Towards Sustainable Synthesis of Polyesters: A QM/MM Study of the Enzymes CalB and AfEST

Plastic pollution is causing an immeasurable damage to marine and land eco-systems. Sustainable alternatives are actively being sought-after, such as biodegradable polyesters obtained by enzymatic synthesis. However, wild type enzymes still pose fundamental efficiency limitations that can be circumvented by protein reengineering approaches.Here we compare in detail the catalytic mechanisms for polycaprolactone synthesis by the enzymes Archaeoglobus fulgidus carboxylesterase (AfEST) and Candida antarctica lipase B (CalB) by performing Quantum mechanics calculations and Quantum Mechanics/Molecular Mechanics Molecular Dynamics simulations. We found that bond forming/breaking events are concerted with proton transfer to or from the catalytic histidine in all the transition states, but with different degrees of coupling between the motions of the atoms involved. Our results give important insights towards the design of new enzyme variants combining good activity with high thermostability.