Ninety percent of lignocellulose-degrading fungi contain genes encoding lytic polysaccharide monooxygenases (LPMOs). These enzymes catalyze the initial oxidative cleavage of recalcitrant polysaccharides after activation by an electron donor. Understanding the source of electrons is fundamental to fungal physiology and will also help with the exploitation of LPMOs for biomass processing. Using genome data and biochemical methods, we characterized and compared different extracellular electron sources for LPMOs: cellobiose dehydrogenase, phenols procured from plant biomass or produced by fungi, and glucose-methanol-choline oxidoreductases that regenerate LPMO-reducing diphenols. Our data demonstrate that all three of these electron transfer systems are functional and that their relative importance during cellulose degradation depends on fungal lifestyle. The availability of extracellular electron donors is required to activate fungal oxidative attack on polysaccharides.

Lytic polysaccharide monooxygenases (LPMOs) are copper-dependent enzymes that catalyze oxidative cleavage of glycosidic bonds using molecular oxygen and an external electron donor. We have used NMR and isothermal titration calorimetry (ITC) to study the interactions of a broad-specificity fungal LPMO, NcLPMO9C, with various substrates and with cellobiose dehydrogenase (CDH), a known natural supplier of electrons. The NMR studies revealed interactions with cellohexaose that center around the copper site. NMR studies with xyloglucans, i.e., branched β-glucans, showed an extended binding surface compared with cellohexaose, whereas ITC experiments showed slightly higher affinity and a different thermodynamic signature of binding. The ITC data also showed that although the copper ion alone hardly contributes to affinity, substrate binding is enhanced for metal-loaded enzymes that are supplied with cyanide, a mimic of O 2 − . Studies with CDH and its isolated heme b cytochrome domain unambiguously showed that the cytochrome domain of CDH interacts with the copper site of the LPMO and that substrate binding precludes interaction with CDH. Apart from providing insights into enzyme-substrate interactions in LPMOs, the present observations shed new light on possible mechanisms for electron supply during LPMO action.

ABSTRACTThe genome ofNeurospora crassaencodes two different cellobiose dehydrogenases (CDHs) with a sequence identity of only 53%. So far, only CDH IIA, which is induced during growth on cellulose and features a C-terminal carbohydrate binding module (CBM), was detected in the secretome ofN. crassaand preliminarily characterized. CDH IIB is not significantly upregulated during growth on cellulosic material and lacks a CBM. Since CDH IIB could not be identified in the secretome, both CDHs were recombinantly produced inPichia pastoris. With the cytochrome domain-dependent one-electron acceptor cytochromec, CDH IIA has a narrower and more acidic pH optimum than CDH IIB. Interestingly, the catalytic efficiencies of both CDHs for carbohydrates are rather similar, but CDH IIA exhibits 4- to 5-times-higher apparent catalytic constants (kcatandKmvalues) than CDH IIB for most tested carbohydrates. A third major difference is the 65-mV-lower redox potential of the hemebcofactor in the cytochrome domain of CDH IIA than CDH IIB. To study the interaction with a member of the glycoside hydrolase 61 family, the copper-dependent polysaccharide monooxygenase GH61-3 (NCU02916) fromN. crassawas expressed inP. pastoris. A pH-dependent electron transfer from both CDHs via their cytochrome domains to GH61-3 was observed. The different properties of CDH IIA and CDH IIB and their effect on interactions with GH61-3 are discussed in regard to the proposedin vivofunction of the CDH/GH61 enzyme system in oxidative cellulose hydrolysis.

The plant-pathogenic fungus Glomerella cingulata (anamorph Colletotrichum gloeosporoides) secretes high levels of an FAD-dependent glucose dehydrogenase (GDH) when grown on tomato juice-supplemented media. To elucidate its molecular and catalytic properties, GDH was produced in submerged culture. The highest volumetric activity was obtained in shaking flasks after 6 days of cultivation (3400 U l "1 , 4.2 % of total extracellular protein). GDH is a monomeric protein with an isoelectric point of 5.6. The molecular masses of the glycoforms ranged from 95 to 135 kDa, but after deglycosylation, a single 68 kDa band was obtained. The absorption spectrum is typical for an FAD-containing enzyme with maxima at 370 and 458 nm and the cofactor is non-covalently bound. The preferred substrates are glucose and xylose. Suitable electron acceptors are quinones, phenoxy radicals, 2,6-dichloroindophenol, ferricyanide and ferrocenium hexafluorophosphate. In contrast, oxygen turnover is very low. The GDH-encoding gene was cloned and phylogenetic analysis of the translated protein reveals its affiliation to the GMC family of oxidoreductases. The proposed function of this quinone and phenoxy radical reducing enzyme is to neutralize the action of plant laccase, phenoloxidase or peroxidase activities, which are increased in infected plants to evade fungal attack.

Cellobiose dehydrogenase (CDH) is an emerging enzyme in the field of bioelectrocatalysis. Due to its flexible cytochrome domain, which acts as a built-in redox mediator, CDH is capable of direct electron transfer (DET) to electrode surfaces. This rare property is employed in mediatorless "third generation" biosensors. The ability of Corynascus thermophilus CDH to oxidize glucose under physiological conditions makes it a promising candidate for miniaturized glucose biosensors or glucose powered biofuel cell anodes. We report for the first time the electrochemical application and characterization of a recombinantly produced CDH in a glucose biosensor. Recombinant CDH from C. thermophilus (rCtCDH) was expressed by the methylotrophic yeast Pichia pastoris (376 U L(-1) , 132 mg L(-1) ). A comparative characterization of rCtCDH and CtCDH shows identical pH optima, K(M) values and heme b midpoint potentials. In contrast, the specific activity of rCtCDH (2.84 U mg(-1) ) and consequently the turnover numbers were ~five-times lower than for CtCDH, which was caused by a sub-stoichiometric occupation of catalytic sites with flavin-adenin-dinukleotid (FAD). The performance of rCtCDH-modified electrodes demonstrates the suitability for electrochemical studies. This opens the possibility to engineer the substrate specificity of C. thermophilus CDH for specific carbohydrates by rational engineering or directed evolution.