Comparison of Enzymes Catalyzing the Hydrolysis of Insoluble Polysaccharides

Abstract

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^

Improvement of the Cellulolytic Activity of Trichoderma reesei Endoglucanase IV with an Additional Catalytic Domain

“…With the exception of EGIII and BG I, most of the T. reesei cellulases share a modular structure composed of a catalytic module (CM), a linker peptide and a cellulose-binding module (CBM) (Wilson et al 1995;Teeri 1997). The CM catalyses the hydrolysis of the glucoside bonds, while the CBM binds to the cellulose chains.…”

A Simple Individual-based Model of Insoluble Polysaccharide Hydrolysis: the Potential for Autosynergism with Dual-activity Glycosidases

“…This behavior is relevant to predominantly exo-acting enzymes which are randomly anchored to their insoluble substrates, i.e. which can associate with both internal and terminal chain linkages (Wilson et al, 1995). The concept of autosynergism with a supporting model does not appear to be in the glycosidase literature.…”

“…In this case, the enzymes are often comprised of two domains, with the catalytic site located in one domain and a separate substrate binding site located in the other (Wilson et al, 1995). The binding domain serves to anchor the enzyme to the surface of the insoluble substrate.…”

“…the matrix, the enzyme was instead moved to the opposite end of the matrix. Insoluble polysaccharide hydrolysing enzymes often have two domains, a catalytic domain containing the active site and a binding domain which serves to anchor the enzyme to its insoluble substrate (Wilson et al, 1995). It thus seems appropriate that the mobility of the enzymes is modeled in such a way that, on average, enzymes will not move far from starting points, unless processive catalytic activity drives them along a substrate chain.…”

Effects of noncatalytic residue mutations on substrate specificity and ligand binding of Thermobifida fusca endocellulase Cel6A

“…As more three‐dimensional structures of cellulases have been determined by X‐ray crystallography [2], there has been considerable progress in identifying specific residues or regions involved in cellulose binding and catalysis [2–4]. The catalytic domain of endocellulase Cel6A from Thermobifida fusca (Cel6Acd) (formerly E2 from Thermomonospora fusca ) has been crystallized and its three‐dimensional structure solved at 0.18 [5], 0.118 [6] and 0.10 nm resolution [7]. Cel6Acd is a modified α/β barrel with a deep active‐site cleft that contains subsites for binding at least four glucosyl units.…”