XAS STUDIES OF THE Cu_A CENTERS OF CYTOCHROME c OXIDASE: A UNIQUE BINUCLEAR COPPER CLUSTER. Ninian J. Blackburn⁺, Simon de Vries^[[section]], Robert A. Scott^{[[paragraph]]}, James Fee^$, Yi Lu^#, Chris Dennison^[[yen]] and Gerard Canters^[[yen]]. ⁺Oregon Graduate Institute of Science and Technology; ^[[section]]Delft University of Technology, ^{[[paragraph]]}University of Georgia; ^$University of California, San Diego; ^#University of Illinois, Urbana-Champaign; ^[[yen]]Leiden Institute of Chemistry

The Cu_A centers of cytochrome c oxidases are unique examples of a new type of binuclear copper cluster. X-ray crystallography of enzymes from beef heart, Paracoccus, and the engineered cyoA fragment of the quinol oxidase of E. Coli have provided a structural description of the site. The coppers are bridged by two cysteine ligands, and have an extremely short Cu-Cu distance of ~2.4 Å. X-ray absorption spectroscopy, which had previously predicted the short Cu-Cu distance, has been used to further refine the structural details of the site, in both the oxidized and reduced forms. Subtle changes are detected in the metrical parameters of the oxidized versus reduced proteins which suggest that the short distance may be the result, in part, of a weak metal-metal bond. These studies have been extended to include Cu_A derivatives of the blue proteins azurin and amicyanin produced by "loop-directed mutagenesis", in which the Cu_A-binding sequence has been introduced into the blue copper proteins.