Does the Imidazole Bridge Break During Catalysis in Cu, Zn Superoxide Dismutase ? Judgement from XAFS and Crystallography

Abstract
The binuclear (Cu, Zn) site of superoxide dismutase is bridged by an imidazole ligand which is thought to dissociate from Cu upon reduction, forming the mechanistic basis of this important enzyme. Conflicting crystallographic structures of the reduced enzyme have been reported in the last 12 months. High resolution XAFS data have been collected for both oxidised and reduced forms of the enzyme using the same buffer systems employed in the crystallographic studies. These data and their comprehensive analysis based on fast curved wave theory with multiple scattering, conclusively show that upon reduction of the enzyme very little change takes place at the Zn site but that Cu becomes three coordinate, thus confirming the XAFS results of Blackburn et al. some 13 years ago.