THE CU-A CENTER OF CYTOCHROME-C-OXIDASE - ELECTRONIC-STRUCTURE AND SPECTRA OF MODELS COMPARED TO THE PROPERTIES OF CU-A DOMAINS

The electronic structure and spectrum of several models of the binucle ar metal site in soluble Cu-A domains of cytochrome-c oxidase have bee n calculated by the use of an extended version of the complete neglect of differential overlap/spectroscopic method. The experimental spectr a have two strong transitions of nearly equal intensity around 500 nm and a near-IR transition close to 800 nm. The model that best reproduc es these features consists of a dimer of two blue (type 1) copper cent ers, in which each Cu atom replaces the missing imidazole on the other Cu atom. Thus, both Cu atoms have one cysteine sulfur atom and one im idazole nitrogen atom as ligands, and there are no bridging ligands bu t a direct Cu-Cu bond. According to the calculations, the two strong b ands in the visible region originate from exciton coupling of the dipo les of the two copper monomers, and the near-IR band is a charge-trans fer transition between the two Cu atoms. The known amino acid sequence bas been used to construct a molecular model of the Cu-A site by the use of a template and energy minimization. In this model, the two liga nd cysteine residues are in one turn of an alpha-helix, whereas one li gand histidine is in a loop following this helix and the other one is in a beta-strand.