Myosin phosphate turnover rate

C. Rate of Phosphate Turnover on Myosin LC20 during Contraction... 

Other results suggest a lower rate of phosphate turnover on LC20. Kitazawa et al. (1991) measured phosphatase activity in a-toxin-permeabilized portal vein from the rate of dephosphorylation in rigor solution with inhibited myosin light chain kinase (MLCK) activity. Their value was 0.017 s i at 15°C, which, corrected for temperature (Mitsui et al., 1994), is about nine times smaller than the value of Butler et al. Reconciliation of these findings will require additional studies. However, the conditions of the experiments were quite different and it should also be pointed out that in the experiments of Butler et al. (1994), only 76% of the light chain phosphate turned over at the fast rate, al-... 

White et aP measured the rates of phosphate release during a single turnover of actomyosin nucleoside triphosphate (NTP) hydrolysis using a double-mixing stopped-flow spectrofluorometer, at very low ionic strength to increase the affinity of myosin-ATP and myo-sin-ADP-Pi to actin. Myosin subfragment 1 and a series of nucleoside triphosphates were mixed and incubated for approximately 1-10 s to allow NTP to bind to myosin and generate a steady-state mixture of myosin-NTP and myosin-NDP-Pi. The steady-state intermediates were then mixed with actin. 

Coupled to the folding of the myosin molecule is a change at the active site that results in "trapping" of nucleotide (Cross et al., 1986). The observed rate of phosphate release from single-turnover experiments, <0.0005 sec i, likely reflects product release from the small amount of extended myosin that is in equilibrium with the folded monomer (Cross et al., 1988). The structural basis for trapping is not known. It has been proposed that after MgATP is cleaved at the active site, phosphate leaves via a "backdoor" mechanism (Yount et al., 1995). Perhaps in the folded monomer the glycine-rich P-loop is stabilized in a conformation that prevents phosphate release. 

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