Investigating the Reaction Mechanism of F420-Dependent Glucose-6-phosphate Dehydrogenase from Mycobacterium tuberculosis: Kinetic Analysis of the Wild-Type and Mutant Enzymes
2016
Oyugi, Mercy A. | Bashiri, Ghader | Baker, Edward N. | Johnson-Winters, Kayunta
F₄₂₀-dependent glucose-6-phosphate dehydrogenase (FGD) catalyzes the conversion of glucose-6-phosphate (G6P) to 6-phosphogluconolactone, using F₄₂₀ cofactor as the hydride transfer acceptor, within mycobacteria. A previous crystal structure of wild-type FGD led to a proposed mechanism suggesting that the active site residues His40, Trp44, and Glu109 could be involved in catalysis. We have characterized the wild-type FGD and five FGD variants (H40A, W44F, W44Y, W44A, and E109Q) by fluorescence binding assays and steady-state and pre-steady-state kinetic experiments. Compared to wild-type FGD, all the variants had lower binding affinities for F₄₂₀, thus suggesting that Trp44, His40, and Glu109 aid in F₄₂₀ binding. While all the variants had decreased catalytic efficiencies, FGD H40A and W44A were the least efficient, having lost ∼1000- and ∼2000-fold activity, respectively. This confirms a crucial catalytic role for His40 in the FGD reaction and suggests that aromaticity at residue 44 aids catalysis. To investigate the proposed roles of Glu109 and His40 in acid–base catalysis, the pH dependence of kinetic parameters has been determined for the E109Q and H40A mutants and compared to those of the wild-type enzyme. The log kcₐₜ–pH profile of wild-type FGD and E109Q revealed two ionizable residues in the enzyme–substrate complex, while H40A displayed only one ionization event. The FGD E109Q variant displayed pH-dependent kinetic cooperativity with respect to the F₄₂₀ cofactor. The multiple-turnover pre-steady-state kinetics were biphasic for wild-type FGD, W44F, W44Y, and E109Q, while the H40A and W44A variants displayed only a single phase because of their reduced catalytic efficiency.
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