Resolution of the electronic transitions of cytochrome c oxidase: evidence for two conformational states of ferrous cytochrome alpha.

Second-derivative absorption spectra are reported for a variety of oxidation and ligation states of bovine cytochrome c oxidase (ferrocytochrome-c:oxygen oxidoreductase, EC 1.9.3.1). The high resolving power of the second-derivative method allows us to assign the individual electronic transitions of cytochrome alpha and cytochrome alpha 3 in many of these states. In the fully reduced enzyme, one observes a single electronic transition at 444 nm, corresponding to the Soret transition for both ferrous cytochrome alpha and ferrous cytochrome alpha 3. When the cytochrome alpha 3 site is occupied by an exogenous ligand (CN or CO), one observes two absorption bands assignable to the ferrous cytochrome alpha chromophore, one at ca, 443 nm and the other at ca, 450 nm. The appearance of the 450-nm band is dependent only on ligand occupancy at the cytochrome alpha 3 site and not on the oxidation state of the cytochrome alpha 3 iron. These results can be interpreted either in terms of a heterogeneous mixture of two ferrous cytochrome alpha conformers in the cytochrome alpha 3-ligated enzyme or in terms of a reduction in the effective molecular symmetry of the ferrous cytochrome alpha site that results in a lifting of the degeneracy of the lowest unoccupied molecular orbital associated with the Soret pi,pi* transition of cytochrome alpha. In either case, the present data indicate that ferrous cytochrome alpha can adopt two distinct conformations. One possible structural difference between these two states could be related to differences in the strength of hydrogen bonding between the ferrous cytochrome alpha formyl oxygen and a proton donor from an unidentified amino acid side chain of the enzyme. The implications of such modulation of hydrogen-bond strength are discussed in terms of possible mechanisms of proton translocation and electron transfer in the enzyme.