Khanok Ratanakhanokchai, Rattiya Waeonukul, Patthra Pason, Chakrit Tachaapaikoon, Khin Lay Kyu, Kazuo Sakka, Akihiko Kosugi and Yutaka Mori

Additional information is available at the end of the chapter http://dx. doi. org/10.5772/51820

1. Introduction

To develop a bio-based economy for sustainable economic growth, it is necessary to produce chemicals and fuels from renewable resources, such as plant biomass. Plant biomass contains a complex mixture of polysaccharides, mainly cellulose and hemicellulose (mainly xylan), and other polysaccharides (Aspinall, 1980). The hemicelluloses, as well as the aromatic polymer lignin, interact with the cellulose fibrils, creating a rigid structure strengthening the plant cell wall. Therefore, complete and rapid hydrolysis of these polysaccharides requires not only cellulolytic enzymes but also the cooperation of xylanolytic enzymes (Thomson, 1993). Many microorganisms that produce enzymes capable of degrading cellulose and hemicellulose have been reported and characterized. Two enzyme systems are known for their degradation of lignocellulose by microorganisms. In many aerobic fungi and bacteria, endoglucanase, exoglucanase, and ancillary enzymes are secreted individually and can act synergistically on lignocellulose. The most thoroughly studied enzymes are the glycosyl hydrolases of Trichoderma reesei (Dashtban et al., 2009). On the other hand, several anaerobic cellulolytic microorganisms such as Clostridium thermocellum (Lamed & Bayer, 1988), C. cellulovorans (Doi et al., 2003), C. josui (Kakiuchi et al., 1998) and C. cellulolyticum (Gal et al., 1997) are known to produce a cell-associated, large extracellular polysaccharolytic multicomponent complex called the cellulosome, in which several cellulolytic and xylanolytic enzymes are tightly bound to a scaffolding protein (core protein). Thus, the cellulosome provides for a large variety of enzymes and attractive enzymatic properties for the degradation of recalcitrant plant biomass. So far, anaerobic microorganisms have been identified as producing the multienzyme complex, cellulosome

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(Doi & Kosugi, 2004; Demain et al., 2005). However, when compared with aerobic enzymes, production of those enzymes by anaerobic culture presents a high cost because of the high price of medium, slow rate of growth and low yield of enzyme, while only a little information has been reported on cellulosome-like multienzyme complex produced by aerobic bacteria (Kim & Kim, 1993; Jiang et al,, 2004; van Dyk et al., 2009). Therefore, the multienzyme complexes, cellulosomes, produced by aerobic bacteria show great potential for improving plant biomass degradation. A facultatively anaerobic bacterium, P. curdlanolyticus strain B-6, is unique in that it produces extracellular xylanolytic-cellulolytic multienzyme complex under aerobic conditions (Pason et al., 2006a, 2006b; Waeonukul et al., 2009b). In the following years, the characteristics, function, genetics and mechanism of the xylanolytic-cellulolytic enzymes system of this bacterium has been the subject of considerable research. In light of new findings in this field, this review will describe the state of knowledge about the multienzyme complex of strain B-6 and its potential biotechnological exploitations.