Cellulose is the major structural polysaccharide of the plant cell wall and the most abundant biopolymer on earth.  Research sponsored by the Energy Biosciences Institute is focused toward harnessing the potential of bioenergy, to make discoveries and commercialize realities out of these, which could benefit the world.  In the Marletta lab we are studying enzymes involved in the degradation and depolymerization of plant cell walls – specifically cellulose degradation.

Cellulose is a linear polymer of β(1-4) linked glucose units.  The individual chains vary in length from several hundred to more than 10,000 glucose monomers.  It is highly crystalline and insoluble in water and most organic solvents because of the extensive hydrogen bonding between chains.  The insolubility and crystalline nature of cellulose make it highly recalcitrant to degradation.

Enzymatic degradation of cellulose is a complex, extracellular process.  The complete hydrolysis of cellulose requires the synergistic action of at least three classes of enzymes: endoglucanases, exoglucanases, and beta-glucosidases.  Many enzymes that degrade cellulose are also multi-domain proteins – commonly consisting of cellulose binding domains that are spatially separated from the catalytic domains.

As a class, the glucohydrolyases have been well characterized including mechanism.  However, when these enzymes are assembled in multimeric complexes termed the cellulosome, little is known about substrate recognition and catalysis.  In fact catalysis is quite slow, no doubt reflecting the nature of the insoluble substrate.  Our initial investigations involve the cellulosome from Clostridium papyrosolvens.

In addition, we are collaborating with the Glass Lab in the Department of Plant and Microbial Biology to identify novel enzymes important in plant cell wall degradation by filamentous fungi using an unbiased functional genomics approach.  Novel enzymes identified in the functional genomics studies are being homologously expressed or directly isolated from the native host and characterized biochemically.  Through these studies we hope 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 and provide nutrients for growth of the fungus.