Myosin enzyme activity

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). 

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).

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... 

S ATP + myosin hght chain <1-12> (<1, 2, 8> event in initiation of smooth-muscle contraction [5] <8> involved in regulation of actin-myosin contractile activity in adrenal medulla [7] <8> obhgatory step in development of active tension in smooth muscle [13] <5> involved in myosin phosphorylation and enzyme secretion [16] <2, 3, 5, 6, 8, 10, 11> involved in muscle contractility and motility of non-muscle cells [33] <2> inhibition of actin-myosin ineraction [36,37]) (Reversibihty 1-12 [1-33]) [1-33]... 

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. 

Myosin sufficiently purified in the usual way does not dephosphorylate any of the numerous phosphate compounds of living muscle other than ATP, and perhaps ITP, nor can it transphosphorylate. Menne (1943) finds that myosin, unlike the other main fractions of muscle, can convert arginine, histidine, glycocyamine, and choline into creatine. The myosin used, however, was only reprecipitated once and subsequently washed, and it is possible that the enzyme activity might be lost on further precipitation. After fractionation and precipitating three times, myosin possesses an appreciable adenylic deaminase activity (Hermann and Josepovits, 1949 Summerson and Meister, 1944). 

This enzyme activity has been observed in myosin and actomyosin, mitochondria, microsomes, and cell membranes. In some cases magnesium ions function as an activator, in others calcium ions, and in still others, both calcium and magnesium are requited. Another form of adenosine-triphosphatase is stimulated by sodium and potassium ions and is inhibited by ouabain. Some forms of the enzyme can hydrolyse inosine triphosphate and other nucleoside-5 -triphosphates. The substrate specificity may depend upon the activating divalent cation and on the presence of monovalent cations. These enzymes are probably important components of a system responsible for facilitating cation transfer in membranes. They should not be confused with adenosine triphosphate pyrophosphatase E.C. 3.6.1.8. 

As a result of this active protein synthesis, the total protein mass is increased, and new proteins appear. The development of new proteins has been established by immunological methods and by determination of enzyme activity. During the development of the chick embryo lens, seven antigenic proteins appear. The antigen reactive groups of myosin appear in the heartforming area of the chick embryo. Fluorescent antibody techniques have demonstrated that myosin is diffusely distributed in the early embryo it is later restricted to the heart and muscle-developing areas [16]. 

Troponin (ca. 5% of the contractile proteins) sits on the actin filaments (cf. Fig. 12.10) and controls the contact between the filaments of myosin and actin during muscle contraction by means of a Ca concentration-dependent change in conformation (cf. 12.3.2.1.5). It is a complex of three components, T, I, and C. Troponin T consists of a peptide chain with 259 amino acid residues and binds to tropomyosin. Troponin I (179 amino acid residues) binds to actin and inhibits various enzyme activities (ATPase). Troponin C (158 amino acid residues) binds Ca ions reversibly through a change in conformation. 

Note that while sulfhydryl amino acids oceui in the intracellular protein myosin, tliey arc probably involved in enzymic activity rather than di.sulfide bonds (Dreizen and Gershman, 1970). 

Contraction of muscle follows an increase of Ca " in the muscle cell as a result of nerve stimulation. This initiates processes which cause the proteins myosin and actin to be drawn together making the cell shorter and thicker. The return of the Ca " to its storage site, the sarcoplasmic reticulum, by an active pump mechanism allows the contracted muscle to relax (27). Calcium ion, also a factor in the release of acetylcholine on stimulation of nerve cells, influences the permeabiUty of cell membranes activates enzymes, such as adenosine triphosphatase (ATPase), Hpase, and some proteolytic enzymes and facihtates intestinal absorption of vitamin B 2 [68-19-9] (28). 

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