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.