Energy-coupling mechanisms in mitochondria: kinetic, spectroscopic, and thermodynamic properties of an energy-transducing form of cytochrome b.
The primary event of coupled electron transfer at phosphorylation site II is identified with a modification in one of the two chemically distinct forms of cytochrome b, designated as the energy-transducing cytochrome b(T). This modification is expressed through a change in the redox midpoint potential and by an increase in its reaction half time with cytochrome c(1). In pigeon heart mitochondria cytochrome b(T) exhibits an absorption maximum at 564 nm and on this basis, it can be distinguished from Keilin's cytochrome b which exhibits an absorption maximum at 560 nm and serves as an electron carrier on the substrate side of cytochrome b(T). Kinetic capability of cytochrome b(T) is evidenced by its rapid electron transfer and energization time of less than 200 msec, its thermodynamic capability-by a 280 mV potential span suitable for providing one of the two electron transfer reactions required in ATP formation. Two secondary events of coupled electron flow may be identified with a charge separation across the lipid structure of the permeability barrier and a change in water structure; both events result in an increased 1-anilino-8-naphthalene-sulfonic acid (ANS) response to the altered environment.