Pseudorotation mechanism of ATP hydrolysis in muscle contraction

Abstract

AN analysis of the reaction mechanism of ATP hydrolysis may elucidate how ATP provides energy for muscle contraction. Kinetic studies of the mechanism of ATP hydrolysis catalysed by myosin and its proteolytic subfragments^1,2, revealed at least two intermediates before the rate limiting step: (1) enzyme-bound ATP (M^*ATP) and (2) an intermediate (M^*ADP·P_i) that yields ADP and P_i when a quenching reagent such as HClO₄ is added. Trentham et al.² proposed that the rate limiting step is the transition from M^*ADP·P_i to the Michaelis product complex M·ADP·P_i. That M^*ADP·P_i is distinct from the Michaelis product complex is supported by observations made using techniques which include spin labelling³, difference ultraviolet absorption spectroscopy⁴, extrinsic probe fluorescence⁵ and intrinsic myosin fluorescence⁶. Each method shows that the conformation of myosin during ATP hydrolysis differs from the conformation induced by the binding of ADP and P_i. This conclusion is supported further by the observation that the free energy change for the reversible reaction M^*ATP M^*ADP·P_i is only −1.3 kcalories per mol (ref. 7). Although comparable in energy with M^*ATP, M^*ADP·P_i nevertheless seems to be closely related to products because it decays to ADP and P_i when exposed to a quenching reagent. Although this decay to ADP and P_i has been widely interpreted as indicating that ATP is already cleaved in M^*ADP·P_i, it could alternatively be interpreted in terms of an uncleaved but labile reaction intermediate.