Ca2+ and Mg2+ binding site engineering increases the degradation of polyethylene terephthalate films by polyester hydrolases from Thermobifida fusca

Several bacterial polyester hydrolases are able to hydrolyze the synthetic polyester polyethylene terephthalate (PET). For an efficient enzymatic degradation of PET, reaction temperatures close to the glass transition temperature of the polymer need to be applied. The esterases TfH, BTA2, Tfu_0882, TfCut1, and TfCut2 produced by the thermophilic actinomycete Thermobifida fusca exhibit PET‐hydrolyzing activity. However, these enzymes are not sufficiently stable in this temperature range for an efficient degradation of post‐consumer PET materials. The addition of Ca2+ or Mg2+ cations to the enzymes resulted in an increase of their melting points between 10.8 and 14.1°C determined by circular dichroism spectroscopy. The thermostability of the polyester hydrolases was sufficient to degrade semi‐crystalline PET films at 65°C in the presence of 10 mM Ca2+ and 10 mM Mg2+ resulting in weight losses of up to 12.9% after a reaction time of 48 h. The residues Asp174, Asp204, and Glu253 were identified by molecular dynamics simulations as potential binding residues for the two cations in TfCut2. This was confirmed by their substitution with arginine, resulting in a higher thermal stability of the corresponding enzyme variants. The generated variants of TfCut2 represent stabilized catalysts suitable for PET hydrolysis reactions performed in the absence of Ca2+ or Mg2+. Efficient post‐consumer recycling of polyethylene terephthalate (PET) requires enzymes that are stable at the appropriate reaction temperature. Calcium was found to stabilize the structure of the polyester hydrolase TfCut2 from Thermobifida fusca and to provide sufficient thermal stability for the degradation of PET films at a reaction temperature of 65°C. These findings could contribute to the development of environmentally compatible post‐consumer PET recycling processes.