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Amylases, cellulases, chitinases, lysozymes and xylanases are all insoluble polysaccharide hydrolases. Many of these enzymes possess a separate substrate binding domain that is pointed to the catalytic domain by a hinge peptide. All of them contain multiple binding sites for sugar monomers (from four to eight) in their active sites and either two or three carboxyl side chains that function in catalysis. There are three amylase families, nine cellulase families, two chitinase families, five lysozyme families and two xylanase families. The enzymes in some families invert the conformation of the glycoside oxygen during hydrolysis while the enzymes in the other families retain the conformation. The structure of an inverting endocellulase, Thermomonospora fusca E2, has been determined by X-ray crystallography at 1.18 Å resolution. It has an open active site cleft, while CBHII which is an exocellulase with the same polypeptide chain fold has its active site in a tunnel. Several E2 mutants have been isolated and characterized to help determine its catalytic mechanism.Insoluble polysaccharides function either as storage polymers or as the major structural components of bacteria, Crustacea, insects, fungi and plants. Because of their specific functions and the abundance of these polymers, many organisms produce enzymes which can degrade them. Most of these enzymes only hydrolyze one type of polysaccharide but some hydrolyze more than one. This either results from a single active site that can bind polymers with related structures such as cellulose and xylan (1) or chitin and bacterial cell walls (2) or because of the presence of two or more different catalytic domains in one protein (3). A given type of enzyme can play quite different roles in different organisms. Thus plants produce cellulases to modify their cell walls to allow growth (4) or to ripen fruits (5) while plant pathogens produce cellulases to allow entry through plant cell walls (6). Many bacteria and fungi produce cellulases to utilize cellulose as a carbon source and they are essential for recycling plant cell walls in nature. Few animals make cellulases; so that ruminants, shipworms and termites utilize the cellulases produced by symbiotic bacteria and fungi to digest cellulose (7-9). Chitinases are produced by Crustacea, fungi and insects to allow the organisms to (X)97-6156/95W18-0001$12.00^

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