Kinetic mechanism of Tritrichomonas foetus inosine 5 '-monophosphate dehydrogenase

IMP dehydrogenase (IMPDH) catalyzes the oxidation of IMP to XMP with conver sion of NAD(+) to NADH. This reaction is the rate-limiting step in de novo guanine nucleotide biosynthesis. IMPDH is a target for antitumor, antiviral , and immunosuppressive chemotherapy. We have determined the complete kinet ic mechanism for IMPDH from Tritrichomonas foetus using ligand binding, iso tope effect, presteady-state kinetic, and rapid quench kinetic experiments. Both substrates bind to the free enzyme, which suggests a random mechanism . LMP binds to the enzyme in two steps. Two steps are also involved when IM P binds to a mutant IMPDH in which the active site Cys is substituted with a Ser. This observation suggests that this second step may be a conformatio nal change of the enzyme. No V-m isotope effect is observed when [2-H-2]NP is the substrate which indicates that hydride transfer is not rate-limiting . This result is confirmed by the observation of a pre-steady-state burst o f NADH production when monitored by absorbance. However, when NADH producti on was monitored by fluorescence, the rate constant for the exponential pha se is 5-10-fold lower than when measured by absorbance. This observation su ggests that the fluorescence of enzyme-bound NADH is quenched and that this transient represents NADH release from the enzyme. The time-dependent form ation and decay of [C-14]E-XMP* intermediates was monitored using rapid que nch kinetics. These experiments indicate that both NADH release and E-XMP* hydrolysis are rate-limiting and suggest that NADH release precedes hydroly sis of E-XMP*.