Troponin and Tropomyosin

Troponin and tropomyosin contraction   in response to Ca²⁺ mediate the regulation of muscle. These 2 proteins are present in the thin filament, alongside the actin, and constitute regarding a third of its mass. Tropomyosin which is an elongated protein  of  70  kDa  that  forms  a  two-stranded  α-helical rod  which  lies  closely parallel  to the  long  axis  of the  thin  filament.  Troponin  is a complicated  of three polypeptide  chains: TnC (18 kDa) which binds Ca²⁺, TnI (24 kDa) which binds to actin & TnT (37 kDa) which binds to tropomyosin.  by a nerve impulse, On muscle stimulation ,Ca²⁺ ions are released  from the sarcoplasmic  reticulum  (a specialized  form of the ER found in muscle cells) into the cytosol, growing  the  cytosolic  Ca²⁺ concentration  from  the  resting  concentration  of less than 1 μM to approximate 10 μM. The Ca²⁺ binds to sites on TnC which causing a conformational change in this polypeptide which is transmitted through the other components of the troponin complex to the tropomyosin. Then the tropomyosin moves out of the way, permitting the S1 head of myosin to interact with the actin and initiate a cycle of contraction.  Therefore, by an allosteric mechanism Ca²⁺ controls muscle contraction involving troponin,  actin , tropomyosin and myosin.