Today’s society is based on synthetic polymers; practically every area of our daily life is permeated with polymers. Unfortunately, the first generations of polymeric materials surrounding us today have been designed for durability rather than recyclability and/or degradability. The very common poly vinyl products such as poly ethylene, poly propylene and poly styrene comprise hydrocarbon backbones devoid of any functionality prone to spontaneous or microbial cleavage (degradation). As a consequence, these polymers can persist in the environment for a very long time, leading to accumulation and environmental pollution.
Poly esters and poly amides principally contain functional groups in their backbone and therefore are principally prone to hydrolase-catalysed cleavage via hydrolysis. Early findings reported poly ester breakdown by hydrolases. However, the catalytic active sites of these enzymes are generally deeply buried within the enzymes and therefore hardly accessible for the polymeric substrates. The recent discovery of so-called PETases has fundamentally changed this situation as PETases bear surface-exposed active sites and therefore can readily interact with the substrate.  Not astonishingly, PETases are currently receiving tremendous interest from academic research and industrial R&D.
Another important class of polymers are epoxy-derived polymers. Here, the polymerisation reaction is achieved by reaction di-oxiranes with diols or diamines resulting in poly glycidyl amines or poly glycidyl ethers (Figure 1). Also the glycidyl ether/amine motif is not attackable by any currently known microbial enzyme rendering epoxy polymers amongst the recalcitrant polymers.One possibility to solve this issue would be to (enzymatically) insert additional functional groups enabling further breakdown. For this, a-hydroxylation appears a promising strategy. Within BIZENTE we propose peroxygenase enzymes (E.C. 1.11.2) to catalyse this transformation by using simple H2O2 as stoichiometric oxidant (Figure 2).
Trials with low molecular weight model compounds (aromatic ethers and/or secondary amines) using the prototypical peroxygenase from Agrocybe aegerita (AaeUPO, PaDa-I) indeed confirmed the principal feasibility of this approach (Figure 3). The desired p-nitro aniline was indeed formed suggesting that the proposed a-hydroxylation strategy (Figure 2) indeed is practical. However, also the undesired ring-hydroxylation side-reaction was observed. Furthermore, so far all trials converting real polymeric substrates using PaDa-I have so far failed. The latter observation can readily be explained by the fact that the active heme group of PaDa-I is deeply buried within the protein shell. The next steps will comprise engineering PaDa-I to overcome these limitations.
By Frank Hollmann, Delft University of Technology
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