The function of the mlr6791 gene from Mesorhizobium loti MAFF303099 has been identified. This gene encodes 3-hydroxy-2-methylpyridine-4,5-dicarboxylate decarboxylase (HMPDdc), an enzyme involved in the catabolism of pyridoxal 5-phosphate (Vitamin B 6 ). This enzyme was overexpressed in Escherichia coli and characterized. HMPDdc is a 26 kDa protein that catalyzes the decarboxylation of 3-hydroxy-2-methylpyridine-4,5-dicarboxylate to 3-hydroxy-2-methylpyridine-5-carboxylate. The K M and k cat were found to be 366 μM and 0.6 s -1 respectively. The structure of this enzyme was determined at 1.9 Å resolution using SAD phasing and belongs to the class II aldolase/adducin superfamily. While the decarboxylation of hydroxy-substituted benzene rings is a common motif in biosynthesis, the mechanism of this reaction is still poorly characterized. The structural studies described here suggest that catalysis of such decarboxylations proceeds by an aldolase-like mechanism.In contrast to our understanding of cofactor biosynthetic pathways, very little is known about cofactor catabolism. Cofactor catabolism is likely to be rare because cofactors are trace metabolites and therefore not good food sources for bacteria. Pyridoxine, 1 (vitamin B 6 ) catabolism is the best understood cofactor catabolic pathway and a small number of bacteria that can grow on vitamin B 6 as the sole source of carbon and nitrogen have been identified (1). Two catabolic pathways have been proposed (2). In the first pathway Figure 1, (pathway A), found in Pseudomonas sp. vitamin B 6 is degraded in eight steps to form succinic semialdehyde, 9, while in the second pathway (pathway B) observed in Pseudomonas IA and in Arthrobacter Cr-7, vitamin B 6 is catabolized in seven steps to 2-(hydroxymethyl)-4-oxobutanoate, 14. A related catabolic pathway in Mesorhizobium loti MAFF303099 which is very similar to the degradative pathway A has recently been discovered. 3-hydroxy-2-methylpyridine-5-carboxylate, 7 is an intermediate in this catabolic pathway. The gene coding for the enzyme producing this metabolite has not previously been discovered and is the subject of this paper.Previously, the genes encoding pyridoxine-4-oxidase (mlr6785), 4-pyridoxolactonase (mlr6805), pyridoxal-4-dehydrogenase (mlr6807) and the 2-methyl-3-hydroxypyridine-5-carboxylic acid oxygenase (mlr6788) were identified in Mesorhizobium loti MAFF303099 (3-6). Recently we have reported the identification of a fifth gene (mlr6793), encoding 4-* To whom correspondence should be addressed at the Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14853. Telephone: (607) 255-4137. E-mail: tpb2@cornell.edu or see3@cornell.edu # These authors contributed equally to the research reported in this paper. ‡ The Brookhaven Protein Data Bank code is 2Z7B. pyridoxic acid dehydrogenase (7). These genes are not part of an operon, but are all close to each other on the M. loti chromosome, Figure 1. This suggested that other PLP catabolic genes might also be found in this r...