Mechanism of Triphosphate Hydrolysis in Aqueous Solution:  QM/MM Simulations in Water Clusters

The mechanism of the hydrolysis reaction of the unprotonated methyl triphosphate (MTP) ester in water clusters has been modeled. The effective fragment potential based quantum mechanical-molecular mechanical (QM/MM) approach has been applied in the simulations. It is shown that the minimum energy reaction path is consistent with an assumption of a two-step dissociative-type process similar to the case of the guanosine triphosphate (GTP) hydrolysis in the Ras-GAP protein complex (Grigorenko, B. L.; Nemukhin, A. V.; Topol, I. A.; Cachau, R. E.; Burt, S. K. Proteins: Struct., Funct., Bioinf. 2005, 60, 495). At the first stage, a unified action of environmental molecular groups and the catalytic water molecule leads to a substantial spatial separation of the gamma-phosphate group from the rest of the molecule. At the second stage, inorganic phosphate H2PO4- is formed from water and the metaphosphate anion PO3- through the chain of proton transfers along hydrogen bonds. The estimated activation barriers for MTP in aqueous solution at both stages (20 and 14 kcal/mol) are substantially higher than the corresponding barriers for the GTP hydrolysis in the protein.