Myosin actomyosin

In summary, therefore, solution and fiber biochemistry have provided some idea about how ATP is used by actomyosin to generate force. Currently, it seems most likely that phosphate release, and also an isomerization between two AM.ADP.Pj states, are closely linked to force generation in muscle. ATP binds rapidly to actomyosin (A.M.) and is subsequently rapidly hydrolyzed by myosin/actomyosin. There is also a rapid equilibrium between M. ADP.Pj and A.M.ADP.Pj (this can also be seen in fibers from mechanical measurements at low ionic strength). The rate limiting step in the ATPase cycle is therefore likely to be release of Pj from A.M.ADP.Pj, in fibers as well as in solution, and this supports the idea that phosphate release is associated with force generation in muscle. 

Properties Myosin proper of Szent-Gyorgyi L-myosin Actomyosin (Szent-Gyorgyi) S-myosin... 

No new X-ray data have become available since the summary in this journal by Bailey in 1944. The X-ray examination of actin (Astbury ei al., 1947) has shown indirectly that the wide angle diagram of myosin (actomyosin-L-myosin mixture) is that of the L-myosin component. The relation between the X-ray data and amino acid analysis was also thoroughly discussed by Bailey (1944) in the same review. Few modifications in the amino-acid composition have been made since that time, and the present position cf. Bailey, 1948) is given in Table X. 

There appear to be no data on the pH dependence of the ATPase activity of pure L-myosin. Banga (1942) finds an optimum at pH about 6 for actomyosin (B myosin) in mixtures of alkali salts and 10 M MgCL. The total salt concentration or ionic strength is not specified here nor in the experiments of Engelhardt (1946), in which the pH optimum of the myosin (L-myosin/actomyosin mixture) in presence of Mg++ was around pH 9, while in the presence of Ca++ there was an additional, much weaker, optimum at 6.3. The activity and the optimum at pH 9 are very much lower with Mg++ than with Ca++. Mommaerts and Seraidarian (1947) give an optimum at pH 6.5 for actomyosin (B-myosin) in alkali salts at ionic strength 0.15-0.17. In the presence... 

Figure 3 Electron spin resonance spectrum of suspended myosin/actomyosin irradiated to 60 kGy at 0°C and scanned at 77 K. Reproduced from Taub et al. [13 ] with permission.

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. 

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. 

Gopal, D., Pavlov, D. I., Levitsky, D. I., et al., 1996. Chemomechanical trans-dnction in the actomyosin molecnlar motor by 2, 3 -dideoxydidehydro-ATP and characterizadon of its interacdon with myosin snbfragment 1 in die presence and absence of acdn. Biochemistry 35 10149-10157. 

A subfamily of Rho proteins, the Rnd family of small GTPases, are always GTP-bound and seem to be regulated by expression and localization rather than by nucleotide exchange and hydrolysis. Many Rho GTPase effectors have been identified, including protein and lipid kinases, phospholipase D and numerous adaptor proteins. One of the best characterized effector of RhoA is Rho kinase, which phosphorylates and inactivates myosin phosphatase thereby RhoA causes activation of actomyosin complexes. Rho proteins are preferred targets of bacterial protein toxins ( bacterial toxins). 

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 ... 

Although this model is very attractive because of its simplicity, further experiments have shown that there are two problems with it (Eisenberg and Hill, 1985). First, myosin.ATP binds as well to actin as does myosin.ADP.Pj such that there is a rapid equilibrium between actomyosin.ATP and myosin.ATP, and between... 

The binding of ATP to actomyosin weakens the binding of actin to myosin such that the dissociation constant decreases from 1(T M to 10 M. There is thus a... 

The hydrolysis of ATP is used to drive movement of the filaments. ATP binds to myosin heads and is hydrolyzed to ADP and P, by the ATPase activity of the actomyosin complex. 

Phosphate in combination with NaCl has a beneficial effect on the waterbinding capacity of processed meat products for a detailed description, see Schmidt.227 The effect of phosphates is suggested to be alterations in pH or ionic strength, sequestration of metal ions, dissociation of actomyosin and depolymerisation of myosin.103,104,228,229 However, before action, added phosphates must be hydrolysed by muscle phosphatases or non-enzymatically. Belton et al.230 studied the hydrolysis of pyrophosphate and tripolyphosphate in comminuted chicken meat using 31P NMR spectroscopy, and found that the rate of hydrolysis was dependent on the length of ageing period of the muscle as well as the presence of NaCl. Li et al.231 studied the hydrolysis of various types of phosphates in intact chicken muscle with a similar approach by 31P NMR spectroscopy and thereby demonstrated differences in rate of hydrolysis of various phosphates. The findings of these studies... 

That myosin, a structural protein, also had enzyme activity as an ATPase, had been shown by Engelhardt and Ljubimova (1939-1941). ATP was now found to dissociate actomyosin producing a marked fall in viscosity the ATP was split to ADP and Pj. Contrasting properties of ATP in muscle systems were also observed. The rigor seen at postmortem occurred as ATP levels fell. The ATPase activity of myosin could be inhibited by mercurials (which block SH groups on cysteine) with ATPase blocked, ATP caused muscle fibers to relax (Weber and Portzehl, 1952). 

The ATPase enzyme activity of actomyosin has been assigned the classification number EC 3.6.1.32. Myosin catalyzes the hydrolysis of ATP to ADP and orthophosphate. In the absence of actin, myosin is a more feeble ATPase. See also Myosin... 

White et aP measured the rates of phosphate release during a single turnover of actomyosin nucleoside triphosphate (NTP) hydrolysis using a double-mixing stopped-flow spectrofluorometer, at very low ionic strength to increase the affinity of myosin-ATP and myo-sin-ADP-Pi to actin. Myosin subfragment 1 and a series of nucleoside triphosphates were mixed and incubated for approximately 1-10 s to allow NTP to bind to myosin and generate a steady-state mixture of myosin-NTP and myosin-NDP-Pi. The steady-state intermediates were then mixed with actin. 

DEBYE-HUCKEL AW BIOMINERALIZATION ACTOMYOSIN ATPase MYOSIN... 

In vitro, the enzyme is able to catalyze crosslinking of whey proteins, soy proteins, wheat proteins, beef myosin, casein, and crude actomyosin (which is refined from mechanically deboned meat), improving functional properties such as the texture of food products [49-53], Bonds formed by transglutaminase exhibit a high resistance to proteolytic degradation [54],... 

That actin and myosin are jointly responsible for contraction was demonstrated long before the fine structure of the myofibril became known. In about 1929, ATP was recognized as the energy source for muscle contraction, but it was not until 10 years later that Engelhardt and Ljubimowa showed that isolated myosin preparations catalyzed the hydrolysis of ATP.138 Szent-Gyorgi139 140 showed that a combination of the two proteins actin (discovered by F. Straub141) and myosin was required for Mg2+-stimulated ATP hydrolysis (ATPase activity). He called this combination actomyosin. 

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