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Cellobiose Dehydrogenase and a Copper-Dependent Polysaccharide Monooxygenase Potentiate Cellulose Degradation by Neurospora crassa
journal contribution
posted on 2011-12-16, 00:00 authored by Christopher
M. Phillips, William T. Beeson, Jamie H. Cate, Michael A. MarlettaThe 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.