Bacterial cellulases and xylanases

Although this review focusses primarily on bacterial cellulases and xylanases, there is considerable overlap in the molecular biology and biochemistry of prokaryotic and fungal forms of these enzymes. Therefore, fungal plant cell wall hydrolases are discussed where comparisons with the corresponding bacterial enzymes are important. Cellulases and xylanases encompass a collection of enzymes whose primary function is to hydrolyse p1,4-glycosidic linkages in the major plant structural polysaccharides, cellulose and xylan. In converting cellulose and xylan to their constituent sugars, these enzymes play an essential role in the digestive processes of herbivores and in the recycling of photosynthetically fixed carbon. The past decade has seen a burgeoning interest in all aspects of the biochemistry and molecular biology of cellulases and xylanases, which at first glance is rather perplexing. As a paradigm for p1,4glycanases, lysozyme is the enzyme of choice; it has been studied in greater detail than cellulases or xylanases and has a clearly understood catalytic mechanism. The recent discoveries from analysis of gene sequences and the three-dimensional structure of cellulase proteins show, however, that there is much to be understood about the enzymic hydrolysis of plant carbohydrate polymers beyond what can be adduced from analogy with lysozyme. To really appreciate the rationale behind current studies of microbial cellulases and xylanases, it is necessary to take a broad view that takes into account not just their intrinsic interest, but also their undoubted commercial potential. The structural polysaccharides cellulose and hemicellulose (xylan being the major