Reaction Mechanism of MHETase, a PET Degrading Enzyme

In 2016, one of the two enzymes involved in the polyethylene terephthalate (PET) degradation pathway of Ideonella sakaiensis 201-F6, MHETase, was found to exhibit a strong ability to degrade the PET monomer mono-(2-hydroxyethyl)­terephthalate (MHET) at room temperature, converting it back into the precursors used in PET production. MHETase engineering to improve efficiency is an active field that suffers from an incomplete characterization of its reaction mechanism. In this paper, we analyze the reaction mechanism of MHETase using umbrella sampling molecular dynamics simulations at the B3LYP/MM level of theory. The combination of a high theoretical level and extensive sampling generated a very robust computational prediction. We found that MHETase catalyzed the conversion of MHET in two steps, with a rate-limiting step activation barrier of ΔG^⧧ = 19.35 ± 0.15 kcal·mol^–1 (from the weighted-histogram analysis). Our calculations are in line with the hypothesis that a transient tetrahedral intermediate mediates the reaction mechanism in each step, which is quite common in the serine hydrolase class. The energy of the first tetrahedral intermediate was similar to that of the reactant state, while the tetrahedral intermediate of the deacylation step was observed to lie closer to the rate-limiting transition state. Nevertheless, both determined tetrahedral states were found to be transient, with activation barriers close to ∼2.0 kcal·mol^–1 relative to the product state of the acylation and deacylation steps, corresponding to a half-life of about 3 ps at 303.15 K.