Mo–Se‐Containing Nicotinate Dehydrogenase

AbstractSe Mo‐containing nicotinate dehydrogenase (NDH) of the anaerobic soil bacteriumEubacterium barkericatalyzes the hydroxylation of nicotinate with water and NADP+to 6‐hydroxynicotinate, one proton, and NADPH. This reaction is the first step in the degradation of nicotinate to pyruvate and propionate in a unique fermentation pathway. NDH is constituted by four different subunits and contains a Mo‐bound pyranopterin cofactor linked to cytosine dinucleotide, two different [2Fe2S] centers, and flavin adenine dinucleotide to facilitate the transfer of a hydride ion on NADP+. NDH is a member of the family of molybdenum hydroxylases and shares several distinctive features with other members of this enzyme family, such as xanthine oxidoreductase, aldehyde oxidase, and the Mo‐containing carbon monoxide dehydrogenase. The reactivity of NDH critically depends on the presence of selenium in the active site, where it is bound as a labile nonselenocysteinyl selenium ligand to molybdenum. The role of the selenium ligand is likely to accelerate the formal hydride transfer from the substrate into the Mo Se bond, displaying a novel reactivity of selenium in nature.