Enzyme catalysis has shown its great power in dealing
with global
poly(ethylene terephthalate) (PET) waste. However, it is still challenging
to design a super enzyme that can treat the sheer volume of worldwide
PET waste. Without a complete understanding of the catalytic mechanism,
it will be difficult to reach this important goal. Here, we systematically
study the PET depolymerization mechanism catalyzed by structurally
different hydrolases. The role of fleeting chiral intermediates was
proved to be crucial. We observed different prochiral selectivities
among these PET hydrolases. While most hydrolases favor Si-face binding, a few hydrolases (e.g., Humicola insolens cutinase) mainly adapt Re-face binding. Interestingly,
we found that Si-face binding leads to higher activity
than Re-face binding in all of the studied hydrolases.
This Si-face selectivity originates from the difficulty
of proton transfer from catalytic histidine residue to the substrate
and the less stability of the oxyanion hole. Since the Si-face binding ratio ranges from 0 to 95%, we infer that all these
hydrolases are not perfectly evolved to degrade PET. Our in silico
results highlight that enlarging binding site residues (e.g., Leu66
and Asn69) will enhance enzymatic depolymerization, which was further
confirmed by our in vitro experiments where both Leu66Phe and Asn69Phe
show significantly increased PET hydrolysis activity. Hopefully, this
work will aid the future rational design of super enzymes to fight
PET pollution.