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

Lytic polysaccharide monooxygenases (LPMOs) are Cu-containing 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-binding peptides that mimic the active site of LPMOs. CD, ATR-FTIR and EPR spectroscopic studies of the peptides and their corresponding copper complexes show that the sequences fold in a β-hairpin conformation and produce complexes with a single Cu ion bound in an LPMO-like environment, confirmed by computational studies. Activity assays were conducted with p-nitrophenyl-β-D-glucopyranoside (PNPG) and demonstrate that the Cu-complexes show LPMO-like activity on the model substrate. Furthermore, the Cu-hairpins can also 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.