Contractile Mechanisms & Energy Sources (S1)
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What is the mechanism of contraction within the smooth muscle?
• Increase in cytosolic Ca2+ • Ca2+ + Calmodulin • Activation of MLCK • Phosphorylation of MLC • Myosin ATPase activates • Attachment of myosin head with actin • Contraction by sliding filament mechanism
How is contraction regulated in smooth muscle?
By the thick filament (myosin) - regulated by Ca2+ via calmodulin and MLCK
How is contraction regulated in skeletal and cardiac muscle?
By thin filaments (actin) - regulated by Ca2+ via troponin complex in skeletal and cardiac muscle
What is the structure of myosin?
Myosin II • 2 myosin heavy chains (MHC) • 4 myosin light chains (MLC)
What is the structure of actin in muscle?
Filamentous actin polymer (of globular G-actin) plus troponin/tropomyosin
What happens when ATP binds to the myosin head?
Causes detatchment of the myosin head from the actin filament
What causes the myosin head to tilt back?
Hydrolysis of ATP to ADP and Pi which makes the myosin head tilt back
What happens when the phosphate dissociates from the myosin head?
The myosin head re-attaches to a different actin molecule
What happens to the filaments when ADP is released?
A power stroke occurs, the tilting of myosin head to rigor formation, causes actin filament to also slide along the myosin = contraction!
What is the calcium regulatory protein that sits on the tropomyosin string (attached to actin)?
Troponin
What happens when calcium binds to the troponin subcomplex?
Structural shape is changed & tropomyosin no longer inhibits the actin filament from binding with the myosin head.
To which troponin subunit does calcium bind to (TnI, TnT, TnC)?
Troponin C