The intrinsic chemical reaction of adenosine triphosphate (ATP) hydrolysis catalyzed by myosin is modeled by using a combined quantum mechanics and molecular mechanics (QM/MM) methodology that achieves a near ab initio representation of the entire model. Starting with coordinates derived from the heavy atoms of the crystal structure (Protein Data Bank ID code 1VOM) in which myosin is bound to the ATP analog ADP⅐VO 4 ؊ , a minimum-energy path is found for the transformation ATP ؉ H 2O 3 ADP ؉ Pi that is characterized by two distinct events: (i) a low activation-energy cleavage of the P ␥OO␤␥ bond and separation of the ␥-phosphate from ADP and (ii) the formation of the inorganic phosphate as a consequence of proton transfers mediated by two water molecules and assisted by the Glu-459 -Arg-238 salt bridge of the protein. The minimum-energy model of the enzyme-substrate complex features a stable hydrogen-bonding network in which the lytic water is positioned favorably for a nucleophilic attack of the ATP ␥-phosphate and for the transfer of a proton to stably bound second water. In addition, the P␥OO␤␥ bond has become significantly longer than in the unbound state of the ATP and thus is predisposed to cleavage. The modeled transformation is viewed as the part of the overall hydrolysis reaction occurring in the closed enzyme pocket after ATP is bound tightly to myosin and before conformational changes preceding release of inorganic phosphate.ATP hydrolysis ͉ enzymatic catalysis ͉ energy profile ͉ quantum mechanics and molecular mechanics simulations T he mechanism of hydrolysis of adenosine triphosphate (ATP) by myosin, leading to adenosine diphosphate (ADP) and inorganic phosphate (P i ), which constitutes one of the most important enzymatic reactions responsible for energy transduction into the directed movements of adjoining actin filaments, continues to remain a subject of active debates (1-16), a significant part of which relates to what constitutes the acceptor of the proton that must be released by the ''lytic'' water in its nucleophilic attack on the ATP ␥-phosphate.In terms of the generally accepted kinetic scheme (1-3), the relevant ATP-myosin transformations may be described by the equationin which M* and M** indicate conformers of myosin. As reported (1-3), reaction (Eq. 1) occurs with a near unit equilibrium constant K Ͻ 10 and the estimated rate constants k ϩ Ն 160 s Ϫ1 and k Ϫ Ն 18 s Ϫ1 . The rate constant k ϩ ϭ 160 s Ϫ1 can be converted to the free-energy activation barrier ⌬G # Ϸ 14.6 kcal/mol at room temperature, T ϭ 300 K, by applying a simple transition-state theory formula (17) k Ϸ 6⅐10 12 exp͓Ϫ⌬G # ͞RT͔. [2]However, noting that the experimental rate constants of reaction (Eq. 1) incorporate contributions from conformational changes in the protein from M* to M** leads us to expect that the activation energy of the intrinsic chemical reactionwhich excludes conformational rearrangements, should be considerably Ͻ14.6 kcal/mol. However, previous attempts (13, 15, 16) to simulate the mechanism of react...

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