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| Description: (a) When cytosolic Ca2+ is low, the cross-bridge myosin-binding sites on actin are blocked by tropomyosin. (b) When cytosolic Ca2+ increases, Ca2+ binds to troponin, which in turn allows tropomyosin to move away from the myosin-binding sites on actin. Tropomyosin is a rod-shaped molecule composed of two intertwined proteins, with a length equal to that of about seven actin molecules. Chains of tropomyosin molecules are arranged end to end along the actin thin filament. In the absence of calcium ions, they partially cover the myosin-binding site on each actin molecule, thereby preventing cross-bridges from making contact with actin. Each tropomyosin molecule is held in this blocking position by troponin, a smaller, globular-shaped protein that is bound to both tropomyosin and actin. One molecule of troponin binds to each molecule of tropomyosin. In this way, troponin and tropomyosin cooperate to block access to myosin-binding sites on actin molecules in the relaxed muscle fibre. Troponin is capable of binding calcium ions. The binding of Ca2+ produces a change in the shape of troponin, whichâ€â€Âthrough troponin's linkage to tropomyosinâ€â€Âallows tropomyosin to move away from the myosin-binding site on each actin molecule. This permits cross-bridge cycling to occur. Conversely, removal of Ca2+ from troponin reverses the process, turning off contractile activity. Thus, the concentration of cytosolic Ca2+ determines the number of ions bound to troponin molecules, which in turn determines the number of actin sites available for cross-bridge binding. The cytosolic concentration of Ca2+, however, is very low in resting muscle. How is the Ca2+ concentration increased so that contraction can occur? This topic is discussed next. Picture Stats: Views: 10371 Filesize: 45.83kB Height: 295 Width: 850 Source: https://biology-forums.com/index.php?action=gallery;sa=view;id=1189 |