In smooth muscle, unlike skeletal muscle, cross-bridge cycling is regulated by phosphorylation of myosin light chains. The key enzyme that phosphorylates myosin and initiates contraction is:

• A Myosin light chain kinase (MLCK), activated by calmodulin-Ca2+ complex ✓
• B Troponin C, which binds Ca2+ and removes tropomyosin inhibition
• C Protein kinase A (PKA), activated by cAMP from beta-2 agonists
• D Calsequestrin, which releases Ca2+ from the sarcoplasmic reticulum

Explanation

Smooth muscle lacks troponin. Instead, Ca2+ (entering through L-type channels or released from SR) binds calmodulin, and the Ca2+-calmodulin complex activates myosin light chain kinase (MLCK). MLCK phosphorylates the regulatory myosin light chain (MLC20), enabling myosin cross-bridge formation with actin and initiating contraction. Beta-2 agonists activate adenylyl cyclase → cAMP → PKA, which phosphorylates and inhibits MLCK (and activates MLCP via BKCa channels), causing smooth muscle relaxation — the basis of bronchodilation.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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