... predicting the fraction of actin in the act-ive state. That model is consistent with the measuredeffects of Ca2+on binding in the presence of ATP[35, 36] , equilibrium binding in the presence of ... regulation of rabbitskeletal muscle contraction. I. Biochemical studies of the interaction of the tropomyosin-troponin complexwith actin and the proteolytic fragments of myosin.J Biol Chem 2 46, 4 866 –4871.39 ... occur in the large co-operative system consisting of myosin, actin, troponin and tropomyosin. Structuralchanges in troponin [4] and tropomyosin [5], in response to either Ca2+or myosin subfragment...