2-methyl-3-hydroxypyridine-5-carboxylic-acid and 5-hydroxynicotinic-acid

Topics: Binding Sites; Catalysis; Catalytic Domain; Crystallography, X-Ray; Hydrogen Bonding; Kinetics; Mesorhizobium; Mixed Function Oxygenases; Models, Molecular; NAD; Nicotinic Acids; Oxidation-Reduction; Oxygen; Protein Interaction Domains and Motifs; Tyrosine

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 Å 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 Kd 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, H2 O and the 3-hydroxyl group of MHPC are important for substrate binding and hydroxylation. By contrast, the Kd 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 flavoenzyme is able to deal with the presence of multiple forms of a substrate in solution and ensure efficient hydroxylation.

Topics: Amino Acid Substitution; Bacterial Proteins; Catalytic Domain; Hydrogen-Ion Concentration; Hydroxylation; Kinetics; Mixed Function Oxygenases; Models, Molecular; Mutagenesis, Site-Directed; Nicotinic Acids; Oxidation-Reduction; Oxygen; Pseudomonas; Recombinant Proteins; Sodium Azide; Spectrophotometry; Substrate Specificity; Tyrosine

The oxygenation reaction of 2-methyl-3-hydroxypyridine-5-carboxylic acid (MHPC) oxygenase with the substrate, MHPC, was investigated. Two oxygenated flavin intermediates C(4a)-hydroperoxy flavin and C(4a)-hydroxy flavin were found, implying that the enzyme functions similarly to flavoprotein hydroxylases. This finding is supported by the results of independent oxygen-18 tracer experiments, which showed that one atom of oxygen from 18O2 and one atom of oxygen from H218O are incorporated in the product. MHPC oxygenase normally catalyzes both the oxygenation and the hydrolytic ring opening of the pyridine ring of MHPC to yield the acyclic compound, alpha-(N-acetylaminomethylene)succinic acid. Using 5-hydroxynicotinic acid (5HN), which has no 2-methyl group, we tested whether the hydrolytic reaction was due to the presence of the 2-methyl group on MHPC (that prevented rearomatization of the initial product) or to the specific properties of MHPC oxygenase. Product analysis of the enzymatic reaction of 5HN and MHPC oxygenase shows that the enzyme catalyzes the hydroxylation and subsequent hydrolysis of the hydroxylated substrate to yield an acyclic product. The investigation of the oxygenation reaction demonstrates that the enzyme uses the same mechanism to catalyze the 5HN reaction as it does in the MHPC reaction.

Topics: Azides; Catalysis; Kinetics; Mixed Function Oxygenases; Nicotinic Acids; Oxidation-Reduction; Oxygen; Oxygen Radioisotopes; Spectrophotometry