in the presence of the aldimine form of the enzyme both show inflexion points in the pH5 and pH8 regions, which may arise from variations in the binding of difluoro-oxaloacetate as specific groups on the enzyme are successively protonated. Difluoro-oxaloacetate also interacts with apoenzyme to form a complex, the dissociation constant of which was determined by 19F n.m.r. The 19F n.m.r. line-width-pH and chemical-shift-pH profiles of difluoro-oxaloacetate in the presence of apoenzyme show a single inflexion point in the region of pH8. The absence, in this case, of an inflexion in the pH5 region indicates that the latter, present in the corresponding profiles for the aldimine form of the enzyme, results from ionization of an enzyme group associated with the pyridoxal phosphate cofactor.