Crystal structure and substrate-binding mode of cellulase 12A from Thermotoga maritima

Cellulases have been used in many applications to treat various carbohydrate-containing materials. Thermotoga maritima cellulase 12A (TmCel12A) belongs to the GH12 family of glycoside hydrolases. It is a β-1,4-endoglucanase that degrades cellulose molecules into smaller fragments, facilitating further utilization of the carbohydrate. Because of its hyperthermophilic nature, the enzyme is especially suitable for industrial applications. Here the crystal structure of TmCel12A was determined by using an active-site mutant E134C and its mercury-containing derivatives. It adopts a β-jellyroll protein fold typical of the GH12-family enzymes, with two curved β-sheets A and B and a central active-site cleft. Structural comparison with other GH12 enzymes shows significant differences, as found in two longer and highly twisted β-strands B8 and B9 and several loops. A unique Loop A3-B3 that contains Arg60 and Tyr61 stabilizes the substrate by hydrogen bonding and stacking, as observed in the complex crystals with cellotetraose and cellobiose. The high-resolution structures allow clear elucidation of the network of interactions between the enzyme and its substrate. The sugar residues bound to the enzyme appear to be more ordered in the -2 and -1 subsites than in the +1, +2 and -3 subsites. In the E134C crystals the bound -1 sugar at the cleavage site consistently show the α-anomeric configuration, implicating an intermediate-like structure.     

X-ray crystallographic study of xylopentaose binding to Pseudomonas fluorescens xylanase A

The structure of the complex between a catalytically compromised family 10 xylanase and a xylopentaose substrate has been determined by X-ray crystallography and refined to 3.2 A resolution. The substrate binds at the C-terminal end of the eightfold betaalpha-barrel of Pseudomonas fluorescens subsp. cellulosa xylanase A and occupies substrate binding subsites -1 to +4. Crystal contacts are shown to prevent the expected mode of binding from subsite -2 to +3, because of steric hindrance to subsite -2. The loss of accessible surface at individual subsites on binding of xylopentaose parallels well previously reported experimental measurements of individual subsites binding energies, decreasing going from subsite +2 to +4. Nine conserved residues contribute to subsite -1, including three tryptophan residues forming an aromatic cage around the xylosyl residue at this subsite. One of these, Trp 313, is the single residue contributing most lost accessible surface to subsite -1, and goes from a highly mobile to a well-defined conformation on binding of the substrate. A comparison of xylanase A with C. fimi CEX around the +1 subsite suggests that a flatter and less polar surface is responsible for the better catalytic properties of CEX on aryl substrates. The view of catalysis that emerges from combining this with previously published work is the following: (1) xylan is recognized and bound by the xylanase as a left-handed threefold helix; (2) the xylosyl residue at subsite -1 is distorted and pulled down toward the catalytic residues, and the glycosidic bond is strained and broken to form the enzyme-substrate covalent intermediate; (3) the intermediate is attacked by an activated water molecule, following the classic retaining glycosyl hydrolase mechanism.     

Components of cellulase from the higher termite,Nasutitermes walkeri

The cellulase from the termite Nasutitermes walkeri consists of two enzymes. Each has broad specificity with predominantly one activity. One enzyme is an endo-gb-1,4-glucanase (EC 3.2.1.4) which predominantly cleaves cellulose randomly to glucose, cellobiose and cellotriose. It hydrolyses cellotetraose to cellobiose but will not hydrolyse cellobiose or cellotriose. The second enzyme component is a β-1,4-glucosidase (EC 3.2.1.21) as its major activity is to hydrolyse cellobiose, cellotriose and cellotetraose to glucose; it has some exoglucosidase activity as glucose is the only product produced from cellulose. Its cellobiase activity is inhibited by glucono-δ-lactone.     

Crystal structure of truncated Fibrobacter succinogenes 1,3-1,4-beta-D-glucanase in complex with beta-1,3-1,4-cellotriose

Fibrobacter succinogenes 1,3-1,4-beta-D-glucanase (Fsbeta-glucanase) catalyzes the specific hydrolysis of beta-1,4 glycosidic bonds adjacent to beta-1,3 linkages in beta-D-glucans or lichenan. This is the first report to elucidate the crystal structure of a truncated Fsbeta-glucanase (TFsbeta-glucanase) in complex with beta-1,3-1,4-cellotriose, a major product of the enzyme reaction. The crystal structures, at a resolution of 2.3 angstroms, reveal that the overall fold of TFsbeta-glucanase remains virtually unchanged upon sugar binding. The enzyme accommodates five glucose residues, forming a concave active cleft. The beta-1,3-1,4-cellotriose with subsites -3 to -1 bound to the active cleft of TFsbeta-glucanase with its reducing end subsite -1 close to the key catalytic residues Glu56 and Glu60. All three subsites of the beta-1,3-1,4-cellotriose adopted a relaxed C(1)4 conformation, with a beta-1,3 glycosidic linkage between subsites -2 and -1, and a beta-1,4 glycosidic linkage between subsites -3 and -2. On the basis of the enzyme-product complex structure observed in this study, a catalytic mechanism and substrate binding conformation of the active site of TFsbeta-glucanase is proposed.     

Analysis of the putative substrate binding region of hyperthermophilic endoglucanase from Pyrococcus horikoshii

A hyperthermophophilic beta-1,4 endoglucanase (family 5, cellulase) was identified in a hyperthermophilic archaeon Pyrococcus horikoshii and found to be capable of hydrolyzing crystalline cellulose at high temperatures. This hyperthermophilic enzyme has promise for applications in biomass utilization, but we have no information regarding the catalytic mechanism or structure of the enzyme. To determine its catalytic mechanism, we examined the roles of amino acids located in a loop near the speculative active site by the alanine scanning method. Ten mutants of the enzyme were constructed and expressed in Escherichia coli. The purified mutant enzymes were assayed for their hydrolytic activities on p-nitrophenyl cellobiose (pNG2), carboxylmethyl cellulose, and avicel. The results showed that His155, Arg156, and Ile162 play an important role in pNG2 binding capacity, and that H155 and I162 are important for catalysis.     

The nature and mode of action of the cellulolytic component C1 of Trichoderma koningii on native cellulose

1. A purified cellulolytic component C(1) was isolated free from associated activities of the cellulase complex and shown to act as a beta-1,4-glucan cellobiohydrolase on both simple and complex forms of native cellulose. 2. The enzyme releases terminal cellobiose units from cellulose, its extent of action being determined principally by the product and by the nature of the substrate. 3. Component C(x) of the cellulase system is not required for the action of component C(1) (cellobiohydrolase). The enzyme synergizes extensively with cellobiase in extending the hydrolysis of native and of less-complex forms of cellulose to at least 70% with the liberation of glucose. 4. The cellobiohydrolase is relatively unstable, with an optimum at pH5 and a K(m) of 0.05mg/ml. The enzyme is inhibited by its product, from which it is released by cellobiase. 5. Of other compounds tested against the cellobiohydrolase the metal ions Cu(2+), Zn(2+), phenylmercuric and Fe(3+) are increasingly effective inhibitors. Glucose has no action at concentrations found inhibitory with cellobiose. 6. The relationship of the enzyme to the entire cellulase complex is discussed.     

Conversion of cellulose to glucose with cellulase of Trichoderma viride ITCC-1433

Trichoderma viride ITCC-1433 secretes a cellulase complex that is rich in β-glucosidase and therefore well suited for the saccharification of cellulosic materials. The cellulase was investigated with respect to optimum conditions of reaction and enzyme stability. Avicelase, CMCase, and β-glucosidase differed considerably in their physicochemical properties. At temperatures above 50°C, β-glucosidase is not very stable. Therefore, as a compromise the conditions of hydrolysis were chosen to be 50°C and pH 4.5. With the crude culture filtrate of T. viride ITCC-1433 a nearly pure glucose solution of 4% is reached from a 10% cellulose suspension. Wood pulp and newsprint are hydrolyzed to a much smaller extent. With an enzyme concentrate up to 8% glucose accumulated in the reaction fluid within 48 hr. At this time the glucose-cellobiose ratio was 75:1. Glucose was demonstrated to be the most potent inhibitor of total hydrolysis. The addition of glucose to the enzyme-substrate solution at zero time completely stopped its own formation and cellobiose and reducing groups (oligosaccharides) accumulated. By removing glucose through an ultrafilter device about 90% saccharification of cellulose to glucose was achieved in 48 hr without any accumulation of cellobiose.     

Probing the active site of cellodextrin phosphorylase from Clostridium stercorarium: kinetic characterization, ligand docking, and site-directed mutagenesis

Cellodextrin phosphorylase from Clostridium stercorarium has been recombinantly expressed in Escherichia coli for the first time. Kinetic characterization of the purified enzyme has revealed that aryl and alkyl β-glucosides can be efficiently glycosylated, an activity that has not yet been described for this enzyme class. To obtain a better understanding of the factors that determine the enzyme's specificity, homology modeling and ligand docking were applied. Residue W168 has been found to form a hydrophobic stacking interaction with the substrate in subsite +2, and its importance has been examined by means of site-directed mutagenesis. The mutant W168A retains about half of its catalytic activity, indicating that other residues also contribute to the binding affinity of subsite +2. Finally, residue D474 has been identified as the catalytic acid, interacting with the glycosidic oxygen between subsites -1 and +1. Mutating this residue results in complete loss of activity. These results, for the first time, provide an insight in the enzyme-substrate interactions that determine the activity and specificity of cellodextrin phosphorylases.     

Catalytic mechanisms and reaction intermediates along the hydrolytic pathway of a plant beta-D-glucan glucohydrolase.

BACKGROUND: Barley beta-D-glucan glucohydrolases represent family 3 glycoside hydrolases that catalyze the hydrolytic removal of nonreducing glucosyl residues from beta-D-glucans and beta-D-glucooligosaccharides. After hydrolysis is completed, glucose remains bound in the active site. RESULTS: When conduritol B epoxide and 2', 4'-dinitrophenyl 2-deoxy-2-fluoro-beta-D-glucopyranoside are diffused into enzyme crystals, they displace the bound glucose and form covalent glycosyl-enzyme complexes through the Odelta1 of D285, which is thereby identified as the catalytic nucleophile. A nonhydrolyzable S-glycosyl analog, 4(I), 4(III), 4(V)-S-trithiocellohexaose, also diffuses into the active site, and a S-cellobioside moiety positions itself at the -1 and +1 subsites. The glycosidic S atom of the S-cellobioside moiety forms a short contact (2.75 A) with the Oepsilon2 of E491, which is likely to be the catalytic acid/base. The glucopyranosyl residues of the S-cellobioside moiety are not distorted from the low-energy 4C(1) conformation, but the glucopyranosyl ring at the +1 subsite is rotated and translated about the linkage. CONCLUSIONS: X-ray crystallography is used to define the three key intermediates during catalysis by beta-D-glucan glucohydrolase. Before a new hydrolytic event begins, the bound product (glucose) from the previous catalytic reaction is displaced by the incoming substrate, and a new enzyme-substrate complex is formed. The second stage of the hydrolytic pathway involves glycosidic bond cleavage, which proceeds through a double-displacement reaction mechanism. The crystallographic analysis of the S-cellobioside-enzyme complex with quantum mechanical modeling suggests that the complex might mimic the oxonium intermediate rather than the enzyme-substrate complex.     

Efficient chemoenzymatic oligosaccharide synthesis by reverse phosphorolysis using cellobiose phosphorylase and cellodextrin phosphorylase from Clostridium thermocellum

Inverting cellobiose phosphorylase (CtCBP) and cellodextrin phosphorylase (CtCDP) from Clostridium thermocellum ATCC27405 of glycoside hydrolase family 94 catalysed reverse phosphorolysis to produce cellobiose and cellodextrins in 57% and 48% yield from α-d-glucose 1-phosphate as donor with glucose and cellobiose as acceptor, respectively. Use of α-d-glucosyl 1-fluoride as donor increased product yields to 98% for CtCBP and 68% for CtCDP. CtCBP showed broad acceptor specificity forming β-glucosyl disaccharides with β-(1→4)- regioselectivity from five monosaccharides as well as branched β-glucosyl trisaccharides with β-(1→4)-regioselectivity from three (1→6)-linked disaccharides. CtCDP showed strict β-(1→4)-regioselectivity and catalysed linear chain extension of the three β-linked glucosyl disaccharides, cellobiose, sophorose, and laminaribiose, whereas 12 tested monosaccharides were not acceptors. Structure analysis by NMR and ESI-MS confirmed two β-glucosyl oligosaccharide product series to represent novel compounds, i.e. β-d-glucopyranosyl-[(1→4)-β-d-glucopyranosyl]_n-(1→2)-d-glucopyranose, and β-d-glucopyranosyl-[(1→4)-β-d-glucopyranosyl]_n-(1→3)-d-glucopyranose (n = 1–7). Multiple sequence alignment together with a modelled CtCBP structure, obtained using the crystal structure of Cellvibrio gilvus CBP in complex with glucose as a template, indicated differences in the subsite +1 region that elicit the distinct acceptor specificities of CtCBP and CtCDP. Thus Glu636 of CtCBP recognized the C1 hydroxyl of β-glucose at subsite +1, while in CtCDP the presence of Ala800 conferred more space, which allowed accommodation of C1 substituted disaccharide acceptors at the corresponding subsites +1 and +2. Furthermore, CtCBP has a short Glu496-Thr500 loop that permitted the C6 hydroxyl of glucose at subsite +1 to be exposed to solvent, whereas the corresponding longer loop Thr637–Lys648 in CtCDP blocks binding of C6-linked disaccharides as acceptors at subsite +1. High yields in chemoenzymatic synthesis, a novel regioselectivity, and novel oligosaccharides including products of CtCDP catalysed oligosaccharide oligomerisation using α-d-glucosyl 1-fluoride, all together contribute to the formation of an excellent basis for rational engineering of CBP and CDP to produce desired oligosaccharides.     

Some kinetic properties of the β-d-glucosidase (cellobiase) in a commercial cellulase product from Penicillium funiculosum and its relevance in the hydrolysis of cellulose

Some kinetic parameters of the β-d-glucosidase (cellobiase, β-d-glucoside glucohydrolase, EC 3.2.1.21) component of Sturge Enzymes CP cellulase [see 1,4-(1,3;1,4)-β-d-glucan 4-glucanohydrolase, EC 3.2.1.4] from Penicillium funiculosum have been determined. The Michaelis constants (K_m) for 4-nitrophenyl β-d-glucopyranoside (4NPG) and cellobiose are 0.4 and 2.1 mM, respectively, at pH 4.0 and 50°C. d-Glucose is shown to be a competitive inhibitor with inhibitor constants (K_i) of 1.7 mM when 4NPG is the substrate and 1 mM when cellobiose is the substrate. Cellobiose, at high concentrations, exhibits a substrate inhibition effect on the enzyme. d-Glucono-1,5-lactone is shown to be a potent inhibitor (K_i = 8 μM; 4NPG as substrate) while d-fructose exhibits little inhibition. Cellulose hydrolysis progress curves using Avicel or Solka Floc as substrates and a range of commercial cellulase preparations show that CP cellulase gives the best performance, which can be attributed to the activity of the β-d-glucosidase in this preparation in maintaining the cellobiose at low concentrations during cellulose hydrolysis.     

Structural changes of the active site tunnel of Humicola insolens cellobiohydrolase, Cel6A, upon oligosaccharide binding.

The mechanisms of crystalline cellulose degradation by cellulases are of paramount importance for the exploitation of these enzymes in applied processes, such as biomass conversion. Cellulases have traditionally been classified into cellobiohydrolases, which are effective in the degradation of crystalline materials, and endoglucanases, which appear to act on "soluble" regions of the substrate. Humicola insolensCel6A (CBH II) is a cellobiohydrolase from glycoside hydrolase family 6 whose native structure has been determined at 1.9 A resolution [Varrot, A., Hastrup, S., Schülein, M., and Davies, G. J. (1999) Biochem. J. 337, 297-304]. Here we present the structure of the catalytic core domain of Humicola insolens cellobiohydrolase II Cel6A in complex with glucose/cellotetraose at 1.7 A resolution. Crystals of Cel6A, grown in the presence of cellobiose, reveal six binding subsites, with a single glucose moiety bound in the -2 subsite and cellotetraose in the +1 to +4 subsites. The complex structure is strongly supportive of the assignment of Asp 226 as the catalytic acid and consistent with proposals that Asp 405 acts as the catalytic base. The structure undergoes several conformational changes upon substrate binding, the most significant of which is a closing of the two active site loops (residues 174-196 and 397-435) with main-chain movements of up to 4.5 A observed. This complex not only defines the polysaccharide-enzyme interactions but also provides the first three-dimensional demonstration of conformational change in this class of enzymes.     

Substrate recognition mechanism of alpha-1,6-glucosidic linkage hydrolyzing enzyme, dextran glucosidase from Streptococcus mutans

We have determined the crystal structure of Streptococcus mutans dextran glucosidase, which hydrolyzes the α-1,6-glucosidic linkage of isomaltooligosaccharides from their non-reducing ends to produce α-glucose. By using the mutant of catalytic acid Glu236→Gln, its complex structure with the isomaltotriose, a natural substrate of this enzyme, has been determined. The enzyme has 536 amino acid residues and a molecular mass of 62,001 Da. The native and the complex structures were determined by the molecular replacement method and refined to 2.2 Å resolution, resulting in a final R-factor of 18.3% for significant reflections in the native structure and 18.4% in the complex structure. The enzyme is composed of three domains, A, B and C, and has a (β/α)₈-barrel in domain A, which is common to the α-amylase family enzymes. Three catalytic residues are located at the bottom of the active site pocket and the bound isomaltotriose occupies subsites −1 to +2. The environment of the glucose residue at subsite −1 is similar to the environment of this residue in the α-amylase family. Hydrogen bonds between Asp60 and Arg398 and O4 atom of the glucose unit at subsite −1 accomplish recognition of the non-reducing end of the bound substrate. The side-chain atoms of Glu371 and Lys275 form hydrogen bonds with the O2 and O3 atoms of the glucose residue at subsite +1. The positions of atoms that compose the scissile α-1,6-glucosidic linkage (C1, O6 and C6 atoms) are identical with the positions of the atoms in the scissile α-1,4 linkage (C1, O4 and C4 atoms) of maltopentaose in the α-amylase structure from Bacillus subtilis. The comparison with the α-amylase suggests that Val195 of the dextran glucosidase and the corresponding residues of α-1,6-hydrolyzing enzymes participate in the determination of the substrate specificity of these enzymes.     

Crystal structure of Thermobifida fusca endoglucanase Cel6A in complex with substrate and inhibitor: the role of tyrosine Y73 in substrate ring distortion

Endoglucanase Cel6A from Thermobifida fusca hydrolyzes the beta-1,4 linkages in cellulose at accessible points along the polymer. The structure of the catalytic domain of Cel6A from T. fusca in complex with a nonhydrolysable substrate analogue that acts as an inhibitor, methylcellobiosyl-4-thio-beta-cellobioside (Glc(2)-S-Glc(2)), has been determined to 1.5 A resolution. The glycosyl unit in subsite -1 was sterically hindered by Tyr73 and forced into a distorted (2)S(o) conformation. In the enzyme where Tyr73 was mutated to a serine residue, the hindrance was removed and the glycosyl unit in subsite -1 had a relaxed (4)C(1) chair conformation. The relaxed conformation was seen in two complex structures of the mutated enzyme, with cellotetrose (Glc(4)) at 1.64 A and Glc(2)-S-Glc(2) at 1.04 A resolution.     

Enzymatic hydrolysis of cellulose to glucose lung immobilized β-glucosidase

The production of sugars by enzymatic hydrolysis of cellulose is a multistep process which includes conversion of the intermediate cellobiose to glucose by β-glucosidase. Aside from its role as an intermediate, cellobiose inhibits the endoglucanase components of typical cellulase enzyme systems. Because these enzyme systems often contain insufficient concentrations of β-glucosidase to prevent accumulation of inhibitory cellobiose, this research investigated the use of supplemental immobilized β-glucosidase to increase yield of glucose. Immobilized β-glucosidase from Aspergillus phoenicis was produced by sorption at controlled-pore alumina with about 90% activity retention. The product lost only about 10% of the original activity during an on-stream reaction period of 500 hr with cellobiose as substrate; maximum activity occurred near pH 3.5 and the apparent activation energy was about 11 kcal/mol. The immobilized β-glucosidase was used together with Trichoderma reesei cellulase to hydrolyze cellulosic materials, such as Solka Floc, corn stove and exploded wood. Increased yields of glucose and greater conversions of cellobiose of glucose were observed when the reaction systems contained supplemental immobilized β-glucosidase.     

Effects of essential carbohydrate/aromatic stacking interaction with Tyr100 and Phe259 on substrate binding of cyclodextrin glycosyltransferase from alkalophilic Bacillus sp. 1011

The stacking interaction between a tyrosine residue and the sugar ring at the catalytic subsite -1 is strictly conserved in the glycoside hydrolase family 13 enzymes. Replacing Tyr100 with leucine in cyclodextrin glycosyltransferase (CGTase) from Bacillus sp. 1011 to prevent stacking significantly decreased all CGTase activities. The adjacent stacking interaction with both Phe183 and Phe259 onto the sugar ring at subsite +2 is essentially conserved among CGTases. F183L/F259L mutant CGTase affects donor substrate binding and/or acceptor binding during transglycosylation [Nakamura et al. (1994) Biochemistry 33, 9929-9936]. To elucidate the precise role of carbohydrate/aromatic stacking interaction at subsites -1 and +2 on the substrate binding of CGTases, we analyzed the X-ray structures of wild-type (2.0 A resolution), and Y100L (2.2 A resolution) and F183L/F259L mutant (1.9 A resolution) CGTases complexed with the inhibitor, acarbose. The refined structures revealed that acarbose molecules bound to the Y100L mutant moved from the active center toward the side chain of Tyr195, and the hydrogen bonding and hydrophobic interaction between acarbose and subsites significantly diminished. The position of pseudo-tetrasaccharide binding in the F183L/F259L mutant was closer to the non-reducing end, and the torsion angles of glycosidic linkages at subsites -1 to +1 on molecule 1 and subsites -2 to -1 on molecule 2 significantly changed compared with that of each molecule of wild-type-acarbose complex to adopt the structural change of subsite +2. These structural and biochemical data suggest that substrate binding in the active site of CGTase is critically affected by the carbohydrate/aromatic stacking interaction with Tyr100 at the catalytic subsite -1 and that this effect is likely a result of cooperation between Tyr100 and Phe259 through stacking interaction with substrate at subsite +2.     

Apo- and cellopentaose-bound structures of the bacterial cellulose synthase subunit BcsZ

Cellulose, a very abundant extracellular polysaccharide, is synthesized in a finely tuned process that involves the activity of glycosyl-transferases and hydrolases. The cellulose microfibril consists of bundles of linear β-1,4-glucan chains that are synthesized inside the cell; however, the mechanism by which these polymers traverse the cell membrane is currently unknown. In Gram-negative bacteria, the cellulose synthase complex forms a trans-envelope complex consisting of at least four subunits. Although three of these subunits account for the synthesis and translocation of the polysaccharide, the fourth subunit, BcsZ, is a periplasmic protein with endo-β-1,4-glucanase activity. BcsZ belongs to family eight of glycosyl-hydrolases, and its activity is required for optimal synthesis and membrane translocation of cellulose. In this study we report two crystal structures of BcsZ from Escherichia coli. One structure shows the wild-type enzyme in its apo form, and the second structure is for a catalytically inactive mutant of BcsZ in complex with the substrate cellopentaose. The structures demonstrate that BcsZ adopts an (α/α)(6)-barrel fold and that it binds four glucan moieties of cellopentaose via highly conserved residues exclusively on the nonreducing side of its catalytic center. Thus, the BcsZ-cellopentaose structure most likely represents a posthydrolysis state in which the newly formed nonreducing end has already left the substrate binding pocket while the enzyme remains attached to the truncated polysaccharide chain. We further show that BcsZ efficiently degrades β-1,4-glucans in in vitro cellulase assays with carboxymethyl-cellulose as substrate.     

Cellulases: Biosynthesis and applications

Strains of Trichoderma, particularly T. reesei and its mutants, are good sources of extracellular cellulase suitable for practical saccharification. They secrete a complete cellulase complex containing endo- and exo-glucanases plus β-glucosidase (cellobiase) which act syngergistically to degrade totally even highly resistant crystalline cellulose to soluble sugars. All strains investigated to date are inducible by cellulose, lactose, or sophorose, and all are repressible by glucose. Induction, synthesis and secretion of the β-glucanase enzymes appear to be closely associated. The composition and properties of the enzyme complex are similar regardless of the strain or inducing substrate although quantities of the enzyme secreted by the mutants are higher. Enzyme yields are proportional to initial cellulose concentration. Up to 15 filter paper cellulase units (20 mg of cellulase protein) per ml and productivities up to 80 cellulase units (130 mg cellulase protein) per litre per hour have been attained on 6% cellulose. The economics of glucose production are not yet competitive due to the low specific activity of cellulase (0.6 filter paper cellulase units/mg protein) and poor efficiency (about 15% of predicted sugar levels in 24 h hydrolyses of 10–25% substrate). As hydrolysis proceeds, the enzyme reaction slows due to increasing resistance of the residue, product inhibition, and enzyme inactivation. These problems are being attacked by use of pretreatments to increase the quantity of amorphous cellulose, addition of β-glucosidase to reduce cellobiose inhibition, and studies of means to overcome instability and increase efficiency of the cellulases. Indications are that carbon compounds derived from enzymatic hydrolysis of cellulose will be used as fermentation and chemical feedstocks as soon as the process economics are favourable for such application.     

The structure and characterization of a modular endo-beta-1,4-mannanase from Cellulomonas fimi

The endo-beta-1,4-mannanase from the soil bacterium Cellulomonas fimi is a modular plant cell wall degrading enzyme involved in the hydrolysis of the backbone of mannan, one of the most abundant polysaccharides of the hemicellulosic network in the plant cell wall. The crystal structure of a recombinant truncated endo-beta-1,4-mannanase from C. fimi (CfMan26A-50K) was determined by X-ray crystallography to 2.25 A resolution using the molecular replacement technique. The overall structure of the enzyme consists of a core (beta/alpha)8-barrel catalytic module characteristic of clan GH-A, connected via a linker to an immunoglobulin-like module of unknown function. A complex with the oligosaccharide mannotriose to 2.9 A resolution has also been obtained. Both the native structure and the complex show a cacodylate ion bound at the -1 subsite, while subsites -2, -3, and -4 are occupied by mannotriose in the complex. Enzyme kinetic analysis and the analysis of hydrolysis products from manno-oligosaccharides and mannopentitol suggest five important active-site cleft subsites. CfMan26A-50K has a high affinity -3 subsite with Phe325 as an aromatic platform, which explains the mannose releasing property of the enzyme. Structural differences with the homologous Cellvibrio japonicus beta-1,4-mannanase (CjMan26A) at the -2 and -3 subsites may explain the poor performance of CfMan26A mutants as "glycosynthases".     

Identification of acceptor substrate binding subsites +2 and +3 in the amylomaltase from Thermus thermophilus HB8

Glycoside hydrolase family 77 (GH77) belongs to the alpha-amylase superfamily (Clan H) together with GH13 and GH70. GH77 enzymes are amylomaltases or 4-alpha-glucanotransferases, involved in maltose metabolism in microorganisms and in starch biosynthesis in plants. Here we characterized the amylomaltase from the hyperthermophilic bacterium Thermus thermophilus HB8 (Tt AMase). Site-directed mutagenesis of the active site residues (Asp293, nucleophile; Glu340, general acid/base catalyst; Asp395, transition state stabilizer) shows that GH77 Tt AMase and GH13 enzymes share the same catalytic machinery. Quantification of the enzyme's transglycosylation and hydrolytic activities revealed that Tt AMase is among the most efficient 4-alpha-glucanotransferases in the alpha-amylase superfamily. The active site contains at least seven substrate binding sites, subsites -2 and +3 favoring substrate binding and subsites -3 and +2 not, in contrast to several GH13 enzymes in which subsite +2 contributes to oligosaccharide binding. A model of a maltoheptaose (G7) substrate bound to the enzyme was used to probe the details of the interactions of the substrate with the protein at acceptor subsites +2 and +3 by site-directed mutagenesis. Substitution of the fully conserved Asp249 with a Ser in subsite +2 reduced the activity 23-fold (for G7 as a substrate) to 385-fold (for maltotriose). Similar mutations reduced the activity of alpha-amylases only up to 10-fold. Thus, the characteristics of acceptor subsite +2 represent a main difference between GH13 amylases and GH77 amylomaltases.