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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.



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.)



A muscle protein, comprising up to 50% of the total muscle proteins; combines with actin to form actomycin.
References in periodicals archive ?
2003) Different effects on human skeletal myosin heavy chain isoform expression: strength vs.
Coordinated expression of myosin heavy chains, metabolic enzymes, and morphological features of porcine skeletal muscle fiber types.
Transcriptional regulation of the type I myosin heavy chain gene in denervated rat soleus.
Consequently, we proposed that the LOC534614 gene was similar to the myosin heavy chain of skeletal muscle.
1] = Gap 1, MHC = myosin heavy chain, MLC = myosin light chain, MYH = myosin heavy polypeptide, NIA = National Institute on Aging, RNA = ribonucleic acid, S = Synthesis, SCI = spinal cord injury, SEM = standard error of the mean, SF-36 = 36-item Short Form (Health Survey), SSWS = self-selected walking speed, TNF-[alpha] = tumor necrosis factor-[alpha], VA = Department of Veterans Affairs, VL = vastus lateralis, V[O.
The outer myoepithelial cell layers are typically positive for p63 and a muscle marker, such as smooth muscle actin, calponin, or smooth muscle myosin heavy chain.
At 14 days after injection, expression of cardiac markers Myosin Heavy Chain and Troponin I was observed on donor cells consistent with the differentiation of adipose tissue-derived regenerative cells into cardiac myocytes.
However, calponin-positive myoepithelial cells were noted in clusters of cells from ductal carcinoma in situ, although the smooth muscle myosin heavy chain reactivity seemed to be absent.
Liew's group was the first to succeed in the isolation and complete sequencing of the human cardiac myosin heavy chain gene which has been associated with Familial Hypertrophic Cardiomyopathy (a cause of heart failure).