Cross-bridge cycling phosphorylation rate

The exchange of the covalently bound phosphate of LC20 during contraction (see Section V) supports a relationship between LC20 phosphorylation and cross-bridge cycling rate. 

A model involving cooperativity has been proposed as an alternative to the Hai and Murphy formulation (Vyas et al., 1992, 1994). The models are similar in that in both models cross-bridges exist in the same four basic states. The novelty of this model is a finite attachment rate of unphosphorylated cross-bridges (kg in Fig. 2B), which is dependent on the number of attached, phosphorylated cross-bridges (AMp). In effect, some slowly cycling "latch-bridges" are postulated to form cooperatively as a result of the attach-... 

One consequence of the simulations using the four state model (Hai and Murphy 1988a, Hai and Murphy 1992) was that the dependence of ATPase on force was non-linear and increased steeply at higher force levels, due to increased rates of both cross-bridge cycling and LC20 phosphorylation turnover. This result was found to be at variance with earlier data on ATP... 

When smooth muscle myosin is bound to F-actin in the absence of other muscle proteins such as tropomyosin, there is no detectable ATPase activity. This absence of activity is quite unlike the situation described for striated muscle myosin and F-actin, which has abundant ATPase activity. Smooth muscle myosin contains fight chains that prevent the binding of the myosin head to F-actin they must be phosphorylated before they allow F-actin to activate myosin ATPase. The ATPase activity then attained hydrolyzes ATP about tenfold more slowly than the corresponding activity in skeletal muscle. The phosphate on the myosin fight chains may form a chelate with the Ca bound to the tropomyosin-TpC-actin complex, leading to an increased rate of formation of cross-bridges between the myosin heads and actin. The phosphorylation of fight chains initiates the attachment-detachment contraction cycle of smooth muscle. 

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