Analysis of wild-type and mutant aspartate aminotransferases using integrated rate equations

Precise, facile initial rate measurements

Progress curve analysis has been used sparingly in studies of enzyme-catalyzed reactions due largely to the complexity of the integrated rate expressions used in data analysis. Using an experimental design that simplifies the analysis, the advantages and limitations of progress curve experiments are explored in a study of four different enzyme-catalyzed reactions. The approach involves relatively simple protocols, requires 20-25% of the materials, and provides 10- to 20-fold signal enhancements compared to analogous initial rate studies. Product inhibition, which complicates integrated rate analysis, was circumvented using cloned, purified enzymes that remove the products and draw the reaction forward. The resulting progress curves can be transformed into the equivalent of thousands of initial rate and [S] measurements and, due to the absence of product inhibition, are plotted in the familiar, linear double-reciprocal format. Allowing product to accumulate during a reaction produces a continuously changing substrate/product ratio that can be used as the basis for obtaining product inhibition constants and to distinguish among the three classical inhibition mechanisms. Algebraic models describing the double-reciprocal patterns obtained from such inhibition studies are presented. The virtual continuum of substrate concentrations that occurs during a progress curve experiment provides a nearly errorless set of relative concentrations that results in remarkably precise data; kinetic constant standard deviations are on the order of 0.5%.

Inhibition and aerobic inactivation kinetics of Desulfovibrio fructosovorans NiFe hydrogenase studied by protein film voltammetry

We have used protein film voltammetry to study the NiFe hydrogenase from Desulfovibrio fructosovorans. We show how measurements of transient activity following the addition in the electrochemical cell of H(2), CO, or O(2) allow simple and virtually instantaneous determinations of the Michaelis constant, inhibition constant, or rate of inactivation, respectively, thus opening new opportunities to study the active site of NiFe hydrogenases. The binding and release of CO occur within a fraction of a second, and we determine and discuss how its affinity for the active site changes as the driving force for the H(+)/H(2) reaction is continuously varied. Inactivation by O(2) is a slow, bimolecular process (with pH-independent rate constant approximately 3 x 10(4) s(-1) M(-1) at 40 degrees C, under one atm of H(2)) that leads to a mixture of fully oxidized states, and unlike the case of CO inhibition, the active site is not fully protected by H(2). This experimental approach could be used to study the reaction of other multicentered metalloenzymes with their gaseous substrates or inhibitors.