KINETICS AND MECHANISM OF HYDROLYSIS OF ADENOSINE 5'-TRIPHOSPHATE CATALYZED BY ZN2- CONFORMATION AND SELECTIVITY .1.( ION )

The pH dependence of the initial rate of the Zn.ATP (1:1) hydrolysis w as studied at 50 degrees C. The Zn2+OH- moiety was found to be active in the hydrolysis producing ADP and P-i only in cyclic conformation of phosphate chain in a dimer (ZnATP(2-))(2)H+OH- or in a monomer ZnATP( 2-)OH(-). The value of the rate constant for the cleavage of the P(gam ma)-OP(beta) bond by the Zn2+OH- ion anack in the monomeric cyclic for m ZnATP(2)-OH- shows the crucial role of the interaction of Zn2+ with N-7 of the adenine base. The analysis of the data for the dimer sugges ts that N-1 atom of the second ATP molecule coordinates proton and for ms a hydrogen bond with gamma-phosphate of the first ATP molecule. Thi s results in the pH-independent channel for the dimer hydrolysis. The selective formation of ADP+P-i in both forms (monomeric and dimeric) a nd dose values of their hydrolysis rate constants indicate that the AT P conformation in these forms are analogous. At pH greater than or equ al to 8.5 AMP and pyrophosphate are produced in parallel with ADP and P-i from the very start of the reaction, the rate of AMP formation inc reasing as pH increases.