Controlled depolymerization of cellulose by light-driven lytic polysaccharide oxygenases

Lytic polysaccharide (mono)oxygenases (LPMOs) perform oxidative cleavage of polysaccharides, and are key enzymes in biomass processing and the global carbon cycle. It has been shown that LPMO reactions may be driven by light, using photosynthetic pigments or photocatalysts, but the mechanism behind this highly attractive catalytic route remains unknown. Here, prompted by the discovery that LPMOs catalyze a peroxygenase reaction more efficiently than a monooxygenase reaction, we revisit these light-driven systems, using an LPMO from Streptomyces coelicolor (ScAA10C) as model cellulolytic enzyme. By using coupled enzymatic assays, we show that H2O2 is produced and necessary for efficient light-driven activity of ScAA10C. Importantly, this activity is achieved without addition of reducing agents and proportional to the light intensity. Overall, the results highlight the importance of controlling fluxes of reactive oxygen species in LPMO reactions and demonstrate the feasibility of light-driven, tunable enzymatic peroxygenation to degrade recalcitrant polysaccharides

Ribonucleotide reductase class I with different radical generating clusters

International audienceRibonucleotide reductase (RNR) catalyzes the rate limiting step in DNA synthesis where ribonucleotides are reduced to their corresponding deoxyribonucleotides. They are formed through a radical-induced reduction of ribonucleotides. Three classes of RNR generate the catalytically active site thiyl radical using different co-factors: a tyrosyl-radical in most cases (class I), homolytic cleavage of deoxyadenosyl-cobalamin (class II), or a glycyl-radical (class III), respectively. Class I RNR has a larger subunit R1/R1E containing the active site and a smaller subunit R2/R2F with (the thiyl-generating power from) a tyrosyl radical or an oxidized iron-manganese cluster and is reviewed herein. Class I is divided into subclasses, Ia (tyrosyl-radical and di-iron-oxygen cluster), Ib (tyrosyl-radical and di-manganese-oxygen cluster) and Ic (an iron-manganese cluster). Presented here is an overview of recent developments in the understanding of class I RNR: metal-ion cluster identities, novel 3D structures, magnetic-optical properties, and reaction mechanisms. It became clear in the last years that the primitive bacterial RNR sources can utilize different metal-ion clusters to fulfil function. Within class Ia that includes members from eukaryotes (mammalians, fish) and some viruses species, the presence of hydrogen bonding interactions from water at different distances with the tyrosyl-radical site can occur. This demonstrates a large versatility in the mechanism to form the thiyl radical