tropomiosina

Tropomyosin is a protein found in muscle cells that plays a crucial role in muscle contraction. It is a coiled-coil dimer composed of two alpha-helical protein chains wound around each other. Tropomyosin molecules are arranged head-to-tail along the actin filaments, which are one of the two main protein components of the muscle sarcomere.

In its resting state, tropomyosin covers the myosin-binding sites on the actin filament. This prevents the myosin motor proteins from interacting with actin, thus inhibiting muscle contraction. When a muscle receives a signal to contract, calcium ions are released into the muscle cell. These calcium ions bind to troponin, a complex of proteins that is associated with tropomyosin. This binding causes a conformational change in troponin, which in turn moves tropomyosin away from the myosin-binding sites on actin.

Once the myosin-binding sites are exposed, myosin heads can bind to actin, initiating the cross-bridge cycle. This cycle involves the binding and unbinding of myosin heads to actin filaments, powered by the hydrolysis of ATP. This process generates the force that causes the muscle to contract. Tropomyosin's position is therefore critical in regulating the interaction between actin and myosin, acting as a molecular switch for muscle contraction. Different isoforms of tropomyosin exist, some of which are found in non-muscle cells and have roles in cell structure and motility.