Abstract
Computer simulation has uncovered the geometrical conditions under which the vertebrate striated muscle sarcomere can contract. First, all thick filaments should have identical structure, namely: three myosin cross-bridges, building a crown, should be aligned at angles of 0°, 120°, 180°, and the successive crowns and the two filament halves should be turned around 120°. Second, all thick filaments should act simultaneously. Third, coordination in action of the myosin cross-bridges should exist, namely: the three cross-bridges of a crown should act simultaneously and the cross-bridge crowns axially 43 and 14.333 nm apart should act, respectively, simultaneously and with a phase shift. Fifth, six thin filaments surrounding the thick filament should be turned around 180° to each other in each sarcomere half. Sixth, thin filaments should be oppositely oriented in relation to the sarcomere middle. Finally, the structure of each of the thin filaments should change in consequence of strong interaction with myosin heads, namely: the axial distance and the angular alignment between neighboring actin monomers should be, respectively, 2.867 nm and 168° instead of 2.75 nm and 166.15°. These conditions ensure the stereo-specific interaction between actin and myosin and good agreement with the data gathered by electron microscopy and X-ray diffraction methods. The results suggest that the force is generated not only by the myosin cross-bridges but also by the thin filaments; the former acts by cyclical unwrapping and wrapping the thick filament backbone, and the latter byelongation.
Highlights
It is well established that the vertebrate striated muscle contracts due to shortening of its quasi-cells, sarcomeres [1,2,3]
The actual spectra [34,35,36,43,44,45,46,47,48,49,50,51,52] reveal the following specificities: (1) the distribution of reflections along separate layer-lines distanced by 1/14.333 nm−1; (2) the appearance of prominent reflections along the meridian; (3) the splitting of the M3 reflection; (4) the intensity increasing of the meridional reflection at the fifteen layer line
Second radical difference is applied to the myosin cross-bridge action; each cross-bridge moves from the thick filament surface towards three of six surrounding thin filaments along a helical trajectory, not in an oar-like manner
Summary
It is well established that the vertebrate striated muscle contracts due to shortening of its quasi-cells, sarcomeres [1,2,3]. Sarcomere contraction is coupled with mutual sliding of the two kinds of filaments, thick (myosin-based) and thin (actin-based). The myofilaments occur as individual rods, but during contraction, they are connected by myosin cross-bridges [8,9,10]. Tension generated during contraction is proportional to the sarcomere length [11], as well as to the number of the cross-bridges [12]. Actin-myosin interaction is a prerequisite of muscle contraction [13,14,15]. The dimensions of each binding-site are much smaller than the dimensions of either actin monomer or myosin head
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