Natural photoredox catalysts promote light-driven lytic polysaccharide monooxygenase reactions and enzymatic turnover of biomass

Abstract

Lytic polysaccharide monooxygenases (LPMOs) catalyze oxidative cleavage of crystalline polysaccharides such as cellulose and chitin and are important for biomass conversion in the biosphere as well as in biorefineries. The target polysaccharides of LPMOs naturally occur in copolymeric structures such as plant cell walls and insect cuticles that are rich in phenolic compounds, which contribute rigidity and stiffness to these materials. Since these phenolics may be photoactive and since LPMO action depends on reducing equivalents, we hypothesized that LPMOs may enable light-driven biomass conversion. Here, we show that redox compounds naturally present in shed insect exoskeletons enable harvesting of light energy to drive LPMO reactions and thus biomass conversion. The primary underlying mechanism is that irradiation of exoskeletons with visible light leads to the generation of H 2 O 2 , which fuels LPMO peroxygenase reactions. Experiments with a cellulose model substrate show that the impact of light depends on both light and exoskeleton dosage and that light-driven LPMO activity is inhibited by a competing H 2 O 2 -consuming enzyme. Degradation experiments with the chitin-rich exoskeletons themselves show that solubilization of chitin by a chitin-active LPMO is promoted by light. The fact that LPMO reactions, and likely reactions catalyzed by other biomass-converting redox enzymes, are fueled by light-driven abiotic reactions in nature provides an enzyme-based explanation for the known impact of visible light on biomass conversion.

Biomass Photoreforming for Hydrogen and Value‐Added Chemicals Co‐Production on Hierarchically Porous Photocatalysts

“…[225][226][227] Lytic polysaccharide monooxygenases (LPMOs) with the copper species as the active sites could be activated by incident light or photocatalysts to degrade recalcitrant cellulose into high-value oligosaccharides and other chemicals. [228][229][230]…”

Polyphenol Oxidase Products Are Priming Agents for LPMO Peroxygenase Activity

“…[34] Worthy of note, light also plays a role in inducing H 2 O 2 generation via photoexcitation of photosensitive phenolic compounds such as lignin and catecholamines. [21,35] During the 3-hour time period that most of the reactions in this work were conducted, there is no sign of photoexcitation of 3-MC or MtPPO7 catalytic products (Figure 2B and D) as no release of oxidized cello-oligosaccharides by NcAA9C is observed. However, it cannot be excluded that minor levels of photoexcitation may have occurred during the 24 hours incubations, since we here see (low) product formation (Supplementary Information Figure S4).…”

“…[17,18] However, in H 2 O 2 -driven reactions, there is a trade-off between fast catalytic rates and H 2 O 2 -induced enzyme inactivation rates. LPMOs are quite promiscuous with regards to electron supply for priming, as they can receive electrons from a large variety of small-molecule reductants (such as ascorbic acid, cysteine, and phenolic compounds), [5,16,[19][20][21] lignin [22] and other redox enzymes (e. g. carbohydrate dehydrogenases). [2][3][4]23] When it comes to soluble phenolic reductants of LPMOs, diphenols (benzenediols) and triphenols (benzenetriols) are better LPMO reductants than monophenols (monohydroxybenzenes) due to Scheme 1.…”

Revisiting the role of electron donors in lytic polysaccharide monooxygenase biochemistry

“…The subsequent activity of the LPMO was greatly enhanced by the presence of ascorbate feeding into a complex reaction in which the likely primary role of the light-absorbing pigment was in generating H 2 O 2 and the initial reduction in the LPMO was triggered by the ascorbate [ 55 ]. Recent work has furthered this view by showing that phenolic compounds present within chitinaceous biomass also produce H 2 O 2 in a light-dependent manner, which may also drive LPMO reactions under such conditions [ 57 ].…”

“…Whichever cosubstrate is utilised by LPMOs, an electron source is required to reduce the active site copper and initiate the oxidative reaction catalysed by these enzymes [7,13]. A diversity of electron donors have been demonstrated as capable of driving the LPMO reaction, be it small-molecule reducing agents [7,10,16,17,[43][44][45][46][47][48][49], other redox enzymes [29,41,[50][51][52][53], or light-absorbing compounds from biotic or abiotic sources [54][55][56][57]. Some of these electron donors generate H 2 O 2 themselves, which has now been implicated in several studies as responsible for what was previously considered the O 2 -dependent activity of LPMOs [33,43,58].…”