Comparison of formation of reactive conformers (NACs) for the Claisen rearrangement of chorismate to prephenate in water and in the E. coli mutase: the efficiency of the enzyme catalysis

The Claisen rearrangements of chorismate (CHOR) in water and at the active site of E. coli chorismate mutase (EcCM) have been compared. From a total of 33 ns molecular dynamics simulation of chorismate in water solvent, seven diaxial conformers I-VII were identified. Most of the time (approximately 99%), the side chain carboxylate of the chorismate is positioned away from the ring due to the electrostatic repulsion from the carboxylate in the ring. Proximity of the two carboxylates, as seen in conformer I, is a requirement for the formation of a near attack conformer (NAC) that can proceed to the transition state (TS). In the EcCM.CHOR complex, the two carboxylates of CHOR are tightly held by Arg28 of one subunit and Arg11* of the other subunit, resulting in the side chain C16 being positioned adjacent to C5 with their motions restricted by van der Waals contacts with methyl groups of Val35 and Ile81. With the definition of NAC as the C5...C16 distance < or =3.7 A and the attack angle < or =30 degrees, it was estimated from our MD trajectories that the free energy of NAC formation is approximately 8.4 kcal/mol above the total ground state in water, whereas in the enzyme it is only 0.6 kcal/mol above the average of the Michaelis complex EcCM.CHOR. The experimentally measured difference in the activation free energies of the water and enzymatic reactions (Delta Delta G(++)) is 9 kcal/mol. It follows that the efficiency of formation of NAC (7.8 kcal/mol) at the active site provides approximately 90% of the kinetic advantage of the enzymatic reaction as compared to the water reaction. Comparison of the EcCM.TSA (transition state analogue) and EcCM.NAC simulations suggests that the experimentally measured 100 fold tighter binding of TSA compared to CHOR does not originate from the difference between NAC and the TS binding affinities, but might be due to the free energy cost to bring the two carboxylates of CHOR together to interact with Arg28 and Arg11* at the active site. The two carboxylates of TSA are fixed by a bicyclic structure. The remaining approximately 10% of Delta Delta G(++) may be attributed to a preferential interaction of Lys39-NH(3)(+) with O13 ether oxygen in the TS.