Metal-Bound Histidine Modes in UV Resonance Raman Spectra of Cu, Zn Superoxide Dismutase

Ultraviolet resonance Raman [UVRR] spectra of Cu, Zn superoxide dismutase [SOD] contain bands arising from vibrations of metal-bound histidine ligands. Spectra in H2O solution reveal several modes of the His61 side chain, which bridges the Cu2+ and Zn2+ ions as imidazolate. The disappearance of these bands signals disruption of the bridge when the pH is lowered to 3.0, or the Cu2+ is reduced to Cu+. Binding of hydroxide [pH 12] or cyanide to the Cu2+ perturbs the imidazolate modes, reflecting geometry changes induced by these strong-field ligands. In D2O solution several additional bands become enhanced which arise from histidine ligands that have undergone NH/D exchange. Some of these are attributed to Cu-bound ligands and others to Zn-bound ligands, on the basis of selective changes accompanying removal and replacement of the metals. Excitation profiles are similar for these bands, and for the bridging imidazolate bands; they are red-shifted relative to nonligating histidine. The detection of site-specific histidine ligand modes gives promise for wide applicability of UVRR spectroscopy in studying histidine ligation in metalloproteins. The single tyrosine residue of SOD, which is a target of active-site-catalyzed nitration by peroxynitrite, is found to have an elevated pKa, 11.4, despite being exposed to solvent.