Actin based regulation skeletal and cardiac muscle

Actin based regulation (skeletal and cardiac muscles) 

Calcium regulates muscle contraction by an allosteric mechanism which is mediated by troponin, tropomyosin and F-actin system. Troponin is composed of three components, troponin I, troponin T and the calcium binding component, troponin C. 

In skeletal muscle in the relaxed state, the sarcoplasm has a high Mg ATP2 -concent rat ion, but the concentration of calcium is below the threshold required for initiation of contraction. The myosin head, under resting conditions is unable to react with actin of the thin filaments because in the absence of calcium the tropomyosin molecule masks the myosin binding site on G-actin monomer or holds it in a conformation that is unreactive, through the action of TN-1 subunit of troponin. One tropomyosin molecule inhibits the myosin binding activity of seven G-actin monomers. 

Relaxation of the muscle is brought about by removal of the ionic calcium from the sarcoplasm. This calcium is transported across the membrane of the sarcoplasmic reticulum, in an energy requiring process. In addition to the calcium pumping ATPase, the sarcoplasmic reticulum also contains a calcium binding protein called calsequestrin (Section 4.3.3). Some of the calcium segregated by the sarcoplasmic reticulum is apparently bound to this protein within the lumen of the sarcoplasmic reticulum. As sequestration of calcium ions into sarcoplasmic reticulum proceeds, more calcium ions dissociate from their binding sites on troponin C, re- 

Smooth muscles are not regulated by the troponin system. Rather it is the phosphorylation of the myosin light chains which appears to be the event which activates smooth muscle contraction. 

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