MULTIFREQUENCY EPR EVIDENCE FOR A BINUCLEAR CU-A CENTER IN CYTOCHROME-C-OXIDASE - STUDIES WITH A CU-63-ENRICHED AND CU-65-ENRICHED, SOLUBLE DOMAIN OF THE CYTOCHROME-BA(3) SUBUNIT-II FROM THERMUS-THERMOPHILUS

We have recorded multi-frequency EPR spectra of Cu-63- and Cu-65-labeled, water-soluble Cu-A-protein from the cytochrome ba(3) of T. thermophilus. The spectrum taken at the highest frequency (34.03 GHz) shows no hyperfine structure and is nominally axial with apparent g(z) similar to 2.18 and g(xy) similar to 2.00. The spectrum taken at the lowest frequency (3.93 GHz) shows a rich hyperfine structure. Analyses of the spectra show that the observed splitting arises from an interaction of the unpaired electron with two Cu nuclei and support the notion that Cu-A is a mixed-valent [Cu(II)/Cu(I)] complex in which the unpaired electronic spin is distributed evenly over the two Cu ions. (C) 1995 Academic Press, Inc.