The high cost of enzymes for saccharification of lignocellulosic biomass is a major barrier to the production of second generation biofuels. Using a combination of genetic and biochemical techniques, we report that filamentous fungi use oxidative enzymes to cleave glycosidic bonds in cellulose. Deletion of cdh-1, the gene encoding the major cellobiose dehydrogenase of Neurospora crassa, reduced cellulase activity substantially, and addition of purified cellobiose dehydrogenases from M. thermophila to the Δcdh-1 strain resulted in a 1.6- to 2.0-fold stimulation in cellulase activity. Addition of cellobiose dehydrogenase to a mixture of purified cellulases showed no stimulatory effect. We show that cellobiose dehydrogenase enhances cellulose degradation by coupling the oxidation of cellobiose to the reductive activation of copper-dependent polysaccharide monooxygenases (PMOs) that catalyze the insertion of oxygen into C-H bonds adjacent to the glycosidic linkage. Three of these PMOs were characterized and shown to have different regiospecifities resulting in oxidized products modified at either the reducing or nonreducing end of a glucan chain. In contrast to previous models where oxidative enzymes were thought to produce reactive oxygen species that randomly attacked the substrate, the data here support a direct, enzyme-catalyzed oxidation of cellulose. Cellobiose dehydrogenases and proteins related to the polysaccharide monooxygenases described here are found throughout both ascomycete and basidiomycete fungi, suggesting that this model for oxidative cellulose degradation may be widespread throughout the fungal kingdom. When added to mixtures of cellulases, these proteins enhance cellulose saccharification, suggesting that they could be used to reduce the cost of biofuel production.

Specialized oxygen-sensing cells in the nervous system generate rapid behavioural responses to oxygen. We show here that the nematode Caenorhabditis elegans exhibits a strong behavioural preference for 5-12% oxygen, avoiding higher and lower oxygen levels. 3',5'-cyclic guanosine monophosphate (cGMP) is a common second messenger in sensory transduction and is implicated in oxygen sensation. Avoidance of high oxygen levels by C. elegans requires the sensory cGMP-gated channel tax-2/tax-4 and a specific soluble guanylate cyclase homologue, gcy-35. The GCY-35 haem domain binds molecular oxygen, unlike the haem domains of classical nitric-oxide-regulated guanylate cyclases. GCY-35 and TAX-4 mediate oxygen sensation in four sensory neurons that control a naturally polymorphic social feeding behaviour in C. elegans. Social feeding and related behaviours occur only when oxygen exceeds C. elegans' preferred level, and require gcy-35 activity. Our results suggest that GCY-35 is regulated by molecular oxygen, and that social feeding can be a behavioural strategy for responding to hyperoxic environments.