We have proposed that the binding of ATP at a site of substantial affinity and specificity could regulate the activity of cytochrome c with its physiological partners and thus the overall efficiency of mitochondrial electron transport. We now describe the use of ATP affinity-labeled protein to test the effect of occupancy of that site, which includes the invariant arginine 91, on the activity of cytochrome c with purified cytochrome c reductase and oxidase and its association with the mitochondrial inner membrane. Electron-transfer activities with the reductase and oxidase were inhibited by site occupancy to 41% and 11-15% of native values, respectively. The marked difference in the degree of inhibition of activity that distinguishes the reactions with the two major physiological partners was sufficient to cause, in whole mitochondria, a demonstrable shift from a situation in which there is a rate-limiting transfer from the reductase to cytochrome c, to a state where rates are more evenly matched for transfers between cytochrome c and the two redox partners. Site occupancy also substantially reduces the ionic strength necessary for half-maximal dissociation of cytochrome c from the membrane. These data imply that the decreased efficiency of electron transfer caused by ATP attachment can be attributed to a decrease in the protein's activity with individual physiological partners, possibly compounded with a decrease in its affinity for the inner mitochondrial membrane, and suggest that feedback regulation by ATP of cellular respiration operates in like manner.
Eukaryotic cytochrome c possesses an ATP-binding site of substantial specificity and high affinity that is conserved between highly divergent species and which includes the invariant residue arginine''. Such evolutionary conservatism strongly suggests a physiological role for ATP binding that demands further investigation. We report the preparation of adducts of the protein and the affinity labels 8-azido adenosine 5'-triphosphate, adenosine S-triphosphate-T,3'-diaIdehyde, and 5'-p-fluorosulfonylbenzoyladenosine. The two former reagents were seen to react at the arginine9'-containing site, yet the reaction of the latter, although specific, occurred elsewhere, suggesting caution is necessary in its use. None of the adducts displayed significant modification of global structure, stability, or physicochemical properties, leading us to believe that the 8-N3-ATP and oATP adducts are good stabilized models of the noncovalent interaction; yet modification led to significant, and sometimes pronounced, effects on biological activity. We therefore propose that the role of ATP binding to this site, which we have shown to occur when the phosphorylation potential of the system is high under the equivalent of physiological conditions, is to cause a decrease in electron flow through the mitochondrial electron transport chain. Differences in the degree of inhibition produced by differences in adduct chemistry suggest that this putative regulatory role is mediated primarily by electrostatic effects.
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