David B. Wilson

11.1 Introduction

An important step in the global carbon cycle is the degradation of cellulose, the most abun­dant form of fixed carbon with 1011 tons produced by plants each year (1). Most terrestrial cellulose is degraded by cellulolytic microorganisms, primarily fungi and bacteria, although some cellulose is recycled by fire and by photodegradation (2). Aerobic microorganisms are responsible for much of the cellulose degradation in soils, but there also are many species of cellulolytic anaerobic soil bacteria such as Clostridium thermocellum, C. cellulovorans, and Acetivibrio cellulosolvens (3-5). Termites and some other insects are very important in cellu­lose degradation, especially in tropical regions, and most cellulose degrading insects contain symbiotic cellulolytic microorganisms, even though many termites and other insects pro­duce cellulases (6, 7). Some very cellulolytic termites utilize aerobic symbiotic cellulolytic fungi to breakdown plant material in their nests and then eat the fungi and residual plant material (8). Ruminants, such as cows, sheep, and deer, also are important cellulose de­grading organisms but all of the cellulose that they utilize is degraded by symbiotic rumen microorganisms, primarily bacteria. The rumen is an extremely anaerobic environment and this area has been reviewed recently (9, 10), so that these organisms will not be discussed further here.

Microorganisms catalyze cellulose degradation by producing enzymes called cellulases, which hydrolyze the (3 -1-4 linkages present in cellulose. Almost all cellulytic microorganisms secrete their cellulases outside their cell wall, as bacteria and fungi are unable to transport insoluble materials, like cellulose, inside the cell. The soluble sugars produced by cellulase digestion of cellulose are transported inside the cell and metabolized. Native cellulose is very resistant to hydrolysis because it is insoluble and contains crystalline regions in which the adjacent cellulose molecules have strong interactions, such as hydrogen bonds and hydrophobic stacking. Thus, the specific activities of individual cellulases are much lower than those of most enzymes. However, in terms of catalytic enhancement, cellulases are very active enzymes, as the half-life of crystalline cellulose in water at neutral pH is estimated to be about 100 million years. It takes concentrated sulfuric acid at 125°C to hydrolyze native cellulose at a reasonable rate. When cellulases are assayed on low molecular weight soluble substrates, they show normal Michaelis-Menten kinetics and some have high specific activities, showing that they are basically similar to other enzymes. However, when cellulases are assayed on insoluble substrates, they have very different properties, as the assays are

Biomass Recalcitrance: Deconstructing the Plant Cell Wall for Bioenergy. Edited by Michael. E. Himmel © 2008 Blackwell Publishing Ltd. ISBN: 978-1-405-16360-6

usually nonlinear with time and with the amount of enzyme. Different studies have produced different explanations for this behavior. For the endocellulase Thermobifidafusca Cel6A, the nonlinearity was shown to be due to substrate heterogeneity (11).