Question

Contractions in smooth muscle cells depend on which one of the following activation

steps?
A. Ca2+-activated phosphorylation of myosin-II
B. Ca2+ binding to troponin
C. GTP binding to myosin-II
D. Actin polymerization

Answer 1

Smooth muscle contractions are initiated by Ca2+-activated phosphorylation of myosin-II via the activation of calmodulin, which then activates myosin light chain kinase to phosphorylate the myosin heads.

Step-by-step explanation:

Contractions in smooth muscle cells depend on Ca2+-activated phosphorylation of myosin-II. This process differs from the mechanism in striated muscles, where Ca2+ binds to troponin. In smooth muscle, external Ca2+ ions and additional Ca2+ released from the sarcoplasmic reticulum bind to calmodulin. The resulting Ca2+-calmodulin complex activates an enzyme called myosin light chain kinase. This enzyme phosphorylates myosin heads, converting ATP to ADP and phosphate, allowing the myosin heads to attach to actin-binding sites and generate contraction.