In striated muscle, Ca2+ regulates contraction by interacting with troponin and tropomyosin. Explain how Ca2+ is able to regulate the timing of contraction as well as affecting the kinetics of contraction

Question

What will be an ideal response?

Answer 1

Answer: Typically, cytosolic Ca2+ levels are very low. When Ca2+ concentration rises, it can bind to troponin. Troponin is actually composed of three protein subunits: TnC binds to the Ca2+, TnI binds to the actin (inhibiting myosin/actin interactions), and TnT binds to the tropomyosin. It is the tropomyosin that physically blocks the myosin binding site on actin. When Ca2+ binds to TnC, it causes a conformational change so that TnC and TnI become more tightly bound to each other. This in turn decreases the interaction between TnI and actin, so that the whole troponin/tropomyosin complex slides over, exposing the myosin binding sites on actin. As long as Ca2+ is present, cross-bridge formation can occur. When Ca2+ is removed from the cytosol, it is also released from TnC, causing the whole complex to return to its original position. Clearly, the point of control is how quickly and tightly TnC binds to Ca2+, thereby allowing the interactions between troponin, tropomyosin, actin, and myosin to occur. By having different isoforms of all these proteins, organisms can regulate these interactions and their sensitivities to physiological variables (pH, temperature, etc.). Ultimately, this affects the time it takes for contractions to occur.