Oxidoreductive Cellulose Depolymerization by the Enzymes Cellobiose Dehydrogenase and Glycoside Hydrolase 61

Abstract

Several members of the glycoside hydrolase 61 (GH61) family of proteins have recently been shown to dramatically increase the breakdown of lignocellulosic biomass by microbial hydrolytic cellulases. However, purified GH61 proteins have neither demonstrable direct hydrolase activity on various polysaccharide or lignacious components of biomass nor an apparent hydrolase active site. Cellobiose dehydrogenase (CDH) is a secreted flavocytochrome produced by many cellulose-degrading fungi with no well-understood biological function. Here we demonstrate that the binary combination of Thermoascus aurantiacus GH61A (TaGH61A) and Humicola insolens CDH (HiCDH) cleaves cellulose into soluble, oxidized oligosaccharides. TaGH61A-HiCDH activity on cellulose is shown to be nonredundant with the activities of canonical endocellulase and exocellulase enzymes in microcrystalline cellulose cleavage, and while the combination of TaGH61A and HiCDH cleaves highly crystalline bacterial cellulose, it does not cleave soluble cellodextrins. GH61 and CDH proteins are coexpressed and secreted by the thermophilic ascomycete Thielavia terrestris in response to environmental cellulose, and the combined activities of T. terrestris GH61 and T. terrestris CDH are shown to synergize with T. terrestris cellulose hydrolases in the breakdown of cellulose. The action of GH61 and CDH on cellulose may constitute an important, but overlooked, biological oxidoreductive system that functions in microbial lignocellulose degradation and has applications in industrial biomass utilization.

A survey of genes encoding H2O2-producing GMC oxidoreductases in 10 Polyporales genomes

“…However, recent studies showed that CDH increases the cellulolysis yield and contributes to the action of lytic polysaccharide monooxygenase (Langston et al 2011).…”

Fungal Traits That Drive Ecosystem Dynamics on Land

“…Moreover, ␤-glucosidases hydrolyze cellobiose to glucose. In addition, the more recently described lytic polysaccharide monooxygenase (i.e., the auxiliary redox enzyme AA9) (59) can degrade relatively recalcitrant forms of cellulose, such as cellulose that is highly crystalline (60) or cross-linked with lignin or other cell wall constituents (61).…”

WITHDRAWN: Identification of an endogenous redox partner for lytic polysaccharide monooxygenase-based oxidative cleavage of polysaccharides

“…These copper-dependent LPMOs utilize presence of reductant molecules for the activation and thereby mediating oxidative degradation of glycosidic bond (12)(13)(14). Studies done on plausible reductant molecules led to the identification of interacting partners, such as small molecule reductants (5,6), lignin derived redox mediator, oxidoreductases or possibly a cosecreted dehydrogenase (4,13,15), photosynthetic pigments (16) etc, which might drive LPMO mediated cellulose degradation. Subsequent studies have also shown that a combination of AA9 protein and a co-secretory CDH (cellobiose dehydrogenase) could cleave pure cellulose (17).…”