Myosin ATPase activity

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. 

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. 

Figure 13.14 The effect of a change in the Ccf ion concentration on in vitro activity of myosin ATPase activity. At rest, the cytosolic ion concentration is about 0.1 xmoL/L, at which concentration myosin ATPase activity is low. Nervous stimulation of the fibre increases the cytosolic Ca ion concentration to about 2-10 xmol/L, with the half maximum change at about 0.5 xmol/L. Hence, on the basis of this property of myosin ATPase in the test tube, the in vivo change in Ca ion concentration should result in almost total activation of the enzyme. Approximately 50% activation of the myosin ATPase, when measured in vitro, occurs at about 0.5 xmol/L Ca ion concentration. A similar Ca ion concentration in the cytosol of the fibre results in 50% of the maximal force of contraction (Appendix 13.2).

Rayment, I. (1996) The structural basis of the myosin ATPase activity. J. Biol. Chem. 271, 15,850-15,853. 

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. 

Mammalian skeletal muscle can be separated Into two distinct fiber populations, based on relative contraction characteristics, and are referred to as slow-twltch (Type I) or fast-twitch (Type II) fibers. The slow-twltch fiber type exhibits a relatively low shortening velocity (27), a low rate of tension development (27). a low myosin ATPase activity (28) and a low rate of calcium sequestration by the sarcoplasmic reticulum (29). The converse Is true for the fast-twitch fibers. Since contraction velocity highly correlates with myosin ATPase activity (30), It Is possible to easily Identify,... 

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. 

The actomyosin or myosin ATPase activity in fish was reduced after subjecting the myofibrils to treatments above 300 MPa for 20 to 30 min. 

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... 

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.,... 

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