Jg. Head et al., CHARACTERIZATION OF THE EQUILIBRIUM BETWEEN BLOCKED AND CLOSED STATESOF MUSCLE THIN-FILAMENTS, European journal of biochemistry, 227(3), 1995, pp. 694-699
We recently proposed a three-state model for the regulation of actomyo
sin interaction by tropomyosin and troponin (Tm . Tn). In this model,
the thin filament exists in rapid equilibrium between the following th
ree states: blocked, which cannot bind myosin significantly; closed, w
hich can bind myosin weakly to form the A-state; open, which can bind
both to form the A-state and isomerize to the strongly bound R-state.
In this study, we demonstrate that the equilibrium between the blocked
and closed states is calcium sensitive with an equilibrium constant,
K-B, of 0.3 and greater than or equal to 10 at pCa 8.9 and 4.6, respec
tively. The pCa dependence of K-B is typical of that for calcium bindi
ng to thin filaments with a mid-point at pCa 5.6 and a Hill coefficien
t of 1.8. K-B is independent of ionic strength over the range 0.4-0.06
hi but increases dramatically below 0.05 M to greater than or equal t
o 10 at 0.01 M suggesting loss of the blocked state at low ionic stren
gth. The blocked state also has reduced occupancy at high temperatures
. K-B in the absence of calcium, increases from 0.2 at 5 degrees C to
0.6 at 40 degrees C.