Practice question

Smooth muscle contraction lacks troponin based thin filament system found in skeletal muscle, so regulatory control migrates to the myosin itself and its energy metabolism. In resting smooth muscle myosin II molecule folded into inactive 10S conformation with low actin activated ATPase activity and sequestered heads. Elevation of cytosolic calcium from 100 nM to near 1 micromolar via L type calcium channel influx and IP3 mediated sarcoplasmic release promotes formation of calcium calmodulin complexes. These activate myosin light chain kinase which phosphorylates serine 19 of the 20 kDa regulatory light chain on each head. Phosphorylation unfolds myosin to extended 6S form and dramatically enhances cross bridge cycling. Each cycle still relies on ATP hydrolysis: ATP binding dissociates rigor actomyosin, hydrolysis to ADP Pi cocks lever arm into pre power stroke state, Pi release drives force generating swing, ADP release resets. Thus ATP turnover supplies mechanical energy while phosphorylation gates whether turnover can occur, integrating calcium signaling with energy metabolism for tonic force maintenance and latch state characteristic of visceral smooth muscle physiology.