Myosinhoveder

Myosinhoveder are the catalytic motor domains of myosin molecules that bind actin filaments and adenosine triphosphate (ATP) to generate mechanical force. They are essential for muscle contraction and for many intracellular transport processes in eukaryotic cells.

Structure: Each head contains a motor domain responsible for ATP hydrolysis and actin binding, and a neck or lever arm that transduces chemical energy into movement. The head binds two light chains—the essential light chain and the regulatory light chain—that help stabilize the structure and modulate activity. The motor domain is highly conserved across myosin classes, while the lever arm length and regulatory features vary.

Mechanism: In the cross-bridge cycle, the head binds strongly to actin when ADP and Pi are present; release of Pi triggers the power stroke, moving the actin filament relative to the myosin, followed by ADP release and ATP binding that causes detachment. Hydrolysis of ATP re-cocks the head for the next cycle. Calcium ions regulate this process in muscle through the troponin-tropomyosin complex, enabling myosin heads to interact with actin when muscles are activated.

Roles and diversity: In muscle tissue, myosin II heads form part of thick filaments and execute contraction. In non-muscle cells, various myosin classes with specialized head domains drive vesicle transport, organelle positioning, and cytokinesis. Regulation of head activity occurs via phosphorylation of light chains and other post-translational modifications.

Evolution and significance: The myosin head is a highly conserved motor domain found in most eukaryotes, reflecting its fundamental role in cellular mechanics and motility.