Regulation of Actomyosin ATPase
Muscle cells have an extremely efficient regulation system which reduces actomyosin ATPase activity by more than one thousand fold in relaxed versus active muscle. A surprising fact is that two quite different regulation systems are used in striated and smooth muscles. The actomyosin ATPase of smooth muscles as well as non muscle cells is activated by a calmodulin dependent protein kinase which specifically phosphorylates a myosin light chain (LC-2), Actomyosin is inhibited by dephosphorylation by a specific phosphatase.1 Activation of enzymes by phosphorylation is a very common control mechanism and its use in muscle regulation is not surprising. The second mechanism apparently evolved to meet the need for faster switching on and off in striated muscles. Although a phosphorylatable light chain has been retained by striated muscle myosin and the level of phosphorylation can be altered by stimulation, phosphorylation no longer activates the actomyosin ATPase. Regulation is obtained by a structural change of the thin filament. The thin filaments in striated muscle contain troponin which is not present in smooth muscle or non-muscle cells.
KeywordsMyosin Light Chain Thin Filament Kinetic Mechanism Myosin Head Product Dissociation
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