Insights into periplasmic nitrate reductase function under single turnover.

Nitrate reductases play pivotal roles in nitrogen metabolism by leveraging the molybdopterin cofactor to facilitate the reduction of nitrate to nitrite. Periplasmic nitrate reductases (NapA) utilize nitrate as a terminal electron acceptor when oxygen is limiting, helping to drive anaerobic metabolism in bacteria. Despite extensive research into NapA homologs, open questions about the mechanism remain especially at the molecular level.

Active Site Characterization of a Nitrate Reductase Variant Provides Insight into the Enzyme Mechanism.

Mo K-edge X-ray absorption spectroscopy (XAS) is used to probe the structure of wild-type nitrate reductase NapA and the C176A variant. The results of extended X-ray absorption fine structure (EXAFS) experiments on NapA support an oxidized Mo(VI) hexacoordinate active site coordinated by a single terminal oxo donor, four sulfur atoms from two separate pyranopterin dithiolene ligands, and an additional S atom from a conserved cysteine amino acid residue. We found no evidence of a terminal sulfido ligand in NapA.