2-Methyl-3-hydroxypyridine-5-carboxylic acid (MHPC) oxygenase (EC 1.14.12.4) from Pseudomonas sp. MA-1 is a flavin-dependent monooxygenase that catalyzes a hydroxylation and aromatic ring cleavage reaction. The functional roles of two residues, Tyr223 and Tyr82, located 5 A away from MHPC, were characterized using site-directed mutagenesis, along with ligand binding, product analysis and transient kinetic experiments. Mutation of Tyr223 resulted in enzyme variants that were impaired in their hydroxylation activity and had K d values for substrate binding 5-10-fold greater than the wild-type enzyme. Because this residue is adjacent to the water molecule that is located next to the 3-hydroxy group of MHPC, the results indicate that the interaction between Tyr223, H 2 O and the 3-hydroxyl group of MHPC are important for substrate binding and hydroxylation. By contrast, the K d for substrate binding of Tyr82His and Tyr82Phe variants were similar to that of the wild-type enzyme. However, only~40-50% of the substrate was hydroxylated in the reactions of both variants, whereas most of the substrate was hydroxylated in the wild-type enzyme reaction. In free solution, MHPC or 5-hydroxynicotinic acid exists in a mixture of monoanionic and tripolar ionic forms, whereas only the tripolar ionic form binds to the wild-type enzyme. The binding of tripolar ionic MHPC would allow efficient hydroxylation through an electrophilic aromatic substitution mechanism. For the Tyr82His and Tyr82Phe variants, both forms of substrates can bind to the enzymes, indicating that the mutation at Tyr82 abolished the selectivity of the enzyme towards the tripolar ionic form. Transient kinetic studies indicated that the hydroxylation rate constants of both Tyr82 variants are approximately two-to 2.5-fold higher than that of the wild-type enzyme. Altogether, our findings suggest that Tyr82 is important for the binding selectivity of MHPC oxygenase towards the tripolar ionic species, whereas the interaction between Tyr223 and the substrate is important for ensuring hydroxylation. These results highlight how the active site of a flavoenAbbreviations
Scheme1.The C 1 -catalyzed reaction thatg enerates FMNH À for monooxygenase-catalyzed reactions.The NADH-regenerating systemm ightb e, for example, glucose/glucose dehydrogenase, glucose 6-phosphate/glucose 6phosphate dehydrogenase, [41,42] or formate/formate dehydrogenase. [43,44] The monooxygenases mightb e, for example, C 2 or bacterialluciferase. Figure 6. Distances between A58/P58 and I14:A )int he wild type, and B) in the A58P variant over 6nsM Ds imulation,a ttemperatures varying from 300-500 K. The interactions between A58/P58 of C) the wild type, and D) the A58P variant and other residues after 6-ns MD simulations.
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