6533b85bfe1ef96bd12bbdcd

RESEARCH PRODUCT

Proteins of Muscle and the Cytoskeleton

subject

description

The contractile elements of striated vertebrate skeletal muscle, the myofibrils, contain thin filaments, which are 6 nm in diameter and consist mainly of actin, and thicker myosin filaments with a diameter of 16 nm (Fig. 10.1). During muscle contraction, the filaments undergo a sliding movement relative to each other (sliding filament mechanism). This is brought about by the reversible formation of bridges between the myosin molecules and the actin filaments, which bind, change their conformation and then dissociate (bridge cycle). The required energy is supplied by the hydrolysis of ATP. The sliding distance (step size) per molecule of ATP hydrolysed is controversial; the most recent measurements give a value of 40 nm [102, 286]. As yet, the molecular processes of bridge formation and change in conformation have not been fully defined [79, 257, 267]. Because one ATP is hydrolysed per bridge cycle, the ATPase activity can be used as an in vitro indication of contraction. Isolated myosin has a very low ATPase activity, but this increases up to 1000-fold after combination with actin to give actomyosin. ATPase is inhibited in relaxed muscle; the relief of ATPase inhibition and the triggering of contraction in activated muscle involves an increase in the sarcoplasmic Ca2+ concentration from about 0.1 μmol/1 at rest to about 10 μmol/l on activation. The calcium-dependent regulation of contraction is brought about by tropomyosin and the troponin complex, which are an integral part of the thin filaments [238].