Practice question

In relaxed skeletal and cardiac muscle actomyosin interaction is prevented not by direct myosin inhibition but by steric blocking of thin filament through tropomyosin troponin complex evolutionarily conserved. Tropomyosin is alpha helical coiled coil dimer about 40 nm long that polymerizes head to tail along major groove of filamentous actin spanning seven actin monomers and forming continuous cable. In low calcium near 100 nM troponin I holds tropomyosin in blocked B state covering myosin binding sites on outer domain of actin. Elevation of calcium to micromolar via ryanodine receptor release allows calcium binding to troponin C N lobe causing structural shift of troponin complex moving tropomyosin azimuthally toward closed C state partially exposing sites then to open M state upon myosin strong binding. Only then can myosin heads attach release Pi and produce force. CapZ caps barbed ends gelsolin and cofilin sever or depolymerize actin but they do not regulate relaxation steric block essential for preventing wasteful ATP consumption at rest.