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(mī`əsĭn), one of the two major proteinprotein,
any of the group of highly complex organic compounds found in all living cells and comprising the most abundant class of all biological molecules. Protein comprises approximately 50% of cellular dry weight.
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 constituents responsible for contraction of muscle. In muscle cells myosin is arranged in long filaments called thick filaments that lie parallel to the microfilaments of actinactin,
a protein abundantly present in many cells, especially muscle cells, that significantly contributes to the cell's structure and motility. Actin can very quickly assemble into long polymer rods called microfilaments.
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. In muscle contraction, filaments of actin alternately chemically link and unlink with those of myosin in a creeping or sliding action. The energy for this reaction is supplied by adenosine triphosphateadenosine triphosphate
(ATP) , organic compound composed of adenine, the sugar ribose, and three phosphate groups. ATP serves as the major energy source within the cell to drive a number of biological processes such as photosynthesis, muscle contraction, and the synthesis of
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. Myosin and actin also function in the motility of diverse non-muscle cells. In slime moldsslime mold
or slime fungus,
a heterotrophic organism once regarded as a fungus but later classified with the Protista. In a recent system of classification based on analysis of nucleic acid (genetic material) sequences, slime molds have been classified in a major group
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, for example, although present in much smaller quantities and forming shorter filaments, the interaction of the two proteins is employed to change cell shape and permit some movements.
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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



a fibrillar protein, a basic component of contractile muscle fibers (myofibrils); it constitutes 40–60 percent of the total muscle protein content. Myosin combines with another myofibrillar protein, actin, to form actomyosin, a primary structural element in the contractile muscular system. Another important property of myosin is its ability to split adenosine triphosphate (ATP) (V. A. Engel’gardt and M. N. Liubimova, 1939). Owing to the ATP-ase activity of myosin, the chemical energy of the high-energy ATP bonds is transformed into the mechanical energy necessary for muscular contraction. Myosin has a molecular weight of approximately 500,000. When acted upon by proteolytic enzymes, myosin decomposes into heavy meromyosin and light meromyosin (approximate molecular weights, 350,000 and 150,000, respectively).

Electron photomicrographs of myosin molecules reveal a bacilliform structure (1,600 × 25 angstroms), with two globular formations at one end. It is conjectured that the two polypeptide chains which make up the myosin molecule are twisted into a spiral. Proteins that are similar to myosin have been discovered in flagella, cilia, and other motile structures in many species of protozoa and bacteria, as well as in the spermatozoids of animals and certain plants.


Poglazov, B. F. Struktura i funktsii sokratitel’nykh belkov. Moscow, 1965.
Finean, J R Rialagicheskie ul’$$$ Moacow, 1970. (Translated from English.)


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


A muscle protein, comprising up to 50% of the total muscle proteins; combines with actin to form actomycin.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
(1989) Inhibitory regulation by calcium ion of myosin ATPase activity: Binding of calcium ion and phosphorylation of myosin.
6) Myosin ATPase activities ATPase Ca-ATPase > K-EDTA-ATPase > activities K-EDTA ATPase > Ca-ATPase > (see, Mg-ATPase Mg-ATPase Section V) Actin-activated Mg-ATPase activities Inhibited by Not affected by [micro]M levels [Ca.sup.2+] of [Ca.sup.2+] Skeletal muscle (b) Subunit 200 Kd (c) Composition 19Kd (c) [Al.sub.l] F 25 Kd (21 Kd (c)) [Al.sub.2] f 17 Kd Tail Structure Assembly Yes (see, 6.0 Section IV) Ca-binding capacity (see, Fig.
McGuigan et al .[sup][22] performed myosin ATPase staining on gastrocnemius of PAD patients and also found similar muscle fiber remodeling.
(a) Representative transverse sections of soleus muscles stained with myosin ATPase preincubated at pH 4.3 from the control (i) and LNAME-treated (ii) groups.
This stored kinetic energy originates from stretching the pigment matrix like a spring, and is corroborated by the lack of effect of the myosin ATPase inhibitor BDM on rapid-phase aggregation, despite a marked effect on the subsequent plateau phase of aggregation velocity (McNamara and Ribeiro, 1999).
When we compare the effects of decavanadate on myosin ATPase, [Ca.sup.2+]-ATPase, actin polymerization, and myosin ATPase activity stimulated by actin, the latter presented the higher decavanadate inhibitory capacity with an [IC.sub.50] value of 0.75 [micro]M, whereas higher inhibitory [IC.sub.50] values were found for [Ca.sup.2+]-ATPase activity (15 [micro]M) and for actin polymerization (68 [micro]M) [3, 12, 13, 49].
Particularly interesting is the proposed backdoor mechanism of [V.sub.10] myosin ATPase inhibition stimulated by actin and also the inhibition of actin polymerization by decavanadate, although the latter process is still to be clarified.
The actin activated Chara myosin ATPase was assayed in STD buffer without the oxygen scavenger system in the presence of 0.4pg/ml Chara myosin and 0.5 mg/ml skeletal muscle F-actin, by measuring inorganic phosphate liberated with the malachite green method.