In smooth muscle contraction, calcium binds to calmodulin instead of troponin. What is the subsequent step that directly initiates the phosphorylation of myosin light chains?
- A Activation of myosin light chain kinase ✓
- B Activation of myosin light chain phosphatase
- C Binding of ATP to the myosin head
- D Conformational change in tropomyosin
Correct answer: A. Activation of myosin light chain kinase
Explanation
Calcium binds to calmodulin, forming a Ca2+-calmodulin complex that activates myosin light chain kinase. This kinase then phosphorylates the myosin light chain, enabling myosin ATPase activity and cross-bridge formation. Myosin light chain phosphatase dephosphorylates myosin to cause relaxation. Tropomyosin does not undergo conformational changes in smooth muscle.
Reference: Ganong's Review of Medical Physiology, 26th ed.
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