Myofibrillar myosin

Thick myofilaments are made primarily of myosin. Myofibrillar myosin is one member of the multi-gene myosin II sub-family in the myosin super-family. Of the 15 types of myosin that have been described, at least 7 occur in mammalian species. Of these seven, three are fairly well characterized. Myosin 1 has a single heavy chain and one to three (usually one) light chain, which is calmodulin. Myosin V has a heavy chain dimer, and carries six light chains, which are usually calmodulin. Both types I and V are motors that carry organelles and vesicles along the actin filaments of the cytoskeleton and probably... 

In addition to the major proteins of striated muscle (myosin, actin, tropomyosin, and the troponins), numerous other proteins play important roles in the maintenance of muscle structure and the regulation of muscle contraction. Myosin and actin together account for 65% of the total muscle protein, and tropomyosin and the troponins each contribute an additional 5% (Table 17.1). The other regulatory and structural proteins thus comprise approximately 25% of the myofibrillar protein. The regulatory proteins can be classified as either myosin-associated proteins or actin-associated proteins. 

The superstructure of smooth muscle actin filaments is differentiated from those of striated muscle by the absence of the troponins and the lateral organization by association of the filaments with dense bodies instead of with the Z-line. How these differences are encoded is again not at all clear. However, the myofibrillar structure and the alignment of the alternating actin and myosin filaments is apparently due primarily to dense bodies and the actin-actinin macrostructures. As the bent dumbbell shaped actins assemble into filaments they are all oriented in the same direction. The S-1 fragments of myosin will bind to actin filaments in vitro and in... 

The smdy of tissue protein breakdown in vivo is difficult, because amino acids released during intracellular breakdown of proteins can be extensively reutilized for protein synthesis within the cell, or the amino acids may be transported to other organs where they enter anabohc pathways. However, actin and myosin are methylated by a posttranslational reaction, forming d-methylliistidine. During intracellular breakdown of actin and myosin, 3-methylhistidine is released and excreted into the urine. The urinary output of the methylated amino acid provides a rehable index of the rate of myofibrillar protein breakdown in the musculature of human subjects. 

Voluntary muscles contain a variety of fibre types which are specialized for particular tasks. Most muscles contain a mixture of fibre types although one type may predominate. All human skeletal muscles are composed of several different muscle fibre types. Up to seven different fibre types have been identified histochemically based on the pH stability of myofibrillar adenosine triphosphatase and on the myosin heavy chain profile. Innumerable fibre type transients exist due to continuing adaptation processes. However, three main... 

In contrast to milk, where samples are primarily derived from cows, meat analysis has to be performed in samples of a widely different animal origin including cattle, lamb, swine, poultry, and fish. Muscle is a complex matrix with a pH of 5.7, composed of muscle fibers, various types of connective tissue, adipose tissue, cartilage, and bones. Sarcoplasmic proteins such as myoglobin, and glycolytic enzymes are soluble in water while the myofibrillar proteins such as myosin and actin are soluble in concentrated salt solutions (14). The connective tissue proteins, collagen and elastin, are insoluble in both solvents. 

The myofibrillar proteins make up 50-60% of the total protein of muscle cells. Insoluble at low ionic strengths, these proteins dissolve when the ionic strength exceeds -0.3 and can be extracted with salt solutions. Analysis of isolated mammalian myofibrils86 shows that nine proteins account for 96% or more of the protein myosin, which constitutes the bulk of the thick filaments, accounts for 43% and actin, the principal component of the thin filaments, 22%. 

Actomyosin. At high salt concentrations ( . . 0.6 M KC1), actin and myosin combine to form actomyosin filaments giving a highly viscous solution. Actomyosin retains the ATPase activity of myosin and demonstrates "super-precipitation" on the addition of ATP (24,34). As expected, there are differences between actomyosins of rabbit and fish with respect to solubility (10,22,35,36), viscosity (46) and ultracentrifugal behavior (477. Since actomyosin is the most readily available form of myofibrillar proteins from fish muscle, its behavior relative to deterioration during frozen storage has been most frequently studied. 

Among the above hypotheses, effects of lipids (4-17,59-62, 69-71,155-159), formaldehyde (160-166), and gas-solid interface TMJ appear to be very important in Gadoid fishes. Denaturation of myofibrillar proteins caused by free fatty acids and/or lipid peroxides must occur during frozen storage. To prove this, Jarenback and Liljemark have shown by electron microscopy that, in muscle stored frozen with added linoleic and linolenic hydroperoxides, myosin became resistant to extraction with salt solution (168). 

The major regulatory proteins located on an actin filament are troponin and tropomyosin, each occupying 5% of the total myofibrillar proteins. Both proteins confer calcium sensitivity on the ATP-actin-myosin interactions (see Section II). There are minor regulatory proteins that modify the fine structures of myosin and actin filaments and also of Z lines. 

Before summarizing the reactions generally occurring in proteins irradiated at low temperatures, it is instructive to review some of the major observations that have been made for certain representative proteins. Those selected for illustration are myoglobin, ribonuclease (RNase), the myofibrillar proteins myosin and actomysin, and gelatin. Wherever possible, comparisons will be made between results for fluid and frozen systems. 

Effects of linoleic acid and linoleic acid hydroperoxides on the myofibrils and the solutions of myofibrillar proteins of cod muscle have been proved using the electron microscopy (80). Linoleic acid hydroperoxides were ten times more effective than linoleic acid in reducing the amount of the protein in KCl-extracts from the myofibrils incubated with the acid or its hydroperoxides. Linoleic acid seemed to prevent the dissolution of the myofibril frame work but appeared not to impair the extraction of myosin while hydroperoxides appeared to cause a retention of A-bands (myosin) in the myofibrils. 

Myosin phosphatases are prepared from both the soluble (cytosolic) and the myofibrillar fraction of smooth muscle. If the source of phosphatase is the soluble fraction, then chromatographic procedures are usually preceded by fractionation with ammonium sulfate. Myosin phosphatase from myofibrils, crude actomyosin, or myosin is solubilized at high ionic strength by 0.6 NaCl plus detergent (Alessi et al.,... 

A "myofibrillar" form of MLCP distinct from those just described was isolated from turkey gizzards by Nowak et al. (1993). This consisted of a monomeric 35-kDa component that bound to unphosphorylated myosin. It was suggested that in vivo this protein is complexed with MLCK and a 63-kDa subunit. The relationship of this MLCP to the trimeric phosphatases is not known. In contrast, several other groups found that PPlc did not bind to unphosphorylated myosin (Alessi et al., 1992 Mitsui et al., 1992 Okubo et al.,... 

Protein ( histidine ) Methyltransferase. An enzyme which methylates histidine in proteins to give primarily 3-methylhistidine residues has been observed in myofibrillar protein and in the sarcoplasmic fraction of muscle homogenates (218). S-Adenosyl-L-methionine serves as the methyl donor for the enzyme. The enzyme has not been solubilized and purified. Very little is known about the substrate specificity of protein-(histidine) methyltransferase. Actins from a wide variety of species consistently contain one 3-N-methylhistidine residue per molecule (191, 219). It appears that myosin from white muscle contains two residues of 3-N-methylhistidine (one residue per heavy chain), whereas myosin from red muscle contains no 3-N-methylhistidine (220). The amino acid sequence around the methylated residue of rabbit skeletal muscle is (221) ... 

The myofibril is comprised of relatively few proteins compared with the 200 or more different proteins that exist in a muscle cell. Table V lists the known myofibril proteins of vertebrate skeletal muscle and the approximate amount of each protein in the myofibril. Note that myosin and actin comprise up to 75% of the myofibrillar proteins and together are necessary and sufficient for in vitro contraction. Contraction of... 

Sotelo, C.G., Pineiro, C., Perez-Martin, R.I., and Gallardo, J.M. 2000. Analysis of fish and squid myofibrillar proteins by capillary sodium dodecyl sulfate gel electrophoresis Actin and myosin quantification. Ear. Food Res. Technol. 211, 443-448. 

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