Resonance Raman spectra of copper(II)-substituted liver alcohol dehydrogenase: a type 1 copper analog

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Abstract
Horse liver alcohol dehydrogenase (LADH) with Cu in place of the catalytic Zn was recently proposed to contain a type 1 site analogous to that in blue copper proteins. Resonance Raman spectra for the Cu-substituted enzyme, Cu(II).cntdot.LADH, and its binary complexes with NADH and pyrazole support this viewpoint. These spectra have 2 dominant features: a sharp beak at .apprx. 415 cm-1, which is believed to be associated with vibration of the single histidine ligand, and a broader, asymmetric band at .apprx. 350 cm-1, whose components are assigned predominantly to vibrational modes of the 2 cysteinate ligands. The high frequency of these transitions, which is reminiscent of the blue copper proteins, is ascribed to that tetrahedral nature of the mental site that produces unusually short Cu.sbd.S bonds and coupled vibrational modes. Solvent exchange with H218O reveals no contribution to the responance Raman spectrum of the water molecule, which is a metal ligand in free Cu(II).cntdot.LADH; however, the spectrum of the binary complex with pyrazole has several new peaks attributable, in part, to pyrazole ligation. The strong similarity among the vibrational spectra demonstrates that the Cu(II) environment in alcohol dehydrogenase maintains its near-tetrahedral geometry in the various enzyme derivatives. The resonance Raman spectrum of Ni(II).cntdot.LADH is close to that of Cu(II).cntdot.LADH and suggests a similar tetrahedral site. The Raman spectra presented here together with available optical and EPR data indicate that Cu(II).cntdot.LADH belongs to the type I copper classification and, thus, can provide new insights into this unusual coordination geometry.

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