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

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RESUMO

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 bT. This modification is expressed through a change in the redox midpoint potential and by an increase in its reaction half time with cytochrome c1. In pigeon heart mitochondria cytochrome bT 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 bT. Kinetic capability of cytochrome bT 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.

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