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The Marletta Laboratory

Cellulose Degradation

People working on this project:
Van Vu, PhD

Cellulose is the most abundant biopolymer on earth and holds an important biological role in maintaining the structural rigidity of plant cell walls. Due to its abundance, cellulosic biomass holds great promise as a feedstock for second generation biofuels and as a part of the Energy Biosciences Institute (EBI) we are working as part of an interdisciplinary team to make that vision of turning plant biomass into fuels a reality.

Currently the bottleneck to generation of such second generation fuels lies in the relatively expensive and slow enzymatic degradation of cellulose to glucose monomers. The extensive hydrogen bonding network within and between chains means that cellulose is an insoluble, heterogenous substrate and hence cellulose active enzymes are very different than enzymes that catalyze reactions on soluble substrates. To add to this complexity most fungi contain dozens of enzymes that are active on cellulose and the reason for such redundancy is largely a mystery. Currently the best mechanistic models of cellulose degradation rely on the activity of 3 types of enzymes: endocellulases that hydrolyze glycosidic bonds within a chain, excocellulases (or cellobiohydrolases) that hydrolyze from the ends of chains, and beta-glucosidases that cleave cellobiose into glucose monomers. We are trying to build on this fundamental understanding of cellulose degradation and understand the roles for other types of carbohydrate active enzymes.

Cellulose Degradation research image

As a model organism we are studying Neurospora crassa, a filamentous fungi and an efficient degrader of plant biomass. Filamentous fungi like Neurospora are responsible for the vast majority of enzymatic degradation of cellulose in nature and they are also the most promising candidates for development of enzymes to generate second generation biofuels. Taking advantage of whole-genome microarrays, RNA-seq technology, and a publicly available whole genome gene deletion set we have identified a number of previously uncharacterized enzymes likely to be involved in cellulose degradation. This search had led us to a family of copper-dependent hydroxylases (previously annotated as GH61 proteins) that are involved in catalyzing a monooxygenase reaction that breaks internal linkages within the cellulose backbone. We are continuing to purify and characterize these enzymes to gain a fundamental understanding of how individual enzymes carry out their respective catalytic functions, and more importantly, how these enzymes work together to efficiently degrade plant cell walls.

Cellulose Degradation research image

Publications

Beeson WT, Phillips CM, Cate JH, Marletta MA. Oxidative cleavage of cellulose by fungal copper-dependent polysaccharide monooxygenases. J. Am. Chem. Soc. 2012, 134: 890-2.

Phillips CM, Beeson WT, Cate JH, Marletta MA. Cellobiose dehydrogenase and a copper-dependent polysaccharide monooxygenase potentiate cellulose degradation by Neurospora crassa. ACS Chem Biol. 2011 6(12),1399-406.

Phillips CM, Iavarone AT, Marletta MA. A quantitative proteomic approach to cellulose degradation by Neurospora crassa. J Proteome Res. 2011, 10: 4177-85.

Beeson WT 4th, Iavarone AT, Hausmann CD, Cate JH, Marletta MA. Extracellular aldonolactonase from Myceliophthora thermophila.Appl Environ Microbiol. 2010, 77:650-6.

Sun J*, Phillips CM*, Anderson CT, Beeson WT, Marletta MA, Glass NL. Expression and characterization of the Neurospora crassa endoglucanase GH5-1. Prot Expr Purif. 2011, 75:147-54.

Tian C*, Beeson WT*, Iavarone AT, Sun J, Marletta MA, Cate JH, Glass NL. Systems analysis of plant cell wall degradation by the filamentous fungi Neurospora crassa. Proc Natl Acad Sci USA. 2009, 106, 22157-62.