MyosinATPase

Myosin ATPase is the enzymatic activity of the myosin motor protein that hydrolyzes ATP to provide the energy required for muscle contraction and other cellular movements. In muscle, this ATPase activity is actin-activated, meaning that interaction with actin filaments stimulates hydrolysis of ATP by the myosin head.

Mechanism: During the cross-bridge cycle, myosin binds ATP, which lowers its affinity for actin and causes dissociation from the filament. The bound ATP is then hydrolyzed to ADP and inorganic phosphate in the myosin head, storing energy in a conformational change known as the recovery stroke. Myosin rebinds to actin; release of Pi strengthens the bond and triggers the power stroke, performing work to shorten the muscle. Release of ADP completes the cycle, after which a new ATP molecule initiates another cycle.

Structure and regulation: The ATPase site resides in the motor (head) domain of myosin and is coordinated with the actin-binding site. The activity is regulated by calcium through the thin filament proteins troponin and tropomyosin, which control access of myosin to actin. Myosin light chains modulate the kinetics of the cycle and the mechanical properties of contraction.

Variation: Myosin ATPase activity varies by isoform and tissue. Fast-twitch (type II) myosins typically exhibit higher ATPase activity and faster cycling than slow-twitch (type I) isoforms, contributing to differences in contraction speed and energy use. The basal ATPase rate is low without actin, but actin markedly increases it.

Clinical and research context: Myosin ATPase is a key parameter in characterizing myosin isoforms and muscle performance, and alterations in ATPase activity are relevant in cardiac and skeletal muscle diseases as well as in non-muscle myosins involved in cellular transport.