The Work of Contraction: Myosin ATPase

Abstract

The fractional cell volume of sarcomeres in the myocyte is very high, ∼0.5 (it is close to 0.8 in some skeletal muscles). Sarcomeres are composed of filaments responsible for force generation and muscle shortening. The thick filaments contain primarily myosin and the thin filaments contain primarily actin and the regulatory proteins tropomyosin and troponin. Sarcomeres also contain other filaments containing large proteins such as titin and nebulin that provide scaffolding and the viscoelastic elements responsible for relaxation. During contraction of a single sarcomere, the I-bands (I for isotropic) containing the thin filaments shorten and move toward the center of the sarcomere while the A-band (A for anisotropic) essentially stays in place. Thus, during a single contraction cycle, the distance between the Z-bands shortens and then returns to normal resting length (∼2.2 nm). The Z-band also undergoes dynamic rearrangement during the contractile cycle (1). The contraction cycle is the story of protein-protein interactions and the use of the free energy of ATP hydrolysis to accomplish both subtle changes in protein conformation and movement of protein assemblies in space.