The specificity and Kd at physiological ionic strength of an ATP-binding site on cytochrome c suit it to a regulatory role.

Cytochrome c binds ATP with marked specificity at a site that contains the evolutionarily invariant residue Arg-91. The binding of ATP to this site was studied using equilibrium gel filtration, equilibrium dialysis and affinity chromatography. At physiological ionic strength the affinity is such that the major change in occupancy coincides with the normal cellular ATP concentration range, and the degree of saturation is proportional to the ratio of [ATP]/[ADP]. The specificity of binding at this site is more a function of the degree of phosphorylation of the nucleotide, than of the nature of the nucleoside moiety. Thus under physiological conditions the degree of occupancy of this site is proportional to the energy state of the cell, providing a means for the regulation of the respiratory chain which is sensitive to cytoplasmic ATP levels.