Probing the Menasemiquinone Binding Mode to Nitrate Reductase A by Selective 2 H and 15 N Labeling, HYSCORE Spectroscopy, and DFT Modeling

Abstract : In vivo specific isotope labeling at the residue or substituent level is used to probe menasemiquinone (MSK) binding to the quinol oxidation site of respiratory nitrate reductase A (NarGHI) from E. coli. 15N selective labeling of His15Nδ or Lys15Nζ in combination with hyperfine sublevel correlation (HYSCORE) spectroscopy unambiguously identified His15Nδ as the direct hydrogen-bond donor to the radical. In contrast, an essentially anisotropic coupling to Lys15Nζ consistent with a through-space magnetic interaction was resolved. This suggests that MSK does not form a hydrogen bond with the side chain of the nearby Lys86 residue. In addition, selective 2H labeling of the menaquinone methyl ring substituent allows unambiguous characterization of the 2H—and hence of the 1H—methyl isotropic hyperfine coupling by 2H HYSCORE. DFT calculations show that a simple molecular model consisting of an imidazole Nδ atom in a hydrogen-bond interaction with a MSK radical anion satisfactorily accounts for the available spectroscopic data. These results support our previously proposed one-sided binding model for MSK to NarGHI through a single short hydrogen bond to the Nδ of His66, one of the distal heme axial ligands. This work establishes the basis for future investigations aimed at determining the functional relevance of this peculiar binding mode.