Tropomüosiin

Tropomyosin is a protein found in muscle cells and other eukaryotic cells. In muscle, it plays a crucial role in the regulation of muscle contraction by binding to actin filaments. Tropomyosin is a dimer composed of two alpha-helical chains that wind around each other to form a coiled-coil structure. This structure allows it to associate with the actin filament in a linear fashion.

In the context of muscle contraction, tropomyosin is part of a larger regulatory complex. When a muscle is at rest, tropomyosin covers the myosin-binding sites on the actin filament, preventing the interaction between actin and myosin, which is necessary for contraction. Upon stimulation by a nerve impulse, calcium ions are released within the muscle cell. These calcium ions bind to troponin, another protein associated with tropomyosin and actin. This binding causes a conformational change in troponin, which in turn shifts the position of tropomyosin on the actin filament. This shift uncovers the myosin-binding sites, allowing myosin to bind to actin and initiate the sliding filament mechanism that leads to muscle contraction. Tropomyosin's position is therefore essential for switching between the "on" and "off" states of muscle contraction. Beyond muscle, tropomyosin isoforms are found in non-muscle cells where they contribute to cytoskeletal organization and cell motility.