The kinetics of weakly- and strongly-binding crossbridges: implications for contraction and relaxation

The properties of the weakly-binding crossbridge formed when MgATP binds to the active site in the absence of Ca2+ were studied and contrasted with the behavior of the strongly-binding crossbridge formed when MgADP binds to the active site. It was found that salt causes a large decrease in the attachment rate-constants of the weakly-binding state and a small decrease in the detachment ones. This results in a decrease in the number of weakly-attached crossbridges with increasing salt. The effect is about 15-fold when K Propionate is used to increase ionic strength from mu = 20 mM to mu = 160 mM. Frog skeletal fibers show fewer weakly binding crossbridges than the rabbit. Taken as a whole, the data suggest a model in which the major role of troponin-tropomyosin, at both normal and low ionic strength, is to prevent the weakly-binding crossbridges of the relaxed fiber from going on through the crossbridge cycle to more strongly-binding states. Upon activation by Ca2+, the weakly-binding crossbridges go on through the cycle to a series of more strongly-binding intermediates, leading to an increase in the number of attached crossbridges and the appearance of force.