Flavin adenine dinucleotide-dependent 4-phospho-D-erythronate dehydrogenase is responsible for the 4-phosphohydroxy-L-threonine pathway in vitamin B6 biosynthesis in Sinorhizobium meliloti

The vitamin B-6 biosynthetic pathway in Sinorhizobium meliloti is similar to that in Escherichia coli K-12; in both organisms this pathway includes condensation of two intermediates, 1-deoxy-D-xylulose 5-phosphate and 4-phosphohydroXy-L-threonine (4PHT). Here, we report cloning of a gene designated pdxR that functionally corresponds to the pdxB gene of E. coli and encodes a dye-linked flavin adenine dinucleotide-dependent 4-phospho-D-erythronate (4PE) dehydrogenase. This enzyme catalyzes the oxidation of 4PE to 3-hydroxy-4-phosphohydroxy-alpha-ketobutyrate and is clearly different in terms of cofactor requirements from the pdxB gene product of E. coli, which is known to be an NAD-dependent enzyme. Previously, we revealed that in S. meliloti IFO 14782, 4PHT is synthesized from 4-hydroxy-L-threonine and that this synthesis starts with glycolaldehyde and glycine. However, in this study, we identified a second 4PHT pathway in S. meliloti that originates exclusively from glycolaldehyde (the major pathway). Based on the involvement of 4PE in the 4PHT pathway, the incorporation of different samples of C-13-labeled glycolaldehyde into pyridoxine molecules was examined using C-13 nuclear magnetic resonance spectroscopy. On the basis of the spectral analyses, the synthesis of 4PHT from glycolaldehyde was hypothesized to involve the following steps: glycolaldehyde is sequentially metabolized to D-erythrulose, D-erythrulose 4-phosphate, and D-erythrose 4-phosphate by transketolase, kinase, and isomerase, respectively; and D-erythrose 4-phosphate is then converted to 4PHT by the conventional three-step pathway elucidated in E. coli, although the mechanism of action of the enzymes catalyzing the first two steps is different.