ATPase activity is present in
Ans. a (Myosin head); (Ref: Ganong, Physiology, 21st ed., 84; Harper 27th ed. Table 48-3.)* FEATURES OF MYOSIN HEAD# Two heavy and four light chains# Cross-bridges and hinges# ATPase activity (It is greater in type II/fast-twitch fibres than in type I/slow-twitch fibres).* Tropomyosin lies on top of active site of actin so that attraction cannot occur between actin & myosin at rest.*Troponin C has strong affinity for Ca*+ and it initiates contraction. SKELETAL MUSCLECARDIAC MUSCLESMOOTH MUSCLE1. Striated.1. Striated.1. Nonstriated.2. No syncytium.2. Syncytial.2. Syncytial.3. Small T tubules.3. Large T tubules.3. Generally rudimentary T tubules.4. Sarcoplasmic reticulum well developed and Ca2+ pump acts rapidly.4. Sarcoplasmic reticulum present and Ca2+ pump acts relatively rapidly.4. Sarcoplasmic reticulum often rudimentary and Ca2+ pump acts slowly.5. Plasmalemma lacks many hormone receptors.5. Plasmalemma contains a variety of receptors.5. Plasmalemma contains a variety of receptors.6. Nerve impulse initiates contraction.6. Has intrinsic rhythmicity.6. Contraction initiated by nerve impulses, hormones, etc.7. Extracellular fluid Ca2+ not important for contraction.7. Extracellular fluid Ca2+ important for contraction.7. Extracellular fluid Ca2+ important for contraction.8. Troponin system present.8. Troponin system present.8. Lacks troponin system; uses regulatory head of myosin.9. Caldesmon not involved.9. Caldesmon not involved.9. Caldesmon is important regulatory protein.10.Very rapid cycling of the cross-bridges.10.Relatively rapid cycling of the cross-bridges.10.Slow cycling of the cross-bridges permits slow, prolonged contraction and less utilization of ATP.
Myosin head
Actin
Troponin
Tropomyosin
General