Melanin, A Fungal Photosensitizer for Cellulose Oxidizing AA9-LPMO Enzymes

Melanin is a class of hetero-polymer pigments commonly found in nature and widely in fungi. Often referred to as the "animal lignin", melanin is a very abundant bioresource and features many catalytically interesting properties. We conceived that, upon light absorbance, the polymer could promote long-distance electron donation to fuel redox enzymatic catalysis or controlled in-situ generation of H2O2. Here, we report on a fungal photo-biocatalytic system extracted from the commercially relevant A. nidulans, where photoactivated melanin acts as an electron donor for the cellulose-degrading AnAA9A and TtAA9E metalloenzymes. Furthermore, there was a stable and significant accumulation of H2O2 when melanin was irradiated by visible light; having the peroxide functioning as a co-substrate for the AA9 LPMO enzymes. Oxidized cellulose-derived oligosaccharides were detected in the dark and under light conditions, confirming the potential of melanin to reduce AA9s. When placed under light conditions, they provided hydrogen peroxide as a co-substrate for AA9s. The use of light to tune the in-situ generation of H2O2 by natural pigments might be pivotal to enable also another peroxide-dependent enzymatic catalysis. Photosensitation of fungal melanin provides photo-excited electrons and H2O2-cosubstrate to fungal AA9-LPMO enzymes. An example of natural photobiocatalytic system with component sorted all from the same organism.+image