Amino myosin

Fibrous proteins are long-chain polymers that are used as structural materials. Most contain specific repetitive amino acid sequences and fall into one of three groups coiled-coil a helices as in keratin and myosin triple helices as in collagen and p sheets as in silk and amyloid fibrils. 

Approximately 500 of the 820 amino acid residues of the myosin head are highly conserved between various species. One conserved region, located approximately at residues 170 to 214, constitutes part of the ATP-binding site. Whereas many ATP-binding proteins and enzymes employ a /3-sheet-a-helix-/3-sheet motif, this region of myosin forms a related a-f3-a structure, beginning with an Arg at (approximately) residue 192. The /3-sheet in this region of all myosins includes the amino acid sequence... 

McLachlan, A., 1984. Structural implications of die myosin amino acid sequence. Annual Review of Biophysics and Bioengineering 13 167—189. 

Figure 1. An unrooted phylogenetic tree of the myosins based on the amino acid sequence comparison of their head domains demonstrating the division of the myosin superfamily into nine classes. The lengths of the branches are proportional to the percent of amino acid sequence divergence and a calibration bar for 5% sequence divergence is shovk n. The different classes of myosins have been numbered using Roman numerals in rough order of their discovery and hypothetical models of the different myosin structures are shown. Question marks indicate either hypothetical or unknown structural features, and only a fraction of the known myosins are shown. (Taken, in modified form, from Cheney et al., 1993).

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. 

Some work has been completed on reaction of proteins with nitrite followed by hydrolysis and analysis for amino acids It has been shown that 3-nitrotyrosine and 3,4-dihydroxyphenylalanine are formed from bovine serum albumin when nitrosation occurs under conditions similar to those found in the human stomach (36), Direct demonstration that nitrite reacts with protein has been made by using NaN02 with bovine serum albumin (pH 5.5, 20 C and 200 ppm nitrite). A 60% loss of the originally added nitrite was observed in one week and nearly half of the nitrite (labelled %) could be recovered from the protein. Similar work with myosin revealed that 10-20% of the incorporated label was present as 3-nitrotyro-sine (J7). 

The fifth was a molecular biologist, who smiled sweetly and pointed out that all the others had missed the point. The frog jumps because of the biochemical properties of its muscles. The muscles are largely composed of two interdigitated filamentous proteins, actin and myosin, and they contract because the protein filaments slide past each other. This property of the actin and myosin is dependent on the amino acid composition of the two proteins, and hence on chemical, and thus on physical properties. In the last analysis, the molecular biologist insisted, following James Watson, we are all nothing but subatomic particles. 

GFP hopo ICBP IP3 Ln3+ mal memal MLCK nota oxine par pdta pmea py quin-2 green fluorescent protein hydroxypyridinon(at)e intestinal calcium-binding protein inositol 1,4,5-triphosphate a lanthanide(III) cation malonate methylmalonate myosin light chain kinase 1,4,7-triazacyclononane-l,4,7-triacetate 8- hydroxyquinoline pyridine-2-azo-4 -dimethylaniline propylene-1,2-diaminetetraacetate 9- [2-(phosphonomethoxy)ethyl] adenine pjrridine pjrridyl 8-amino-2- [(2-amino-5-methylphenoxy )methyl] -6-methoxyquinoline-ATJV -tetraacetate 2- [ [2-[his(carboxymethyl)amino]-5-methyl-phenoxy] methyl] -6-methoxy-8- [bis(carboxymethyl) amino] quinoline]... 

The long tail of myosin contains a high proportion of the amino acids leucine, isoleucine, aspartate and glutamate. These are released upon the degradation of myosin by intracellular proteases and peptidases and they provide nitrogen for the synthesis of glutamine. It is then stored in muscle and is a very important fuel for immune cells (Chapter 17). 

Modified amino acids, which occur in special proteins such as hydroxyproline in collagen and 3-methylhistidine in actin and myosin, can be used as indicators of the degradation of these proteins. 

Myosin is quantitatively the most important protein in the myofibrils, representing 65% of the total. It is shaped like a golf club (bottom right). The molecule is a hexamer consisting of two identical heavy chains (2 X 223 kDa) and four light chains (each about 20 kDa). Each of the two heavy chains has a globular head at its amino end, which extends into a tail about 150 nm long in which the two chains are intertwined to form a superhelix. The small subunits are attached in the head area. Myosin is present as a bundle of several hundred stacked molecules in the form of a thick myosin filament. The head portion of the molecule acts as an ATPase, the activity of which is modulated by the small subunits. 

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