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, whi ch bridges the Cu2+ and Zn2+ ions as imidazolate. The disappearance of thes e 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 becom e enhanced which arise from histidine ligands that have undergone NH/D exch ange. Some of these are attributed to Cu-bound ligands and others to Zn-bou nd ligands, on the basis of selective changes accompanying removal and repl acement of the metals. Excitation profiles are similar for these bands, and for the bridging imidazolate bands; they are redshifted relative to nonlig ating histidine. The detection of site-specific histidine ligand modes give s 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 h ave an elevated pK(a), 11.4, despite being exposed to solvent.