Site-Site Interaction on Mitochondrial F1-ATPase. Functional Symmetry of the High-Affinity Nucleotide Binding Sites

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Abstract
Interactions between the high affinity binding sites on mitochondrial F1 were analysed by combined use of the nucleotide analogues 3''-O-(1-naphthoyl)-ADP (N-ADP) and 2'',3''-O-(2,4,6-trinitrophenyl)-ADP (TNP-ADP). The binding behaviour of F1 with respect to these ligands was studied by measuring the fluorescence of F1 and of TNP-ADP and the fluorescence anisotropy of N-ADP. A total of 3 high affinity binding sites can be occupied by TNP-ADP. By exchange experiments, it could be shown that binding of TNP-ADP to such a site considerably accelerates the dissociation of a ligand bound to a neighbouring site. These results support the notion that the functional behaviour of F1 is symmetric: during the catalytic cycle any individual site can successively assume different affinity states as has been predicted by hypotheses such as the binding change model.

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