Studies on the Mechanism of Site I Energy Conservation

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Abstract
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 (> 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.

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