Proton NMR study of the heme environment in bacterial quinol oxidases

The heme environment and ligand binding properties of two relatively large membrane proteins containing multiple paramagnetic metal centers, cytochrome bo₃ and bd quinol oxidases, have been studied by high field proton nuclear magnetic resonance (NMR) spectroscopy. The oxidized bo₃ enzyme displays well-resolved hyperfine-shifted ¹H NMR resonance assignable to the low-spin heme b center. The observed spectral changes induced by addition of cyanide to the protein were attributed to the structural perturbations on the low-spin heme (heme b) center by cyanide ligation to the nearby high-spin heme (heme o) of the protein. The oxidized bd oxidase shows extremely broad signals in the spectral region where protons near high-spin heme centers resonate. Addition of cyanide to the oxidized bd enzyme induced no detectable perturbations on the observed hyperfine signals, indicating the insensitive nature of this heme center toward cyanide. The proton signals near the low-spin heme b₅₅₈ center are only observed in the presence of 20% formamide, consistent with a critical role of viscosity in detecting NMR signals of large membrane proteins. The reduced bd protein also displays hyperfine-shifted ¹H NMR signals, indicating that the high-spin heme centers (hemes b₅₉₅ and d) remain high-spin upon chemical reduction. The results presented here demonstrate that structural changes of one metal center can significantly influence the structural properties of other nearby metal center(s) in large membrane paramagnetic metalloproteins.