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Tropomyosin is a two-stranded alpha-helical, coiled coil protein found in many animal and fungal cells. In animals, it is an important component of the muscular system which works in conjunction with troponin to regulate muscle contraction.
In a relaxed muscle, tropomyosin blocks the attachment site for the myosin crossbridge, thus preventing contraction. When the muscle cell is stimulated to contract by an action potential , calcium channels open in the sarcoplasmic membrane and release calcium into the sarcoplasm.
Tropomyosin alpha-1 chain is a protein that in humans is encoded by the TPM1 gene. [5] This gene is a member of the tropomyosin (Tm) family of highly conserved, widely distributed actin-binding proteins involved in the contractile system of striated and smooth muscles and the cytoskeleton of non-muscle cells.
Depiction of smooth muscle contraction. Muscle contraction is the activation of tension-generating sites within muscle cells. [1] [2] In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as when holding something heavy in the same position. [1]
β-Tropomyosin, also known as tropomyosin beta chain is a protein that in humans is encoded by the TPM2 gene. [5] [6] β-tropomyosin is striated muscle-specific coiled coil dimer that functions to stabilize actin filaments and regulate muscle contraction.
Tropomyosin alpha-3 chain is a protein that in humans is encoded by the TPM3 gene. [ 5 ] [ 6 ] This gene encodes a member of the tropomyosin family of actin-binding proteins involved in the contractile system of striated and smooth muscles and the cytoskeleton of non-muscle cells.
When the muscle fibre is relaxed (before contraction), the myosin head has ADP and phosphate bound to it. When a nerve impulse arrives, Ca 2+ ions cause troponin to change shape; this moves the troponin + tropomyosin complex away, leaving the myosin binding sites open. The myosin head now binds to the actin myofilament.
During muscle contraction, tropomyosin shifts to expose the myosin-binding site on an actin filament, allowing the interaction between actin and myosin microfilaments to occur. The initiation of contraction involves calcium ions binding to troponin, prompting a reaction that displaces tropomyosin from the actin filament binding sites.