ENERGY-COUPLING MECHANISMS IN MITOCHONDRIA: KINETIC, SPECTROSCOPIC, AND THERMODYNAMIC PROPERTIES OF AN ENERGY-TRANSDUCING FORM OF CYTOCHROME b^*

Abstract

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.