1H NMR study of the magnetic properties and electronic structure of the hydroxide complex of substrate-bound heme oxygenase from Neisseria meningitidis: influence of the axial water deprotonation on the distal H-bond network

The substrate and active site residues of the low-spin hydroxide complex of the protohemin complex of Neisseria meningitidis heme oxygenase (NmHO) have been assigned by saturation transfer between the hydroxide and previously characterized aquo complex. The available dipolar shifts allowed the quantitation of both the orientation and anisotropy of the paramagnetic susceptibility tensor. The resulting positive sign, and reduced magnitude of the axial anisotropy relative to the cyanide complex, dictate that the orbital ground state is the conventional "d(pi)" (d(2)(xy)(d(xz), d(yz))(3)); and not the unusual "d(xy)" (d(2)(xz)d(2)(yz)d(xy)) orbital ground state reported for the hydroxide complex of the homologous heme oxygenase (HO) from Pseudomonas aeruginosa (Caignan, G.; Deshmukh, R.; Zeng, Y.; Wilks, A.; Bunce, R. A.; Rivera, M. J. Am. Chem. Soc. 2003, 125, 11842-11852) and proposed as a signature of the HO distal cavity. The conservation of slow labile proton exchange with solvent from pH 7.0 to 10.8 confirms the extraordinary dynamic stability of NmHO complexes. Comparison of the diamagnetic contribution to the labile proton chemical shifts in the aquo and hydroxide complexes reveals strongly conserved bond strengths in the distal H-bond network, with the exception of the distal His53 N(epsilon)(1)H. The iron-ligated water is linked to His53 primarily by a pair of nonligated, ordered water molecules that transmit the conversion of the ligated H-bond donor (H(2)O) to a H-bond acceptor (OH(-)), thereby increasing the H-bond donor strength of the His53 side chain.