Response of Heme Symmetry to the Redox State of Bovine Cytochrome c Oxidase

Abstract

Second-derivative absorption spectroscopy was employed to monitor the response of effective symmetry of cytochromes a and a to the redox and ligation states of bovine cytochrome c oxidase (CcO). The Soret band π → π* electronic transitions were used to display the changes in symmetry of these chromophores induced by the reduction of CcO inhibited by the exogenous ligands and during catalytic turnover. The second derivative of the difference absorption spectra revealed only a single Soret band for the oxidized cytochromes a and a and cyanide-ligated oxidized cytochrome a. In contrast, two absorption bands were resolved in ferrous cytochrome a and ferrous cytochrome a ligated with cyanide. A transition from one-band spectrum to two-band spectrum indicates the lowering of symmetry of these hemes due to the alteration of their immediate surroundings. It is suggested that the changes in polarity occurring in the vicinity of these cofactors are main reason for the split of the Soret band of both ferrous cytochrome a and cyanide-bound ferrous cytochrome a.

Individual heme a and heme a3 contributions to the Soret absorption spectrum of the reduced bovine cytochrome c oxidase

“…The same applies to the recent second-derivative analysis of the Soret CcO spectra confirming ferrous heme a Soret splitting but arguing against splitting of the ferric hemes a and a3 Soret bands [45]. On this basis, the authors proposed that the ferric hemes are symmetric but their reduction changes the protein conformation distorting heme planes and splitting the B0x-and B0y-transitions.…”