Myosin/paramyosin

The secondary structures can also unite the supersecondary structures or secondary structure aggregates. Two a helices are enmeshed with each other via the side chains in what are called double a helices they wind about each other with a period of about 18 nm. Such double a helices are found, for example, with a-keratin, myosin, paramyosin, and tropomyosin. The pleated-sheet-helix structure is another supersecondary structure it consists of three pleated-sheet strands and two a helices. Such supersecondary structures occur with phosphorylases, phosphoglycerate-kinases, and some dehydrogenases. 

In a previous section we mentioned the significance of myosin filament structure. In nematodes two forms of myosin-II, myosin A and B, are required for proper filament stmcture (Epstein, 1988). The two forms of myosin are expressed at the proper time to allow for correct filament assembly. An accessory protein called paramyosin is also required for correct filament assembly. In vertebrate cardiac muscle, there are also two isoforms of myosin-II a-myosin and p-myosin. The proper ratio of these two proteins is of utmost importance for proper muscle activity. The incorrect synthesis of a- and P-myosins results in a severe cardiac disorder known as hypertrophic cardiomyopathy. Genetic transmission of the disease occurs in about 55% of families. The inherited condition is called familial hypertrophic cardiomyopathy (FHC), and this condition is a leading cause of sudden death in young athletes. 

Cohen, C., and Parry, D. A. D. (1998). A conserved C-terminal assembly region in paramyosin and myosin rods./. Struct. Biol. 122, 180-187. 

Paramyosin, a major structural component of thick filaments. It occurs exclusively in invertebrate organisms, where it is widely distributed. Paramyosin is found in varying quantities in different muscles types ranging from smooth to cross-striated muscles. It interacts with the core proteins within the thick myofilaments as well as with the surrounding myosin components, thus stabilizing the thick myofilaments. It is not a component of the cytoskeleton [L. Winkelman, Comp. Biochem. Physiol. B 1976, 55, 391 P. R. Deitiker, H. F. Epstein, J. Cell Biol. 1993, 123, 303]. 

The amino acid composition shows relatively high amounts of Arg (12%) and Lys (9%) and little Pro. The paramyosin molecule consists of two peptide chains (M ) 95,000-125,000), each of which is 120 nm long, has a helical structure and is twisted to a rod. In fact, two disulfide bonds contribute to the stability of the molecule. It forms the core in the thick filaments and is surrounded by myosin. In the production of gels, it influences the rheological properties and is the reason why gels made from mollusk meat are more elastic and more cohesive than gels made from fish protein. 

Suiimi is a concentrate of insoluble muscle proteins (ca. 20%). It forms a solid cohesive gel with water (ca. 80%), which solidifies when warm For production, lean fish meat is ground at 5-10 °C and extracted with water until basically only myosin, actin, actomyosin and small amounts of collagen remain. The addition of paramyosin (cf. 13.1.4.2.2) intensifies the structure of the gel. In the further processing of Surimi to Kamboko, starch (ca. 5%), egg white, flavor enhancers, colorants and aroma substances are added, whereby an attempt is made to imitate crab or mussel meat. The resulting mixture is solidified by denaturation of the proteins first at 40-50 °C and then at 80-90 °C. Fibrous structures are produced by extrusion. 

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