Gene Identification and Characterization of the Pyridoxine Degradative Enzyme 4-Pyridoxic Acid Dehydrogenase from the Nitrogen-fixing Symbiotic Bacterium Mesorhizobium loti MAFF303099†

摘要

The gene encoding 4-pyridoxic acid dehydrogenase was identified as mlr6792 in a chromosome of a nitrogen-fixing symbiotic bacterium Mesorhizobium loti MAFF303099. The enzyme is the fourth enzyme in the vitamin B₆ (pyridoxine)-degradation pathway I. The recombinant enzyme with a his-tag over-expressed in Escherichia coli cells was a membrane-bound protein, and purified to homogeneity. The enzyme was a monomeric protein with a molecular weight of 59,000, and a flavoprotein containing one mole of FAD per mole of subunit. The optimum pH and temperature, and K_m for 4-pyridoxic acid were pH 8.5 and 30°C, and 29 μM, respectively. The enzyme was a glucose-methanol-choline (GMC) family protein with two signature patterns, FAD-binding residues, a putative active site histidine residue and a probable transmembrane segment.