Demembranated Muscle Fibers Catalyze a More Rapid Exchange between Phosphate and Adenosine Triphosphate than Actomyosin Subfragment 1

The rate of ATP ⇌ Pi exchange, that is, the incorporation of medium P_iinto ATP during the net hydrolysis of ATP, has been measured for rabbit psoas muscle fibers, myofibrils, and actomyosin subfragment 1 (acto-Sl). The maximum exchange rate in fibers at saturating [Pi] is 0.04 s^-1per myosin head at 8 °C, pH 7, and an ionic strength of 0.2 M. The dependence of the rate on P_iconcentration can be approximated by a hyperbola with an apparent dissociation constant (k_m) of 3 mM. Myofibrils catalyze ATP ⇄ Pi exchange with a similar K_mbut at a slightly lower rate. In contrast, the soluble acto-Sl system, in which ATP hydrolysis is not coupled to tension generation, catalyzes exchange at a rate 500 times lower than that of fibers at low P_iconcentration, and the K_mfor P_iis greater than 50 mM. The difference between the ATP ⇌ P_iexchange of fibers and of acto-Sl is discussed in terms of a model in which Pi binds to a force-generating state AM′₷ADP and, due to mechanical constraint, the average free energy of this state is higher in the fiber than in acto-Sl.