Year of Publication

2012

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Engineering

Department

Chemical and Materials Engineering

First Advisor

Dr. Barbara Knutson

Abstract

Cellulose from energy crops or agriculture residues can be utilized as a sustainable energy resource to produce biofuels such as ethanol. The process of converting cellulose into solvents and biofuels requires the saccharification of cellulose into soluble, fermentable sugars. However, challenges to cellulosic biofuel production include increasing the activity of cellulose-degrading enzymes (cellulases) and increasing solvent (ethanol) yield while minimizing the co-production of organic acids. This work applies novel surface analysis techniques and fermentation reactor perturbations to quantify, manipulate, and model enzymatic and metabolic processes critical to the efficient production of cellulosic biofuels.

Surface analysis techniques utilizing cellulose thin film as the model substrate are developed to quantify the kinetics of cellulose degradation by cellulase as well as the interactions with cellulase at the interfacial level. Quartz Crystal Microbalance with Dissipation (QCM-D) is utilized to monitor the change in mass of model cellulose thin films cast. The time-dependent frequency response of the QCM simultaneously measures both enzyme adsorption and hydrolysis of the cellulose thin film by fungal cellulases, in which a significant reduction in the extent of hydrolysis can be observed with increasing cellobiose concentrations. A mechanistic enzyme reaction scheme is successfully applied to the QCM frequency response for the first time, describing adsorption/desorption and hydrolysis events of the enzyme, inhibitor, and enzyme/inhibitor complexes. The effect of fungal cellulase concentration on hydrolysis is tested using the QCM frequency response of cellulose thin films. Atomic Force Microscopy (AFM) is also applied for the first time to the whole cell cellulases of the bacterium C. thermocellum, where the effect of temperature on hydrolysis activity is quantified.

Fermentation of soluble sugars to desirable products requires the optimization of product yield and selectivity of the cellulolytic bacterium, Clostridium thermocellum. Metabolic tools to map the phenotype toward desirable solvent production are developed through environmental perturbation. A significant change in product selectivity toward ethanol production is achieved with exogenous hydrogen and the addition of hydrogenase inhibitors (e.g. methyl viologen). These results demonstrate compensatory product formation in which the shift in metabolic activity can be achieved through environmental perturbation without permanent change in the organism’s genome.

Share

COinS