Thick filament myosin

The shortening of a sarcomere (Eigure 17.22) involves sliding motions in opposing directions at the two ends of a myosin thick filament. Net sliding motions in a specific direction occur because the thin and thick filaments both have... 

Actin filaments are the thinnest of the cytoskeletal filaments, and therefore also called microfilaments. Polymerized actin monomers form long, thin fibers of about 8 nm in diameter. Along with the above-mentioned function of the cytoskeleton, actin interacts with myosin ( thick ) filaments in skeletal muscle fibers to provide the force of muscular contraction. Actin/Myosin interactions also help produce cytoplasmic streaming in most cells. 

Contractile proteins which form the myofibrils are of two types myosin ( thick filaments each approximately 12 nm in diameter and 1.5 (im long) and actin ( thin filaments 6nm diameter and 1 (Am in length). These two proteins are found not only in muscle cells but widely throughout tissues being part of the cytoskeleton of all cell types. Filamentous actin (F-actin) is a polymer composed of two entwined chains each composed of globular actin (G-actin) monomers. Skeletal muscle F-actin has associated with it two accessory proteins, tropomyosin and troponin complex which are not found in smooth muscle, and which act to regulate the contraction cycle (Figure 7.1). 

Myosin Thick Filaments Slide along Actin Thin Filaments... 

Much is known about the steps in the biochemical reaction of ATP breakdown by myosin and how these relate to the production of force by the crossbridge. However, since it is no longer attached to the myosin thick filament, myosin SI cannot be an adequate model for a strained crossbridge. Thus data from muscle fibers (e.g., the dependence of phosphate affinity on strain) must also be considered. In this review we attempt to summarize the currently known structural data on myosin and produce a synthesis of this with the biochemical data. We start with an analysis of the polymorphism of the myosin crossbridge and relate this to the crossbridge cycle proposed by Lymn and Taylor (1971). 

Fig. 3. Schematic diagram showing the interaction of the myosin thick filaments and the actin thin filaments during skeletal muscle contraction.

Fig. 4. Mechanism for the generation of force in muscle as an S1 head of a myosin thick filament interacts with an actin thin filament.

The less dense region of the A band, also known as the H zone (not shown in Fig. 5-29b), is the region in which the myosin thick filaments do not overlap the actin thin filaments. When the sarcomere contracts (see Fig. 5-29c), the H zone and the I band decrease in width. 

B. The myosin thick filament expands, while the actin thin filament contracts to achieve maximal force of contraction. 

Actin (thin) filaments and myosin (thick) filaments myosin heads form bridges during contraction filaments are stable in sarcomeres and stereocilia, but often labile in cytoplasm. 

In skeletal muscle cells, actin thin filaments and myosin thick filaments are organized Into highly ordered structures, called sarcomeres (see Figure 19-22). The (+) end of the thin filaments Is attached to the Z disk, the demarcation between adjacent sarcomeres. 

Myosin Thick filament 2MyHC > (200 kDa)-l-2MyLCl/3 (20 kDa)-F2MyLC2 (20 kDa) total 500 kDa 5 fast, > 3 slow and 2 smooth muscle isoforms > 30 genes mainly gene duplication, some alternative splicing Contraction... 

FIGURE 22.3 Contractile apparatus in cells, (a) A sarcomere is ahighly organized contractile unit in muscle cells that uses actin thin filaments and myosin thick filaments to shorten the length of a muscle, (b) A stress fiber is a loosely organized contractile structure in nonmuscle cells that is essential for migration and contraction. 

Movement myosin Thick filaments in muscle fiber... 

Davis, J. S. (1993). Myosin thick filaments and subunit exchange a stochastic simulation based on the kinetics of assembly. Biochemistry, 32,4035-42. 

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