A kinetic model of the co-operative binding of calcium and ADP to scallop (Argopecten irradians) heavy meromyosin

Analysis of the kinetics of ATP and ADP binding to scallop (Argopecten irradians) heavy meromyosin (HMM) showed that the only calcium-dependent process is the rate of ADP release. At physiological ionic strength calcium accelerated ADP release about 20-fold. Notably in the absence of calcium only one ADP bound HMM, with an affinity of 0.5-1 microM. The second nucleotide site remained unoccupied at up to 50 microM ADP yet could bind ATP rapidly. The calcium dependence of ADP-release rates showed that calcium binds co-operatively to scallop HMM with an affinity of 0.78 microM and a Hill coefficient of 1.9. Detailed interpretation of the data suggests that HMM exists in equilibrium between the on and off states and that calcium and ADP modulate the equilibrium between the two states. The on state is favoured in the presence of calcium and in the absence of both calcium and nucleotide. The off state is favoured by ADP (or ADP * P(i)) in the absence of calcium. A detailed co-operative model of the interaction of ADP and calcium with HMM is presented.