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The elucidation of entropic contribution to enzyme catalysis has been debatedover decades. The recent experimentally measured activation enthalpy andentropy, for chorismate rearrangement reaction in PchB brings up a hotlydebated issue whether the chorismate mutase catalyzed reaction isentropy-driven reaction. Extensive configurational sampling combined withquantum mechanics/molecular mechanics molecular dynamics (QM/MM MD) provides anapproach to calculate entropic contribution in condensed phase reactions.Complete reaction pathway is exploited by QM/MM MD simulations at DFT andSCC-DFTB levels. The overall entropy change calculated at SCC-DFTB level QM/MMMD simulations, is close agreement with the experimental value. Conformationanalysis indicates that the self-ordering of chorismate in the active site ofPchB also contributes to total entropy change. This entropy penalty includingconformational transformation entropy and activation entropy cannot beintuitively speculated from the crystal structure that only acts as astationary state along the reaction pathway of PchB catalyzed reaction. This isthe first time to use QM/MM MD simulations to calculate the activation entropyfrom the temperature dependence of reliable free energy profiles with extensivesimulation time. The reasonable insight in enthalpy/entropy scheme clarifiesthe detailed entropy change and provides a quantitative tool to thecontradicted experimental results.

The Activation Entropy Change in Enzymatic Reaction Catalyzed by Isochorismate-Pyruvate Lyase of Pseudomonas Aeruginosa PchB