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Tropomyosin is a protein that is involved in the contraction of skeletal muscle. It is, in fact, the compound responsible for preventing muscles from contracting when they are at rest. This protein acts as a block during the chemical process that produces muscle contraction by wrapping itself around chains of another protein found in muscle cells known as actin. A third protein called myosin must be able to bind to sites along this actin protein in order for muscles to contract. It is this binding of the two proteins that tropomyosin obstructs.
To understand the role of this protein in preventing muscle contraction, one should first understand the mechanism, known as sliding filament theory, that causes muscles to contract. Within muscle cells, which make up muscle fibers arranged in bundles, actin and myosin are arranged in alternating filaments. Myosin is the motor or movement protein that generates the force behind muscle contraction by sliding back and forth along the actin filaments within a structure inside the muscle cell known the sarcomere — one of thousands — that can expand and contract as a unit. During muscle contraction, the myosin proteins slide past the actin, discharging calcium ions that cause each myosin protein to bind to an adjacent site on the actin filament. When this happens, the myosin pulls itself past its neighboring actin, causing a collective shortening of the sarcomere that produces a contraction of the muscle cell.
If the muscle is in a resting state and no muscle contraction is required, tropomyosin wraps itself around the actin filaments, blocking the binding sites and thereby preventing the myosin from binding to the actin so that no muscle contraction may occur. A single tropomyosin molecule blocks seven binding sites on the actin molecule. It does so with the assistance of a protein complex called troponin, which is actually three proteins, each of which plays a different role in blocking or initiating muscle contraction. One, troponin T, joins with tropomyosin to block the sites of myosin attachment. Another, troponin I, attaches to the actin itself to hold these two in place across the binding sites.
A third type of troponin, troponin C, helps the process of contraction to begin all over again by attaching to calcium ions. It is the release of these calcium ions from channels inside the muscle cell that stimulates contraction. When they are released, they bind to troponin C, which moves the tropomyosin-troponin T out of the way so that the myosin once again has access to the binding sites on actin, and contraction of the sarcomere can begin again.
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