Elucidating the Structures of the Low- and High-pH Mo(V) Species in Respiratory Nitrate Reductase: A Combined EPR, 14,15N HYSCORE, and DFT Study

Respiratory nitrate reductases (Nars), members of the prokaryotic Mo/W-bis Pyranopterin Guanosine dinucleotide (Mo/W-bisPGD) enzyme superfamily, are key players in nitrate respiration, a major bioenergetic pathway widely used by microorganisms to cope with the absence of dioxygen. The two-electron reduction of nitrate to nitrite takes place at their active site, where the molybdenum ion cycles between Mo(VI) and Mo(IV) states via a Mo(V) intermediate. The active site shows two distinct pH-dependent Mo(V) electron paramagnetic resonance (EPR) signals whose structure and catalytic relevance have long been debated. In this study, we use EPR and HYSCORE techniques to probe their nuclear environment in Escherichia coli Nar (EcNar). By using samples prepared at different pH and through different enrichment strategies in 98Mo and 15N nuclei, we demonstrate that each of the two Mo(V) species is coupled to a single nitrogen nucleus with similar quadrupole characteristics. Structure-based density functional theory ...