Myosin head cooperativity

The two heads of smooth muscle myosin interact cooperatively. Studies with the soluble two-headed HMM fragment clearly demonstrate that, whereas the phosphorylation of the two heads by myosin light chain kinase was random, phosphorylation of both heads was required for the MgATPase activity of either head to be activated by actin (Sellers et al., 1983 Per-sechini and Hartshorne, 1981). This cooperative activation of the two heads, coupled with the random phosphorylation, meant that at 50% phosphorylation (i.e., 1 mol Pj/mol HMM) there was only 25% of the... 

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