Ketopantoate reductase (EC 18.104.22.168), an enzyme in the pantothenate biosynthetic pathway, catalyzes the NADPH-dependent reduction of α-ketopantoate to form D-(-)-pantoate. The enzyme exhibits high specificity for ketopantoate, with V and V/K for ketopantoate being 5- and 365-fold higher than those values for α-ketoisovalerate and 20- and 648-fold higher than those values for α-keto-Β-methyl-n-valerate, respectively. For pyridine nucleotides, V/K for Β-NADPH is 3-500-fold higher than that for other nucleotide substrates. The magnitude of the primary deuterium kinetic isotope effects on V and V/K varied substantially when different ketoacid and pyridine nucleotide substrates were used. The small primary deuterium kinetic isotope effects observed using NADPH and NHDPH suggest that the chemical step is not rate-limiting, while larger primary deuterium isotope effects were observed for poor ketoacid and pyridine nucleotide substrates, indicating that the chemical reaction has become partially or completely rate-limiting. The pH dependence of DV using ketopantoate was observed to vary from a value of 1.1 at low pH to a value of 2.5 at high pH, while the magnitude of DV/KNADPH and DV/KKP were pH-independent. The value of DV is large and pH-independent when α-keto-Β-methyl-n-valerate was used as the ketoacid substrate. Solvent kinetic isotope effects of 2.2 and 1.2 on V and V/K, respectively, were observed with α-keto-Β-methyl-n-valerate. Rapid reaction analysis of NADPH oxidation using ketopantoate showed no "burst" phase, suggesting that product-release steps are not rate-limiting and the cause of the small observed kinetic isotope effects with this substrate pair. Large primary deuterium isotope effects on V and V/K using 3-APADPH in steady-state experiments, equivalent to the isotope effect observed in single turnover studies, suggests that chemistry is rate-limiting for this poorer reductant. These results are discussed in terms of a kinetic and chemical mechanism for the enzyme.
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