De novo designed β-hairpin peptides mimicking the LPMO copper-binding histidine brace motif

Lytic polysaccharide monooxygenases (LPMOs) are Cu-based enzymes that play a crucial role in lignocellulosic biomass degradation for use in biofuel production. These enzymes carry out the selective oxidation of C-H bonds in the sugar units, leading to the cleavage of the glycosidic bond. Creating LPMO mimics facilitates the study of the mechanism of action, the characterisation of the reactive species responsible for the C-H bond activation and the potential scale up for industrial application. Here we report the synthesis, characterisation and activity assays of two novel Cu-peptides complexes that mimic the LPMO active site. We carried out CD, ATR-FTIR and EPR spectroscopic studies of the peptides and their corresponding copper complexes, showing that the sequences fold in a β-hairpin conformation and produce complexes with a single copper ion bound in an LPMO-like environment, confirmed by computational studies. Activity assays were conducted with p-nitrophenyl-β-D-glucopyranoside (PNPG) and demonstrated that the Cu-complexes can perform LPMO-like activity on the model substrate. Furthermore, the Cu-hairpins were also able to perform light-driven oxidation of phosphoric acid swollen cellulose (PASC) in the presence of melanin, similarly to some LPMO enzymes, an activity that is unreported for any LPMO mimic characterised so far. This work is the first example of a β-hairpin LPMO mimic and paves the way to further exploration of small peptide mimics of this key class of metalloenzymes.