Studies on the mechanism of site I energy conservation.

1. Of the several iron-sulfur centers detected in the site I segment of the respiratory chain, centers N-2 and N-1a alone exhibit apparent phosphate-potential dependent half-reduction potentials, indicating their possible involvement in energy conservation. 2. At high phosphate potential, the apparent half-reduction potential of center N-2 shifts positively by 125 +/- 20 mV, while center N-1a shifts negatively by approximately 60 mV. 3. The redox state of individual iron-sulfur centers in various metabolic states was analyzed. Center N-2 is highly reduced (greater than 90%) in "state 4" mitochondria, while center N-1a stays mostly oxidized. 4. In a submitochondrial system, ATP addition induced reduction of center N-2 if the Eh of the suspension was poised from the high potential side of site I using the succinate/fumarate couple. In contrast, center N-2 was oxidized upon energization, if the Eh of the system was poised from the low potential side using the NADH/NAD couple. 5. Based on these redox behaviors of center N-2, a tentative hypothesis of site I energy transduction was proposed.