Resolution of bovine heart cytochrome c oxidase into smaller complexes by controlled subunit denaturation

Bovine heart cytochrome c oxidase has been partially denaturated under mild conditions with 0.1-0.25% lithium dodecyl sulfate and 0.05% Triton X-100. From its reactivity towards CO and CN-, an unmasking of the heme a was inferred in this enzyme. The catalytic activity was lost during the denaturation and small spectral differences became visible. Spectra and ligand binding properties of the denatured enzyme were reversed by dilution in 2% Triton X-100. This suggests that during the denaturation procedure the hemes were not displaced from their original sites. By gel filtration of the partially denatured enzyme the following complexes of subunits were obtained: I-III, I-II-III, II-IV-V-VI-VII and IV-V-VI-VII. The first three complexes retained almost all the heme, and their spectral characteristics were very similar to those of the partially denatured cytochrome c oxidase. The data, in combination with the information that subunit III does not contain heme [Saraste et al. (1980) FEBS Lett. 114, 35-38], suggest that the hemes are attached to subunit I and II. After denaturation of cytochrome c oxidase under more drastic conditions some of the heme was also found to be associated with the smaller subunits, but its spectral characteristics were radically altered, becoming almost identical to those of free heme.