Kinetics of ATP Hydrolysis by Actomyosin

TABLE I Kinetic Parameters of the Interaction of Actin with Heavy Meromyosin 

In the presence of actin, the scheme is necessarily more complex. In the absence of ATP, myosin s affinity for actin is very large and, under most conditions, myosin would be fully complexed with actin. Binding of ATP greatly weakens the affinity of myosin for actin and dissociation of the two proteins rapidly ensues at low protein concentrations. The extent of the ATP-induced dissociation of actin and myosin is a function of the ionic conditions and protein concentration and a rapid equilibrium exists between the two states. Hydrolysis of ATP can occur either by the dissociated myosin or by the actomyosin complex. The net result of ATP binding and hydrolysis is a rapid equilibrium between the four species shown in Fig. 8 in the box marked weak binding states. The rate of hydrolysis of ATP to give the bound products is fast and is not affected by phosphorylation of HMM (J. R. Sellers, unpublished data). 

Pi release also precedes ADP release in the presence of actin. The identification of the rate-limiting step has not been entirely clarified, even for the widely studied skeletal muscle actomyosin system (Hibberd and Trentham, 1986). There are two possibilities that may depend on experimental conditions. One possibility is that Pj release, or an isomerization of AM-ADP-Pj that precedes Pj release, is the rate-limiting step (Stein, 

Other studies suggest that, under some conditions, the hydrolysis of ATP may be rate limiting (Tesi et al, 1990). In any case, release of P, results in an increased affinity of myosin for actin and marks the transition between the weakly and strongly bound states. The affinity of M-ADP for actin is high enough that most of the myosin-ADP complex would be bound to actin. ADP release is fast compared to Pj release and returns myosin to the starting point of the cycle (J. R. Sellers, unpublished data). 

FIGURE 8 Kinetic scheme for the hydrolysis of ATP by smooth muscle actomyosin. M = myosin (or its subfragments) A = actin T = ATP D = ADP Pi = inorganic phosphate. 

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