Studies on the Mechanism of Site I Energy Conservation

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. 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. The redox state of individual iron‐sulfur centers in various metabolic states was analyzed. Center N‐2 is highly reduced (> 90%) in “state 4” mitochondria, while center N‐1a stays mostly oxidized. 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. Based on these redox behaviors of center N‐2, a tentative hypothesis of site I energy transduction was proposed.