Actin myosin ATPase activation

Szent-Gyorgyi further showed that the viscosity of an actomyosin solution was lowered by the addition of ATP, indicating that ATP decreases myosin s affinity for actin. Kinetic studies demonstrated that myosin ATPase activity was increased substantially by actin. (For this reason, Szent-Gyorgyi gave the name actin to the thin filament protein.) The ATPase turnover number of pure myosin is 0.05/sec. In the presence of actin, however, the turnover number increases to about 10/sec, a number more like that of intact muscle fibers. 

However, release of ADP and P from myosin is much slower. Actin activates myosin ATPase activity by stimulating the release of P and then ADP. Product release is followed by the binding of a new ATP to the actomyosin complex, which causes actomyosin to dissociate into free actin and myosin. The cycle of ATP hydrolysis then repeats, as shown in Figure 17.23a. The crucial point of this model is that ATP hydrolysis and the association and dissociation of actin and myosin are coupled. It is this coupling that enables ATP hydrolysis to power muscle contraction. 

Table 49—7 summarizes and compares the regulation of actin-myosin interactions (activation of myosin ATPase) in striated and smooth muscles. 

In smooth muscle, myosin crossbridges have less myosin ATPase activity than those of skeletal muscle. As a result, the splitting of ATP that provides energy to "prime" the crossbridges, preparing them to interact with actin, is markedly reduced. Consequently, the rates of crossbridge cycling and tension development are slower. Furthermore, a slower rate of calcium removal causes the muscle to relax more slowly. 

Oliveira, D. M., Nakaie, C. R., Sousa, A. D., Farah, C. S., and Reinach, F. C. (2000). Mapping the domain of troponin T responsible for the activation of ac to myosin ATPase activity. Identification of residues involved in binding to actin./. Biol. Chem. 275, 27513-27519. 

Calponin is another polypeptide monomer (M.W. 32,000) that can inhibit actin-activated myosin ATPase activity. In contrast to CaD, CaP exerts its effect in the absence of tropomyosin and completely inhibits motility in a 2/3 ratio with actin. CaP inhibits myosin binding to actin, but does so by reducing the affinity of actin for myosin rather than competing for the same site. CaP can be phosphorylated by PKC and CaMKII, both of which reverse CaP s inhibitory activity. As with caldesmon, many questions remain. The ratio of CaP to actin actually observed in smooth muscle is in the range 1 10 to 1 16, far from the 2/3 ratio found to produce near-complete inhibition of motility. Therefore, the importance of CaP and its regulation by phosphorylation is still debatable. 

At present, the exact biological function of this protein is not known. When tested for its ability to reverse caldesmon s inhibition of the actin-activated myosin ATPase activity, 12-kDa CaBP had no significant effect on the ATPase activity when the mole ratio of 12-kDa... 

It has been established for several years that the major mechanism for regulation of contraction in smooth muscle is myosin phosphorylation (Hart-shorne, 1987). Phosphorylation of the two 20,000-dalton light chains of myosin (LC20) activates the actin-dependent ATPase activity of myosin and this initiates the contractile response. Dephosphorylated myosin is associated with relaxed muscle. In this scheme there are two key enzymes the myosin light chain kinase (MLCK) and the myosin light chain phosphatase (MLCP). Obviously a balance of these two activities determines the level of myosin phosphorylation. 

Phosphorylation of caldesmon in vitro by CaM-kinase II (or PKC) has been shown to interfere with caldesmon binding to F-actin, and results in a reversal of the caldesmon inhibitory effect on actin-activated myosin ATPase activity (Ngai and Walsh, 1987). This has led to speculation that these kinases may be in-... 

Activated myosin light chain kinase phosphorylates the 20-kDa light chain of myosin on serine 19. This phosphorylation is associated with an increase in the actin-activated myosin ATPase activity (Ikebe et al.,... 

Finally, the close similarity in the ATPase activity of actomyosin and L-myosin solutions is evidence that, in presence of ATP, L-myosin is free. When bound to actin, its ATPase activity differs from that in the free form (cf. Section III, 5d). 

FIGURE 22.2 Actin-myosin power stroke cycle. At the top of the cycle, myosin releases from actin when ATP becomes bound. Myosin ATPase activity causes the dissociation of ATP into ADP and Pj, which changes the conformation of myosin for the power stroke. When myosin binds to actin, Pj is released and the head of myosin pulls on actin to impart a force on actin. The remaining ADP is then released and the power stroke cycle begins anew. 

The molecular events of contraction are powered by the ATPase activity of myosin. Much of our present understanding of this reaction and its dependence on actin can be traced to several key discoveries by Albert Szent-Gyorgyi at the University of Szeged in Hungary in the early 1940s. Szent-Gyorgyi showed that solution viscosity is dramatically increased when solutions of myosin and actin are mixed. Increased viscosity is a manifestation of the formation of an actomyosin complex. 

The myosins are a superfamily of proteins that have the ability to convert energy released by ATP is hydrolysis into mechanical work. There are many forms of myosin, all of which have ATPase activity and an actin-binding site that is located... 

Myosin as an ATPase Activation of Myosin ATPase by Actin Lymn and Taylor Model 1971 Eisenberg and Hill Model 1985... 

The simplest mechanism to explain the much faster rate of dissociation of actomyosin-S-1 by ATP than that of ATP cleavage is that actin activates the myosin ATPase by accelerating the rate at which ADP and Pj are released. That is when ATP is added to actomyosin-S-1, ATP rapidly binds and dissociates actomyosin, myosin ATPase then hydrolyzes ATP to form myosin-ADP.Pj, this state then reattaches to actin and phosphate is released much faster from actomyosin. ADP.Pj than it is from myosin.ADP.Pj, as shown in the scheme below ... 

S-1 (molecular mass approximately 115 kDa) does exhibit ATPase activity, binds L chains, and in the absence of ATP will bind to and decorate actin with arrowheads (Figure 49-5). Both S-1 and HMM exhibit ATPase activity, which is accelerated 100- to 200-fold by complexing with F-actin. As discussed below, F-actin greatly enhances the rate at which myosin ATPase releases its products, ADP and Pj. Thus, although F-actin does not affect the hydrolysis step per se, its ability to promote release of the products produced by the ATPase activity greatly accelerates the overall rate of catalysis. 

When smooth muscle myosin is bound to F-actin in the absence of other muscle proteins such as tropomyosin, there is no detectable ATPase activity. This absence of activity is quite unlike the situation described for striated muscle myosin and F-actin, which has abundant ATPase activity. Smooth muscle myosin contains fight chains that prevent the binding of the myosin head to F-actin they must be phosphorylated before they allow F-actin to activate myosin ATPase. The ATPase activity then attained hydrolyzes ATP about tenfold more slowly than the corresponding activity in skeletal muscle. The phosphate on the myosin fight chains may form a chelate with the Ca bound to the tropomyosin-TpC-actin complex, leading to an increased rate of formation of cross-bridges between the myosin heads and actin. The phosphorylation of fight chains initiates the attachment-detachment contraction cycle of smooth muscle. 

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