(1) The results of the analysis of Zn(II) activation and inactivation of E. coli glutamine synthetase suggest that in addition to binding at the structural site, Zn(II) binds tightly to the catalytic site and supports the activity of the enzyme. The enzyme also possesses a Zn(II) inhibitory site in each subunit and the Zn(II) inhibition process is highly cooperative. The affinity of Zn(II) binding to the catalytic site and inhibitory site varies as a function of the fractional saturation of the second Gln binding on each subunit. (2) An ESR study of a spin-labeled Tempo-ATP adenylylated glutamine synthetase revealed that the covalently bound Tempo-AMP exhibits a similar rotational correlation time as that of free Tempo-ATP, which indicates that the adenylylation site is located on the surface of the protein. Using the paramagnetic effect of Mn(II) on the ESR signal of the spin-labeled Tempo-enzyme, we showed that the distances between the adenylyl site and the two metal ion sites are changed due to substrate binding and formation of a reaction intermediate. (3) Study of actomyosin ATPase mechanism revealed that a "six-state" kinetic model is requried to explain the actomyosin ATPase reaction.