Glycoside hydrolases and glycosyltransferases. Families, modules, and implications for genomics

Alternate intron processing of family 5 endoglucanase transcripts from the genus Phytophthora

Twenty-one homologs of family 5 endo-(1-4)-beta-glucanase genes (EGLs) were identified and characterized in the oomycete plant pathogens Phytophthora infestans, P. sojae, and P. ramorum, providing the first comprehensive analysis of this family in Phytophthora. Phylogenetic analysis revealed that these genes constitute a unique eukaryotic group, with closest similarity to bacterial endoglucanases. Many of the identified EGL copies were clustered in a few genomic regions, and contained from zero to three introns. Using reverse transcription PCR to study in vitro and in planta gene expression levels of P. sojae, we detected partially processed RNA transcripts retaining one or more of their introns. In some cases, the positions of intron/exon splicing sites were also found to be variable. The relative proportions of these transcripts remain apparently unchanged under various growing conditions, but differ among orthologous copies of the three Phytophthora species. The alternate processing of introns in this group of EGLs generates both coding and non-coding RNA isoforms. This is the first report on Phytophthora family 5 endoglucanases, and the first record for alternative intron processing of oomycete transcripts.

Characterization of Arabidopsis AtUGT85A and AtGUS gene families and their expression in rapidly dividing tissues

In humans, uridine 5'-diphosphate glucuronosyltransferase (UGT) operates in opposition to glucuronidase (GUS) to control activity of diverse metabolites such as hormones by reversible conjugation with glucuronic acid. Previous data revealed that, as in mammals, these enzymes are required for plant life in that a UGT from Pisum sativum (PsUGT1) controls plant development by opposing endogenous GUS activity thereby modulating the duration of the cell cycle. Here we report that a small family of genes (AtUGT85A1, 2, 3, 4, 5, and 7) homologous to pea PsUGT1 exists in the Arabidopsis genome. The AtUGT85A-encoded proteins are predicted to be membrane-associated enzymes. Three genes (AtGUS1, AtGUS2, and AtGUS3) that are homologous to a GUS-encoding gene from Scutellaria baicalensis were identified. The AtGUS-encoded proteins are predicted to be secretory (AtGUS1) and membrane-associated (AtGUS2 and AtGUS3) enzymes. Both AtUGT85A and AtGUS genes, like PsUGT1, exhibit localized, tissue-specific expression, mainly in areas of active cell division with possible involvement in cell cycle regulation.

Small cysteine-rich peptides resembling antimicrobial peptides have been under-predicted in plants

Multicellular organisms produce small cysteine-rich antimicrobial peptides as an innate defense against pathogens. While defensins, a well-known class of such peptides, are common among eukaryotes, there are other classes restricted to the plant kingdom. These include thionins, lipid transfer proteins and snakins. In earlier work, we identified several divergent classes of small putatively secreted cysteine-rich peptides (CRPs) in legumes [Graham et al. (2004)Plant Physiol. 135, 1179-97]. Here, we built sequence motif models for each of these classes of peptides, and iteratively searched for related sequences within the comprehensive UniProt protein dataset, the Institute for Genomic Research's 33 plant gene indices, and the entire genomes of the model dicot, Arabidopsis thaliana, and the model monocot and crop species, Oryza sativa (rice). Using this search strategy, we identified approximately 13,000 plant genes encoding peptides with common features: (i) an N-terminal signal peptide, (ii) a small divergent charged or polar mature peptide with conserved cysteines, (iii) a similar intron/exon structure, (iv) spatial clustering in the genomes studied, and (v) overrepresentation in expressed sequences from reproductive structures of specific taxa. The identified genes include classes of defensins, thionins, lipid transfer proteins, and snakins, plus other protease inhibitors, pollen allergens, and uncharacterized gene families. We estimate that these classes of genes account for approximately 2-3% of the gene repertoire of each model species. Although 24% of the genes identified were not annotated in the latest Arabidopsis genome releases (TIGR5, TAIR6), we confirmed expression via RT-PCR for 59% of the sequences attempted. These findings highlight limitations in current annotation procedures for small divergent peptide classes.

Multiple dextranases from the yeast Lipomyces starkeyi

The soil yeast Lipomyces starkeyi (NCYC 1436) secretes dextranase activity into the growth medium. Resolution of a dextranase-active protein fraction by SDS-PAGE produced three protein bands, of 66 kDa, 68 kDa and 78 kDa, and isoelectric focusing of the same fraction resulted in seven protein bands, of pIs 3.50, 3.85, 4.20, 4.80, 4.85, 5.00 and 5.30. Dextranase activity was demonstrated for all the isoelectric forms, and for the 78 kDa species in the presence of SDS. Amino acid compositions of the 66 kDa, 68 kDa and 78 kDa protein bands were determined, and the N-termini of the 66 kDa and 78 kDa protein bands were sequenced: the first two amino acids at the N-terminus of each protein were alanine and valine, respectively; an alanine-valine pair is seen early in the N-terminal coding sequences of the dextranases and the isopullulanase produced by the phylogenetically disparate organisms contributing to glycosyl hydrolase family 49.

Family 18 chitolectins: comparison of MGP40 and HUMGP39

Glycosidase and lectins both bind sugars, but only the glycosidases have catalytic activity. The glycosidases occur among over 100 evolved protein families and Family 18 is one of the two chitinases (EC 3, 2.1.14) families. Interestingly, lectins are also in this evolutionary group of Family 18 glycosidase proteins. The proteins belonging to the enzymatically inactive class are referred to as chitolectins and have a binding site that is highly similar to the catalytic Family 18 enzymes. We present a comparison of the recently obtained structures of two Family 18 chitolectins, MGP40 [A.K. Mohanty, G. Singh, M. Paramasivam, K. Saravanan, T. Jabeen, S. Sharma, S. Yadav, P. Kaur, P. Kumar, A. Srinivasan, T.P. Singh, Crystal structure of a novel regulatory 40kDa mammary gland protein (MGP-40) secreted during involution, J. Biol. Chem. 278 (2003) 14451-14460.] and HumGP39 [F. Fusetti, T. Pijning, K.H. Kalk, E. Bos, B.W. Dijkstra, Crystal structure and carbohydrate-binding properties of the human cartilage glycoprotein-39, J. Biol. Chem. 278 (2003) 37753-37760; D.R. Houston, D.R. Anneliese, C.K. Joanne, D.M.V. Aalten, Structure and ligand-induced conformational change of the 39kDa glycoprotein from human articular chondrocytes, J. Biol. Chem. 278 (2003) 30206-30212.] with a focus on the glycosidase active site. We compare the sequence and the structure of these two Family 18 protein classes. The difference between the active and inactive protein is a glutamic acid which acts as the essential acid/base residue for chitin cleavage and is replaced with leucine or glutamine in the chitolectins. Furthermore, a mechanism for the interaction between the chitolectin and oligosaccharides was proposed.

Bacterial glycosidases for the production of universal red blood cells

Enzymatic removal of blood group ABO antigens to develop universal red blood cells (RBCs) was a pioneering vision originally proposed more than 25 years ago. Although the feasibility of this approach was demonstrated in clinical trials for group B RBCs, a major obstacle in translating this technology to clinical practice has been the lack of efficient glycosidase enzymes. Here we report two bacterial glycosidase gene families that provide enzymes capable of efficient removal of A and B antigens at neutral pH with low consumption of recombinant enzymes. The crystal structure of a member of the alpha-N-acetylgalactosaminidase family reveals an unusual catalytic mechanism involving NAD+. The enzymatic conversion processes we describe hold promise for achieving the goal of producing universal RBCs, which would improve the blood supply while enhancing the safety of clinical transfusions.

Purification and characterization of a highly thermostable α-l-Arabinofuranosidase from Geobacillus caldoxylolyticus TK4

The gene encoding an alpha-L: -arabinofuranosidase from Geobacillus caldoxylolyticus TK4, AbfATK4, was isolated, cloned, and sequenced. The deduced protein had a molecular mass of about 58 kDa, and analysis of its amino acid sequence revealed significant homology and conservation of different catalytic residues with alpha-L: -arabinofuranosidases belonging to family 51 of the glycoside hydrolases. A histidine tag was introduced at the N-terminal end of AbfATK4, and the recombinant protein was expressed in Escherichia coli BL21, under control of isopropyl-beta-D-thiogalactopyranoside-inducible T7 promoter. The enzyme was purified by nickel affinity chromatography. The molecular mass of the native protein, as determined by gel filtration, was about 236 kDa, suggesting a homotetrameric structure. AbfATK4 was active at a broad pH range (pH 5.0-10.0) and at a broad temperature range (40-85 degrees C), and it had an optimum pH of 6.0 and an optimum temperature of 75-80 degrees C. The enzyme was more thermostable than previously described arabinofuranosidases and did not lose any activity after 48 h incubation at 70 degrees C. The protein exhibited a high level of activity with p-nitrophenyl-alpha-L: -arabinofuranoside, with apparent K (m) and V (max) values of 0.17 mM and 588.2 U/mg, respectively. AbfATK4 also exhibited a low level of activity with p-nitrophenyl-beta-D: -xylopyranoside, with apparent K (m) and V (max) values of 1.57 mM and 151.5 U/mg, respectively. AbfATK4 released L: -arabinose only from arabinan and arabinooligosaccharides. No endoarabinanase activity was detected. These findings suggest that AbfATK4 is an exo-acting enzyme.

How pollen tubes grow

Sexual reproduction of flowering plants depends on delivery of the sperm to the egg, which occurs through a long, polarized projection of a pollen cell, called the pollen tube. The pollen tube grows exclusively at its tip, and this growth is distinguished by very fast rates and reaches extended lengths. Thus, one of the most fascinating aspects of pollen biology is the question of how enough cell wall material is produced to accommodate such rapid extension of pollen tube, and how the cell wall deposition and structure are regulated to allow for rapid changes in the direction of growth. This review discusses recent advances in our understanding of the mechanism of pollen tube growth, focusing on such basic cellular processes as control of cell shape and growth by a network of cell wall-modifying enzymes, molecular motor-mediated vesicular transport, and intracellular signaling by localized gradients of second messengers.

A multifunctional hybrid glycosyl hydrolase discovered in an uncultured microbial consortium from ruminant gut

A unique multifunctional glycosyl hydrolase was discovered by screening an environmental DNA library prepared from a microbial consortium collected from cow rumen. The protein consists of two adjacent catalytic domains. Sequence analysis predicted that one domain conforms to glycosyl hydrolase family 5 and the other to family 26. The enzyme is active on several different beta-linked substrates and possesses mannanase, xylanase, and glucanase activities. Site-directed mutagenesis studies on the catalytic residues confirmed the presence of two functionally independent catalytic domains. Using site-specific mutations, it was shown that one catalytic site hydrolyzes beta-1,4-linked mannan substrates, while the second catalytic site hydrolyzes beta-1,4-linked xylan and beta-1,4-linked glucan substrates. Polysaccharide Analysis using Carbohydrate gel Electrophoresis (PACE) also confirmed that the enzyme has discrete domains for binding and hydrolysis of glucan- and mannan-linked polysaccharides. Such multifunctional enzymes have many potential industrial applications in plant processing, including biomass saccharification, animal feed nutritional enhancement, textile, and pulp and paper processing.

Expression differences between normal and indeterminate1 maize suggest downstream targets of ID1, a floral transition regulator in maize

The INDETERMINATE1 (ID1) transcription factor is a key regulator of the transition to flowering in maize. ID1 is expressed in immature leaves where it controls the production or transmission of leaf-derived florigenic signals. Loss-of-function id1 mutants make many more leaves than normal plants and produce aberrant flowers; however, they exhibit no obvious developmental defects in early growth stages. A maize oligonucleotide microarray was used to assess the molecular differences between immature leaves of wild-type and id1 mutant plants prior to the floral transition. This analysis revealed 55 genes with a significant 2-fold difference in expression; 22 are down-regulated and 33 are up-regulated in id1 mutants. Most prominent is a novel family of three beta-glucosidase genes that are most closely related to sorghum dhurrinases. These genes, termed Zmdhr1, Zmdhr2, and Zmdhr3, are undetectable in immature leaves of id1 mutants and are expressed exclusively in normal immature leaves in a pattern identical to the ID1 gene. Other down-regulated genes include a group of four zinc finger protein-encoding genes that are unrelated to ID1. A significant number of genes up-regulated in id1 mutant immature leaves have potential roles in photosynthesis and carbon fixation, substantiating a possible connection between floral induction and assimilate partitioning. Finally, expression of these genes was compared in florally induced versus uninduced teosinte, a photoperiod-sensitive progenitor of day-neutral maize. Only a few genes showed expression differences, suggesting that ID1 acts in a novel autonomous floral induction pathway that is distinct from the photoperiod induction pathway.

Molecular cloning and characterization of a novel beta-agarase, AgaB, from marine Pseudoalteromonas sp. CY24

Agarases are generally classified into glycoside hydrolase families 16, 50, and 86 and are found to degrade agarose to frequently generate neoagarobiose, neoagarotetraose, or neoagarohexaose as the main products. In this study we have cloned a novel endo-type beta-agarase gene, agaB, from marine Pseudoalteromonas sp. CY24. The novel agarase encoded by agaB gene has no significant sequence similarity with any known proteins including all glycoside hydrolases. It degrades agarose to generate neoagarooctaose and neoagarodecaose as the main end products. Based on the analyses of enzymatic kinetics and degradation patterns of different oligosaccharides, the agarase AgaB appears to have a large substrate binding cleft that accommodates 12 sugar units, with 8 sugar units toward the reducing end spanning subsites +1 to +8 and 4 sugar units toward the non-reducing end spanning subsites -4 to -1, and enzymatic cleavage taking place between subsites -1 and +1. In addition, 1H NMR analysis shows that this enzyme hydrolyzes the glycosidic bond with inversion of anomeric configuration, in contrast to other known agarases that are retaining. Altogether, AgaB is structurally and functionally different from other known agarases and appears to represent a new family of glycoside hydrolase.

Bioconversion of lignocellulose materials

One of the most economically viable processes for the bioconversion of many lignocellulosic waste is represented by white rot fungi. Phanerochaete chrysosporium is one of the important commercially cultivated fungi which exhibit varying abilities to utilize different lignocellulosic as growth substrate. Examination of the lignocellulolytic enzyme profiles of the two organisms Phanerochaete chrysosporium and Rhizopus stolonifer show this diversity to be reflected in qualitative variation in the major enzymatic determinants (ie cellulase, xylanase, ligninase and etc) required for substrate bioconversion. For example P. chrysosporium which is cultivated on highly lignified substrates such as wood (or) sawdust, produces two extracellular enzymes which have associated with lignin deploymerization. (Mn peroxidase and lignin peroxidase). Conversely Rhizopus stolonifer which prefers high cellulose and low lignin containg substrates produce a family of cellulolytic enzymes including at least cellobiohydrolases and β-glucosidases, but very low level of recognized lignin degrading enzymes.

Fusion of carbohydrate binding modules from Thermotoga neapolitana with a family 10 xylanase from Bacillus halodurans S7

Xylanase A of Thermotoga neapolitana contains binding domains both at the N- and C-terminal ends of the catalytic domain. In the N-terminal position it contains two carbohydrate-binding modules (CBM) which belong to family 22. These CBMs bind xylan but not to cellulose. The gene encoding the mature peptide of these CBMs was fused with an alkaline active GH10 xylanase from Bacillus halodurans S7 and expressed in Escherichia coli. The (His)(6) tagged hybrid protein was purified by immobilized metal affinity chromatography and characterized. Xylan binding by the chimeric protein was influenced by NaCl concentration and pH of the binding medium. Binding increased with increasing salt concentration up to 200 mM. Higher extent of binding was observed under acidic conditions. The fusion of the CBM structures enhanced the hydrolytic efficiency of the xylanase against insoluble xylan, but decreased the stability of the enzyme. The optimum temperature and pH for the activity of the xylanase did not change.

Two xyloglucan xylosyltransferases catalyze the addition of multiple xylosyl residues to cellohexaose

Xyloglucan (XyG) is the principal hemicellulose found in the primary cell walls of most plants. XyG is composed of a beta-(1,4)-glucan backbone that is substituted in a regular pattern with xylosyl residues, which are added by at least one and likely two or three xylosyltransferase (XT) enzymes. Previous work identified seven Arabidopsis thaliana candidate genes, one of which (AtXT1) was shown to encode a protein with XT activity (Faik, A., Price, N. J., Raikhel, N. V., and Keegstra, K. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 7797-7802). We expressed both AtXT1 and a second closely related gene, now called AtXT2, in insect cells and demonstrated that both have XT activity for cellopentaose and cellohexaose acceptor substrates. Moreover, we showed that cellohexaose was a significantly better acceptor substrate than cellopentaose. Product structural characterization showed that AtXT1 and AtXT2 preferentially added the first xylosyl residue to the fourth glucosyl residue from the reducing end of both acceptors. Furthermore, when the ratio of UDP-xylose to cellohexaose and the reaction time were increased, both AtXT1 and AtXT2 added a second xylosyl residue adjacent to the first, which generated dixylosylated cellohexaose. On the basis of these results, we concluded that AtXT1 and AtXT2 have the same acceptor specificities and generate the same products in vitro. The implications of these results for understanding in vivo XyG biosynthesis are considered.

Expression and purification of dalcochinase, a β-glucosidase from Dalbergia cochinchinensis Pierre, in yeast and bacterial hosts

The coding sequence of the mature dalcochinase, a beta-glucosidase from Dalbergia cochinchinensis Pierre, was cloned and expressed in various systems. Expression in Escherichia coli resulted in an insoluble protein, which could be made soluble by co-expression with bacterial chaperonin GroESL. However, the enzyme had no activity. Recombinant expression in Pichia pastoris and Saccharomyces cerevisiae yielded an active enzyme. Dalcochinase was expressed under methanol induction in P. pastoris, since this was much more efficient than constitutive expression in P. pastoris or in S. cerevisiae. Addition of 0.5% casamino acids to the culture medium stabilized the pH of the culture and increased the protein yield by 3- to 5-folds. Insertion of a polyhistidine-tag either after the N-terminal alpha factor signal sequence or at the C-terminus failed to assist in purification by immobilized metal-ion affinity chromatography (IMAC) due to post-translational processing at both termini. A new construct of dalcochinase with an N-terminal truncation following the propeptide and eight histidine residues enabled its purification by IMAC, following hydrophobic interaction chromatography. The purified recombinant dalcochinase was apparently composed of differently post-translationally modified forms, but had kinetic properties and pH and temperature optima comparable to natural dalcochinase. The procedures reported here overcome the limitation in enzyme supply from natural sources, and allow further studies on structure-function relationships in this enzyme.

Mutation of a conserved tryptophan in the chitin-binding cleft of Serratia marcescens chitinase A enhances transglycosylation

Family 18 chitinases have the signature peptide DGXDXDXE forming the fourth beta-strand in the (beta/alpha)8-barrel of their catalytic domain. The carboxyl-end glutamic acid, E315 in Serratia marcescens chitinase A, serves as the acid/base during chitin hydrolysis, and the side-chain of the preceding aspartic acid, D313, helps to position correctly the N-acetyl moiety of the glycosyl sugar undergoing hydrolysis. Chitin substrates are bound within a long cleft across the top of the barrel, whose floor consists of aromatic residues that hydrophobically stack with every other GlcNAc. Alanine substitution of the conserved Trp167 at the -3 subsite in Serratia marcescens chitinase A enhanced transglycosylation. Higher oligosaccharides were formed from both chitin tetra- and pentasaccharide, and the only hydrolytic product from chitin trisaccharide was the disaccharide. Greater retention of the glycosyl fragment at the active site of the -3 mutant of Serratia marcescens chitinase A might favor transglycosylation due to a stabilized conformation of its D313.

Poplar carbohydrate-active enzymes. Gene identification and expression analyses

Over 1,600 genes encoding carbohydrate-active enzymes (CAZymes) in the Populus trichocarpa (Torr. & Gray) genome were identified based on sequence homology, annotated, and grouped into families of glycosyltransferases, glycoside hydrolases, carbohydrate esterases, polysaccharide lyases, and expansins. Poplar (Populus spp.) had approximately 1.6 times more CAZyme genes than Arabidopsis (Arabidopsis thaliana). Whereas most families were proportionally increased, xylan and pectin-related families were underrepresented and the GT1 family of secondary metabolite-glycosylating enzymes was overrepresented in poplar. CAZyme gene expression in poplar was analyzed using a collection of 100,000 expressed sequence tags from 17 different tissues and compared to microarray data for poplar and Arabidopsis. Expression of genes involved in pectin and hemicellulose metabolism was detected in all tissues, indicating a constant maintenance of transcripts encoding enzymes remodeling the cell wall matrix. The most abundant transcripts encoded sucrose synthases that were specifically expressed in wood-forming tissues along with cellulose synthase and homologs of KORRIGAN and ELP1. Woody tissues were the richest source of various other CAZyme transcripts, demonstrating the importance of this group of enzymes for xylogenesis. In contrast, there was little expression of genes related to starch metabolism during wood formation, consistent with the preferential flux of carbon to cell wall biosynthesis. Seasonally dormant meristems of poplar showed a high prevalence of transcripts related to starch metabolism and surprisingly retained transcripts of some cell wall synthesis enzymes. The data showed profound changes in CAZyme transcriptomes in different poplar tissues and pointed to some key differences in CAZyme genes and their regulation between herbaceous and woody plants.

Alpha-L-arabinofuranosidases: the potential applications in biotechnology

Recently, alpha-L-arabinofuranosidases (EC3.2.1.55) have received increased attention primarily due to their role in the degradation of lignocelluloses as well as their positive effect on the activity of other enzymes acting on lignocelluloses. As a result, these enzymes are used in many biotechnological applications including wine industry, clarification of fruit juices, digestion enhancement of animal feedstuffs and as a natural improver for bread. Moreover, these enzymes could be used to improve existing technologies and to develop new technologies. The production, mechanisms of action, classification, synergistic role, biochemical properties, substrate specificities, molecular biology and biotechnological applications of these enzymes have been reviewed in this article.

Catalytic properties of the bifunctional soybean beta-glucan-binding protein, a member of family 81 glycoside hydrolases

The beta-glucan-binding protein (GBP) of soybean (Glycine max L.) has been shown to contain two different activities. As part of the plasma membrane-localized pathogen receptor complex, it binds a microbial cell wall elicitor, triggering the activation of defence responses. Additionally, the GBP is able to hydrolyze beta-1,3-glucans, as present in the cell walls of potential pathogens. The substrate specificity, the mode of action, and the stereochemistry of the catalysis have been elucidated. This defines for the first time the inverting mode of the catalytic mechanism of glycoside hydrolases belonging to family 81.

Correlation of temperature induced conformation change with optimum catalytic activity in the recombinant G/11 xylanase A from Bacillus subtilis strain 168 (1A1)

The 1.7A resolution crystal structure of recombinant family G/11 beta-1,4-xylanase (rXynA) from Bacillus subtilis 1A1 shows a jellyroll fold in which two curved beta-sheets form the active-site and substrate-binding cleft. The onset of thermal denaturation of rXynA occurs at 328 K, in excellent agreement with the optimum catalytic temperature. Molecular dynamics simulations at temperatures of 298-328 K demonstrate that below the optimum temperature the thumb loop and palm domain adopt a closed conformation. However, at 328 K these two domains separate facilitating substrate access to the active-site pocket, thereby accounting for the optimum catalytic temperature of the rXynA.

Identification of a novel bean α-amylase inhibitor with chitinolytic activity

Zabrotes subfasciatus is a devastating starch-dependent storage bean pest. In this study, we attempted to identify novel alpha-amylase inhibitors from wild bean seeds, with efficiency toward pest alpha-amylases. An inhibitor named Phaseolus vulgaris chitinolytic alpha-amylase inhibitor (PvCAI) was purified and mass spectrometry analyses showed a protein with 33330 Da with the ability to form dimers. Purified PvCAI showed significant inhibitory activity against larval Z. subfasciatus alpha-amylases with no activity against mammalian enzymes. N-terminal sequence analyses showed an unexpected high identity to plant chitinases from the glycoside hydrolase family 18. Furthermore, their chitinolytic activity was also detected. Our data provides compelling evidence that PvCAI also possessed chitinolytic activity, indicating the emergence of a novel alpha-amylase inhibitor class.

Analysis of glycoside hydrolase family 98: catalytic machinery, mechanism and a novel putative carbohydrate binding module

Glycoside hydrolases (GHs) are diverse enzymes of biotechnological and medical importance. Bioinformatics contributes to our understanding of GH structure and function in various ways, including dissection of their typically modular structures and detection of the distant evolutionary relationships between families that often allow for prediction of catalytic sites. Here these twin strands are applied to the recently described GH98 family, the founder member of which is a blood group glycotope-cleaving endo-beta-galactosidase of potential medical importance from Clostridium perfringens. Three domains can be discerned including a central catalytic TIM barrel domain in which putative catalytic residues can be assigned. Distant homologies and domain contexts suggest that the N-terminal domain is a novel carbohydrate binding module.

Microbial dextran-hydrolyzing enzymes: fundamentals and applications

Dextran is a chemically and physically complex polymer, breakdown of which is carried out by a variety of endo- and exodextranases. Enzymes in many groups can be classified as dextranases according to function: such enzymes include dextranhydrolases, glucodextranases, exoisomaltohydrolases, exoisomaltotriohydrases, and branched-dextran exo-1,2-alpha-glucosidases. Cycloisomalto-oligosaccharide glucanotransferase does not formally belong to the dextranases even though its side reaction produces hydrolyzed dextrans. A new classification system for glycosylhydrolases and glycosyltransferases, which is based on amino acid sequence similarities, divides the dextranases into five families. However, this classification is still incomplete since sequence information is missing for many of the enzymes that have been biochemically characterized as dextranases. Dextran-degrading enzymes have been isolated from a wide range of microorganisms. The major characteristics of these enzymes, the methods for analyzing their activities and biological roles, analysis of primary sequence data, and three-dimensional structures of dextranases have been dealt with in this review. Dextranases are promising for future use in various scientific and biotechnological applications.

Microbial dextran-hydrolyzing enzymes: fundamentals and applications

Dextran is a chemically and physically complex polymer, breakdown of which is carried out by a variety of endo- and exodextranases. Enzymes in many groups can be classified as dextranases according to function: such enzymes include dextranhydrolases, glucodextranases, exoisomaltohydrolases, exoisomaltotriohydrases, and branched-dextran exo-1,2-alpha-glucosidases. Cycloisomalto-oligosaccharide glucanotransferase does not formally belong to the dextranases even though its side reaction produces hydrolyzed dextrans. A new classification system for glycosylhydrolases and glycosyltransferases, which is based on amino acid sequence similarities, divides the dextranases into five families. However, this classification is still incomplete since sequence information is missing for many of the enzymes that have been biochemically characterized as dextranases. Dextran-degrading enzymes have been isolated from a wide range of microorganisms. The major characteristics of these enzymes, the methods for analyzing their activities and biological roles, analysis of primary sequence data, and three-dimensional structures of dextranases have been dealt with in this review. Dextranases are promising for future use in various scientific and biotechnological applications.

Emergence of a subfamily of xylanase inhibitors within glycoside hydrolase family 18

The xylanase inhibitor protein I (XIP-I), recently identified in wheat, inhibits xylanases belonging to glycoside hydrolase families 10 (GH10) and 11 (GH11). Sequence and structural similarities indicate that XIP-I is related to chitinases of family GH18, despite its lack of enzymatic activity. Here we report the identification and biochemical characterization of a XIP-type inhibitor from rice. Despite its initial classification as a chitinase, the rice inhibitor does not exhibit chitinolytic activity but shows specificities towards fungal GH11 xylanases similar to that of its wheat counterpart. This, together, with an analysis of approximately 150 plant members of glycosidase family GH18 provides compelling evidence that xylanase inhibitors are largely represented in this family, and that this novel function has recently emerged based on a common scaffold. The plurifunctionality of GH18 members has major implications for genomic annotations and predicted gene function. This study provides new information which will lead to a better understanding of the biological significance of a number of GH18 'inactivated' chitinases.

Carbohydrate-binding modules: fine-tuning polysaccharide recognition

The enzymic degradation of insoluble polysaccharides is one of the most important reactions on earth. Despite this, glycoside hydrolases attack such polysaccharides relatively inefficiently as their target glycosidic bonds are often inaccessible to the active site of the appropriate enzymes. In order to overcome these problems, many of the glycoside hydrolases that utilize insoluble substrates are modular, comprising catalytic modules appended to one or more non-catalytic CBMs (carbohydrate-binding modules). CBMs promote the association of the enzyme with the substrate. In view of the central role that CBMs play in the enzymic hydrolysis of plant structural and storage polysaccharides, the ligand specificity displayed by these protein modules and the mechanism by which they recognize their target carbohydrates have received considerable attention since their discovery almost 20 years ago. In the last few years, CBM research has harnessed structural, functional and bioinformatic approaches to elucidate the molecular determinants that drive CBM-carbohydrate recognition. The present review summarizes the impact structural biology has had on our understanding of the mechanisms by which CBMs bind to their target ligands.

Carbohydrate-active enzymes involved in the secondary cell wall biogenesis in hybrid aspen

Wood formation is a fundamental biological process with significant economic interest. While lignin biosynthesis is currently relatively well understood, the pathways leading to the synthesis of the key structural carbohydrates in wood fibers remain obscure. We have used a functional genomics approach to identify enzymes involved in carbohydrate biosynthesis and remodeling during xylem development in the hybrid aspen Populus tremula x tremuloides. Microarrays containing cDNA clones from different tissue-specific libraries were hybridized with probes obtained from narrow tissue sections prepared by cryosectioning of the developing xylem. Bioinformatic analyses using the sensitive tools developed for carbohydrate-active enzymes allowed the identification of 25 xylem-specific glycosyltransferases belonging to the Carbohydrate-Active EnZYme families GT2, GT8, GT14, GT31, GT43, GT47, and GT61 and nine glycosidases (or transglycosidases) belonging to the Carbohydrate-Active EnZYme families GH9, GH10, GH16, GH17, GH19, GH28, GH35, and GH51. While no genes encoding either polysaccharide lyases or carbohydrate esterases were found among the secondary wall-specific genes, one putative O-acetyltransferase was identified. These wood-specific enzyme genes constitute a valuable resource for future development of engineered fibers with improved performance in different applications.

Parallel substrate binding sites in a beta-agarase suggest a novel mode of action on double-helical agarose

Agarose is a gel-forming polysaccharide with an alpha-L(1,4)-3,6-anhydro-galactose, beta-D(1,3)-galactose repeat unit, from the cell walls of marine red algae. beta-agarase A, from the Gram-negative bacterium Zobellia galactanivorans, is secreted to the external medium and degrades agarose with an endo-mechanism. The structure of the inactive mutant beta-agarase A-E147S in complex with agaro-octaose has been solved at 1.7 A resolution. Two oligosaccharide chains are bound to the protein. The first one resides in the active site channel, spanning subsites -4 to -1. A second oligosaccharide binding site, on the opposite side of the protein, was filled with eight sugar units, parallel to the active site. The crystal structure of the beta-agarase A with agaro-octaose provides detailed information on agarose recognition in the catalytic site. The presence of the second, parallel, binding site suggests that the enzyme might be able to unwind the double-helical structure of agarose prior to the catalytic cleavage.

Effect of dimer dissociation on activity and thermostability of the alpha-glucuronidase from Geobacillus stearothermophilus: dissecting the different oligomeric forms of family 67 glycoside hydrolases

The oligomeric organization of enzymes plays an important role in many biological processes, such as allosteric regulation, conformational stability and thermal stability. alpha-Glucuronidases are family 67 glycosidases that cleave the alpha-1,2-glycosidic bond between 4-O-methyl-D-glucuronic acid and xylose units as part of an array of hemicellulose-hydrolyzing enzymes. Currently, two crystal structures of alpha-glucuronidases are available, those from Geobacillus stearothermophilus (AguA) and from Cellvibrio japonicus (GlcA67A). Both enzymes are homodimeric, but surprisingly their dimeric organization is different, raising questions regarding the significance of dimerization for the enzymes' activity and stability. Structural comparison of the two enzymes suggests several elements that are responsible for the different dimerization organization. Phylogenetic analysis shows that the alpha-glucuronidases AguA and GlcA67A can be classified into two distinct subfamilies of bacterial alpha-glucuronidases, where the dimer-forming residues of each enzyme are conserved only within its own subfamily. It seems that the different dimeric forms of AguA and GlcA67A represent the two alternative dimeric organizations of these subfamilies. To study the biological significance of the dimerization in alpha-glucuronidases, we have constructed a monomeric form of AguA by mutating three of its interface residues (W328E, R329T, and R665N). The activity of the monomer was significantly lower than the activity of the wild-type dimeric AguA, and the optimal temperature for activity of the monomer was around 35 degrees C, compared to 65 degrees C of the wild-type enzyme. Nevertheless, the melting temperature of the monomeric protein, 72.9 degrees C, was almost identical to that of the wild-type, 73.4 degrees C. It appears that the dimerization of AguA is essential for efficient catalysis and that the dissociation into monomers results in subtle conformational changes in the structure which indirectly influence the active site region and reduce the activity. Structural and mechanistic explanations for these effects are discussed.

Mechanism-based Fluorescent Labeling of β-Galactosidases

(4-N-5-Dimethylaminonaphthalene-1-sulfonyl-2-difluoromethylphenyl)-beta-d-galactopyranoside was synthesized and successfully tested on beta-galactosidases from Xanthomonas manihotis (Wong-Madden, S. T., and Landry, D. Glycobiology (1995) 5, 19-28 and Taron, C. H., Benner, J. S., Hornstra, L. J., and Guthrie, E. P. (1995) Glycobiology 5, 603-610), Escherichia coli (Jacobson, R. H., Zhang, X. J., DuBose, R. F., and Matthews, B. W. (1994) Nature 369, 761-766), and Bacillus circulans (Fujimoto, H., Miyasato, M., Ito, Y., Sasaki, T., and Ajisaka, K. (1988) Glycoconj. J. 15, 155-160) for the rapid identification of the catalytic site. Reaction of the irreversible inhibitor with enzymes proceeded to afford a fluorescence-labeled protein suitable for further high throughput characterization by using antidansyl antibody and matrix-assisted laser desorption ionization time-of-flight/time-of-flight (MALDI-TOF/TOF). Specific probing by a fluorescent aglycon greatly facilitated identification of the labeled peptide fragments from beta-galactosidases. It was demonstrated by using X. manihotis beta-galactosidase that the Arg-58 residue, which is located within a sequence of 56IPRAYWKD63, was labeled by nucleophilic attack of the guanidinyl group. This sequence including Arg-58 (Leu-46 to Tyr-194) was similar to that (Met-1 to Tyr-151) of Thermus thermophilus A4, which is the first known structure of glycoside hydrolases family 42 (Hidaka, M., Fushinobu, S., Ohtsu, N., Motoshima, H., Matsuzawa, H., Shoun, H., and Wakagi, T. (2002) J. Mol. Biol. 322, 79-91). A catalytic glutamic acid (Glu-537) of E. coli beta-galactosidase was proved to be labeled by the same procedure, suggesting that the modification site with this irreversible substrate might depend both on the nucleophilicity of the amino acids and their spatial arrangement in the individual catalytic cavity. Similarly, a Glu-259 in 257TLEE260 was selectively labeled using B. circulans beta-galactosidase, indicating that Glu-259 is one of the nucleophiles in the active site. The present method can be readily extended to other glycosidases and should greatly aid the high throughput proteomics of many glycoside hydrolases showing both retaining- and inverting-type mechanisms.

Expressed sequence tag (EST) analysis of Gregarine gametocyst development

Gregarines are protozoan parasites of invertebrates in the phylum Apicomplexa. We employed an expressed sequence tag strategy in order to dissect the molecular processes of sexual or gametocyst development of gregarines. Expressed sequence tags provide a rapid way to identify genes, particularly in organisms for which we have very little molecular information. Analysis of approximately 1800 expressed sequence tags from the gametocyst stage revealed highly expressed genes related to cell division and differentiation. Evidence was found for the role of degradation and recycling in gametocyst development. Numerous additional genes uncovered by expressed sequence tag sequencing should provide valuable tools to investigate gametocyst development as well as for molecular phylogenetics, and comparative genomics in this important group of parasites.

Crystal Complex Structures Reveal How Substrate is Bound in the −4 to the +2 Binding Sites of Humicola grisea Cel12A

As part of an ongoing enzyme discovery program to investigate the properties and catalytic mechanism of glycoside hydrolase family 12 (GH 12) endoglucanases, a GH family that contains several cellulases that are of interest in industrial applications, we have solved four new crystal structures of wild-type Humicola grisea Cel12A in complexes formed by soaking with cellobiose, cellotetraose, cellopentaose, and a thio-linked cellotetraose derivative (G2SG2). These complex structures allow mapping of the non-covalent interactions between the enzyme and the glucosyl chain bound in subsites -4 to +2 of the enzyme, and shed light on the mechanism and function of GH 12 cellulases. The unhydrolysed cellopentaose and the G2SG2 cello-oligomers span the active site of the catalytically active H.grisea Cel12A enzyme, with the pyranoside bound in subsite -1 displaying a S31 skew boat conformation. After soaking in cellotetraose, the cello-oligomer that is found bound in site -4 to -1 contains a beta-1,3-linkage between the two cellobiose units in the oligomer, which is believed to have been formed by a transglycosylation reaction that has occurred during the ligand soak of the protein crystals. The close fit of this ligand and the binding sites occupied suggest a novel mixed beta-glucanase activity for this enzyme.

Recognition of Fold and Sugar Linkage for Glycosyltransferases by Multivariate Sequence Analysis

Glycosyltransferases (GTs) are among the largest groups of enzymes found and are usually classified on the basis of sequence comparisons into many families of varying similarity (CAZy systematics). Only two different Rossman-like folds have been detected (GT-A and GT-B) within the small number of established crystal structures. A third uncharacterized fold has been indicated with transmembrane organization (GT-C). We here use a method based on multivariate data analyses (MVDAs) of property patterns in amino acid sequences and can with high accuracy recognize the correct fold in a large data set of GTs. Likewise, a retaining or inverting enzymatic mechanism for attachment of the donor sugar could be properly revealed in the GT-A and GT-B fold group sequences by such analyses. Sequence alignments could be correlated to important variables in MVDA, and the separating amino acid positions could be mapped over the active sites. These seem to be localized to similar positions in space for the alpha/beta/alpha binding motifs in the GT-B fold group structures. Analogous, active-site sequence positions were found for the GT-A fold group. Multivariate property patterns could also easily group most GTs annotated in the genomes of Escherichia coli and Synechocystis to proper fold or organization group, according to benchmarking comparisons at the MetaServer. We conclude that the sequence property patterns revealed by the multivariate analyses seem more conserved than amino acid types for these GT groups, and these patterns are also conserved in the structures. Such patterns may also potentially define substrate preferences.

Quantification of uridine 5'-diphosphate (UDP)-glucose by high-performance liquid chromatography and its application to a nonradioactive assay for nucleoside diphosphate kinase using UDP-glucose pyrophosphorylase as a coupling enzyme

We describe a method for the detection and quantification of nucleoside diphosphate kinase (NDPK). NDPK catalyzes the transfer of the gamma-phosphate of cytidine 5'-triphosphate on uridine 5'-diphosphate (UDP) to produce uridine 5'-triphosphate (UTP). The method uses a nonradioactive coupled enzyme assay in which UTP produced by NDPK is utilized by UDP-glucose pyrophosphorylase. This latter enzyme synthesizes UDP-glucose and inorganic phosphate in the presence of glucose 1-phosphate. UDP-glucose is detected at 260 nm after separation of the reaction mixture by high-performance liquid chromatography (HPLC) on a strong anion-exchange column. The assay is reliable, specific, and linear with respect to time and enzyme amount. Using 15 min incubation time, the method allows detection of NDPK activity below 10 pmol/min. It can be used to analyze kinetic behavior and to quantify NDPK from a wide variety of animal, microbial, and plant sources. It also provides an alternative to radiometric assays and an improvement on pyruvate kinase-linked spectrophotometric assays, which can be hampered by pigments present in crude extracts. Furthermore, we show that the HPLC method developed here can be directly used to assay enzymes for which UDP-glucose is a product.

A putative double role of a chitinase in a cnidarian: pattern formation and immunity

Chitinases are enzymes that degrade chitin, the second most abundant polymer in nature. They are ubiquitous among living organisms where they play a role in development, food-digestion and innate immunity. We have cloned and characterized the first cnidarian chitinase cDNA from the hydroid Hydractinia. The Hydractinia chitinase exhibits a typical secreted family 18 hydrolases primary structure. In situ hybridization and RT-PCR experiments showed that it is exclusively expressed in ectodermal tissues of the animal, only following metamorphosis while undetectable in embryonic and larval stages. Most prominent expression was observed in the stolonal compartment of colonies, structures that are covered by a chitinous periderm. Chitinase mRNA was detected in new branching points along stolons and in hyperplastic stolons indicating a role of the enzyme in pattern formation and allorecognition. It was also expressed in polyps where it was mostly restricted to their basal portion. This expression pattern suggests that HyChit1 also fulfills a role in host defense, probably against fungal and nematode pathogens. Endodermal expression of HyChit1 has never been observed, suggesting that the enzyme does not participate in food-digestion.

A novel family of carbohydrate-binding modules identified with Ruminococcus albus proteins

We recently showed that some of the enzymes underpinning cellulose solubilization by Ruminococcus albus 8 lack the conventional type of dockerin module characteristic of cellulosomal proteins and instead, bear an "X" domain of unknown function at their C-termini. We have now subcloned and expressed six X domains and showed that five of them bind to xylan, chitin, microcrystalline and phosphoric-acid swollen cellulose, as well as more heterogenous substrates such as alfalfa cell walls, banana stem and wheat straw. The X domain that did not bind to these substrates was derived from a family-5 glycoside hydrolase (Cel5G), which possesses two X domains in tandem. Whereas the internal X domain failed to bind to the substrates, the recombinant dyad exhibited markedly enhanced binding relative to that observed for the C-terminal X domain alone. The evidence supports a distinctive carbohydrate-binding role of broad specificity for this type of domain, and we propose a novel family (designated family 37) of carbohydrate-binding modules that appear to be peculiar to R. albus.

The Humicola grisea Cel12A enzyme structure at 1.2 A resolution and the impact of its free cysteine residues on thermal stability

As part of a program to discover improved glycoside hydrolase family 12 (GH 12) endoglucanases, we have extended our previous work on the structural and biochemical diversity of GH 12 homologs to include the most stable fungal GH 12 found, Humicola grisea Cel12A. The H. grisea enzyme was much more stable to irreversible thermal denaturation than the Trichoderma reesei enzyme. It had an apparent denaturation midpoint (T(m)) of 68.7 degrees C, 14.3 degrees C higher than the T. reesei enzyme. There are an additional three cysteines found in the H. grisea Cel12A enzyme. To determine their importance for thermal stability, we constructed three H. grisea Cel12A single mutants in which these cysteines were exchanged with the corresponding residues in the T. reesei enzyme. We also introduced these cysteine residues into the T. reesei enzyme. The thermal stability of these variants was determined. Substitutions at any of the three positions affected stability, with the largest effect seen in H. grisea C206P, which has a T(m) 9.1 degrees C lower than that of the wild type. The T. reesei cysteine variant that gave the largest increase in stability, with a T(m) 3.9 degrees C higher than wild type, was the P201C mutation, the converse of the destabilizing C206P mutation in H. grisea. To help rationalize the results, we have determined the crystal structure of the H. grisea enzyme and of the most stable T. reesei cysteine variant, P201C. The three cysteines in H. grisea Cel12A play an important role in the thermal stability of this protein, although they are not involved in a disulfide bond.

Ab Initio Structure Determination and Functional Characterization Of CBM36

The enzymatic degradation of polysaccharides harnesses multimodular enzymes whose carbohydrate binding modules (CBM) target the catalytic domain onto the recalcitrant substrate. Here we report the ab initio structure determination and subsequent refinement, at 0.8 A resolution, of the CBM36 domain of the Paenibacillus polymyxa xylanase 43A. Affinity electrophoresis, isothermal titration calorimetry, and UV difference spectroscopy demonstrate that CBM36 is a novel Ca(2+)-dependent xylan binding domain. The 3D structure of CBM36 in complex with xylotriose and Ca(2+), at 1.5 A resolution, displays significant conformational changes compared to the native structure and reveals the molecular basis for its unique Ca(2+)-dependent binding of xylooligosaccharides through coordination of the O2 and O3 hydroxyls. CBM36 is one of an emerging spectrum of carbohydrate binding modules that increasingly find applications in industry and display great potential for mapping the "glyco-architecture" of plant cells.

The crystal structure of the family 6 carbohydrate binding module from Cellvibrio mixtus endoglucanase 5a in complex with oligosaccharides reveals two distinct binding sites with different ligand specificities

Glycoside hydrolases that release fixed carbon from the plant cell wall are of considerable biological and industrial importance. These hydrolases contain non-catalytic carbohydrate binding modules (CBMs) that, by bringing the appended catalytic domain into intimate association with its insoluble substrate, greatly potentiate catalysis. Family 6 CBMs (CBM6) are highly unusual because they contain two distinct clefts (cleft A and cleft B) that potentially can function as binding sites. Henshaw et al. (Henshaw, J., Bolam, D. N., Pires, V. M. R., Czjzek, M., Henrissat, B., Ferreira, L. M. A., Fontes, C. M. G. A., and Gilbert, H. J. (2003) J. Biol. Chem. 279, 21552-21559) show that CmCBM6 contains two binding sites that display both similarities and differences in their ligand specificity. Here we report the crystal structure of CmCBM6 in complex with a variety of ligands that reveals the structural basis for the ligand specificity displayed by this protein. In cleft A the two faces of the terminal sugars of beta-linked oligosaccharides stack against Trp-92 and Tyr-33, whereas the rest of the binding cleft is blocked by Glu-20 and Thr-23, residues that are not present in CBM6 proteins that bind to the internal regions of polysaccharides in cleft A. Cleft B is solvent-exposed and, therefore, able to bind ligands because the loop, which occludes this region in other CBM6 proteins, is much shorter and flexible (lacks a conserved proline) in CmCBM6. Subsites 2 and 3 of cleft B accommodate cellobiose (Glc-beta-1,4-Glc), subsite 4 will bind only to a beta-1,3-linked glucose, whereas subsite 1 can interact with either a beta-1,3- or beta-1,4-linked glucose. These different specificities of the subsites explain how cleft B can accommodate beta-1,4-beta-1,3- or beta-1,3-beta-1,4-linked gluco-configured ligands.

Molecular analysis of 10 coding regions from Arabidopsis that are homologous to the MUR3 xyloglucan galactosyltransferase

Plant cell walls are composed of a large number of complex polysaccharides, which contain at least 13 different monosaccharides in a multitude of linkages. This structural complexity of cell wall components is paralleled by a large number of predicted glycosyltransferases in plant genomes, which can be grouped into several distinct families based on conserved sequence motifs (B. Henrissat, G.J. Davies [2000] Plant Physiol 124: 1515-1519). Despite the wealth of genomic information in Arabidopsis and several crop plants, the biochemical functions of these coding regions have only been established in a few cases. To lay the foundation for the genetic and biochemical characterization of putative glycosyltransferase genes, we conducted a phylogenetic and expression analysis on 10 predicted coding regions (AtGT11-20) that are closely related to the MUR3 xyloglucan galactosyltransferase of Arabidopsis. All of these proteins contain the conserved sequence motif pfam 03016 that is the hallmark of the beta-d-glucuronosyltransferase domain of exostosins, a class of animal enzymes involved in the biosynthesis of the extracellular polysaccharide heparan sulfate. Reverse transcriptase-polymerase chain reaction and promoter:beta-glucuronidase studies indicate that all AtGT genes are transcribed. Although six of the 10 AtGT genes were expressed in all major plant organs, the remaining four genes showed more restricted expression patterns that were either confined to specific organs or to highly specialized cell types such as hydathodes or pollen grains. T-DNA insertion mutants in AtGT13 and AtGT18 displayed reductions in the Gal content of total cell wall material, suggesting that the disrupted genes encode galactosyltransferases in plant cell wall synthesis.

Molecular cloning and characterization of a cDNA encoding the N-acetyl-β-D-glucosaminidase homologue ofParacoccidioidesbrasiliensis

A cDNA encoding the N-acetyl-beta-D-glucosaminidase (NAG) protein of Paracoccidioides brasiliensis, Pb NAG1, was cloned and characterized. The 2663-nucleotide sequence of the cDNA consisted of a single open reading frame encoding a protein with a predicted molecular mass of 64.73 kDa and an isoeletric point of 6.35. The predicted protein includes a putative 30-amino-acid signal peptide. The protein as a whole shares considerable sequence similarity with 'classic' NAG. The primary sequence of Pb NAG1 was used to infer phylogenetic relationships. The amino acid sequence of Pb NAG1 has 45, 31 and 30% identity, respectively, with homologous sequences from Trichoderma harzianum, Aspergillus nidulans and Candida albicans. In particular, striking homology was observed with the active site regions of the glycosyl hydrolase group of proteins (family 20). The expected active site consensus motif G X D E and catalytic Asp and Glu residues at positions 373 and 374 were found, reinforcing that Pb NAG1 belongs to glycosyl hydrolase family 20. The nucleotide sequence of Pb nag1 and its flanking regions have been deposited, along with the amino acid sequence of the deduced protein, in GenBank under accession number AF419158.

Family 18 chitinase-oligosaccharide substrate interaction: subsite preference and anomer selectivity of Serratia marcescens chitinase A

The sizes and anomers of the products formed during the hydrolysis of chitin oligosaccharides by the Family 18 chitinase A (ChiA) from Serratia marcescens were analysed by hydrophilic interaction chromatography using a novel approach in which reactions were performed at 0 degrees C to stabilize the anomer conformations of the initial products. Crystallographic studies of the enzyme, having the structure of the complex of the ChiA E315L (Glu315-->Leu) mutant with a hexasaccharide, show that the oligosaccharide occupies subsites -4 to +2 in the substrate-binding cleft, consistent with the processing of beta-chitin by the release of disaccharide at the reducing end. Products of the hydrolysis of hexa- and penta-saccharides by wild-type ChiA, as well as by two mutants of the residues Trp275 and Phe396 important in binding the substrate at the +1 and +2 sites, show that the substrates only occupy sites -2 to +2 and that additional N -acetyl-D-glucosamines extend beyond the substrate-binding cleft at the reducing end. The subsites -3 and -4 are not used in this four-site binding mode. The explanation for these results is found in the high importance of individual binding sites for the processing of short oligosaccharides compared with the cumulative recognition and processive hydrolysis mechanism used to digest natural beta-chitin.

Identification and analysis of catalytic TIM barrel domains in seven further glycoside hydrolase families

Fold recognition results allocate catalytic triose phosphate isomerase (TIM) barrels to seven previously unassigned glycoside hydrolase (GH) families, numbers 29, 44, 50, 71, 84, 85 and 89, enabling prediction of catalytic residues. Modelling of GH family 50 suggests that it may be the common evolutionary ancestor of families 42 and 14. TIM barrels now comprise the catalytic domains of more than half of the assigned GH families, and catalyse a much larger variety of GH reactions than any other catalytic domain architecture. Only 327 GH sequences still have no structurally identified catalytic domain.

Structural and thermodynamic dissection of specific mannan recognition by a carbohydrate binding module, TmCBM27

The C-terminal 176 amino acids of a Thermotoga maritima mannanase (Man5) constitute a carbohydrate binding module (CBM) that has been classified into CBM family 27. The isolated CBM27 domain, named TmCBM27, binds tightly (K(a)s 10(5)-10(6) M(-1)) to beta-1, 4-mannooligosaccharides, carob galactomannan, and konjac glucomannan, but not to cellulose (insoluble and soluble) or soluble birchwood xylan. The X-ray crystal structures of native TmCBM27, a TmCBM27-mannohexaose complex, and a TmCBM27-6(3),6(4)-alpha-D-galactosyl-mannopentaose complex at 2.0 A, 1.6 A, and 1.35 A, respectively, reveal the basis of TmCBM27's specificity for mannans. In particular, the latter complex, which is the first structure of a CBM in complex with a branched plant cell wall polysaccharide, illustrates how the architecture of the binding site can influence the recognition of naturally substituted polysaccharides.

Microbial hemicellulases

Hemicellulases are a diverse group of enzymes that hydrolyze hemicelluloses--one of the most abundant groups of polysaccharide in nature. These enzymes have many biotechnological applications and their structure/function relationships are a subject of intense research. During the past year, new high-resolution structures of catalytic and non-catalytic domains of hemicellulases have been elucidated, and, together with biochemical studies, they reveal the principles of catalysis and specificity for these enzymes.

Crystal structure of rice alpha-galactosidase complexed with D-galactose

alpha-Galactosidases catalyze the hydrolysis of alpha-1,6-linked galactosyl residues from galacto-oligosaccharides and polymeric galacto-(gluco)mannans. The crystal structure of rice alpha-galactosidase has been determined at 1.5A resolution using the multiple isomorphous replacement method. The structure consisted of a catalytic domain and a C-terminal domain and was essentially the same as that of alpha-N-acetylgalactosaminidase, which is the same member of glycosyl hydrolase family 27. The catalytic domain had a (beta/alpha)8-barrel structure, and the C-terminal domain was made up of eight beta-strands containing a Greek key motif. The structure was solved as a complex with d-galactose, providing a mode of substrate binding in detail. The d-galactose molecule was found bound in the active site pocket on the C-terminal side of the central beta-barrel of the catalytic domain. The d-galactose molecule consisted of a mixture of two anomers present in a ratio equal to their natural abundance. Structural comparisons of rice alpha-galactosidase with chicken alpha-N-acetylgalactosaminidase provided further understanding of the substrate recognition mechanism in these enzymes.

An evolving hierarchical family classification for glycosyltransferases

Glycosyltransferases are a ubiquitous group of enzymes that catalyse the transfer of a sugar moiety from an activated sugar donor onto saccharide or non-saccharide acceptors. Although many glycosyltransferases catalyse chemically similar reactions, presumably through transition states with substantial oxocarbenium ion character, they display remarkable diversity in their donor, acceptor and product specificity and thereby generate a potentially infinite number of glycoconjugates, oligo- and polysaccharides. We have performed a comprehensive survey of glycosyltransferase-related sequences (over 7200 to date) and present here a classification of these enzymes akin to that proposed previously for glycoside hydrolases, into a hierarchical system of families, clans, and folds. This evolving classification rationalises structural and mechanistic investigation, harnesses information from a wide variety of related enzymes to inform cell biology and overcomes recurrent problems in the functional prediction of glycosyltransferase-related open-reading frames.

Comparison of family 12 glycoside hydrolases and recruited substitutions important for thermal stability

As part of a program to discover improved glycoside hydrolase family 12 (GH 12) endoglucanases, we have studied the biochemical diversity of several GH 12 homologs. The H. schweinitzii Cel12A enzyme differs from the T. reesei Cel12A enzyme by only 14 amino acids (93% sequence identity), but is much less thermally stable. The bacterial Cel12A enzyme from S. sp. 11AG8 shares only 28% sequence identity to the T. reesei enzyme, and is much more thermally stable. Each of the 14 sequence differences from H. schweinitzii Cel12A were introduced in T. reesei Cel12A to determine the effect of these amino acid substitutions on enzyme stability. Several of the T. reesei Cel12A variants were found to have increased stability, and the differences in apparent midpoint of thermal denaturation (T(m)) ranged from a 2.5 degrees C increase to a 4.0 degrees C decrease. The least stable recruitment from H. schweinitzii Cel12A was A35S. Consequently, the A35V substitution was recruited from the more stable S. sp. 11AG8 Cel12A and this T. reesei Cel12A variant was found to have a T(m) 7.7 degrees C higher than wild type. Thus, the buried residue at position 35 was shown to be of critical importance for thermal stability in this structural family. There was a ninefold range in the specific activities of the Cel12 homologs on o-NPC. The most and least stable T. reesei Cel12A variants, A35V and A35S, respectively, were fully active. Because of their thermal tolerance, S. sp. 11AG8 Cel12A and T. reesei Cel12A variant A35V showed a continual increase in activity over the temperature range of 25 degrees C to 60 degrees C, whereas the less stable enzymes T. reesei Cel12A wild type and the destabilized A35S variant, and H. schweinitzii Cel12A showed a decrease in activity at the highest temperatures. The crystal structures of the H. schweinitzii, S. sp. 11AG8, and T. reesei A35V Cel12A enzymes have been determined and compared with the wild-type T. reesei Cel12A enzyme. All of the structures have similar Calpha traces, but provide detailed insight into the nature of the stability differences. These results are an example of the power of homolog recruitment as a method for identifying residues important for stability.

Dynamic fluorescence studies of beta-glycosidase mutants from Sulfolobus solfataricus: effects of single mutations on protein thermostability

Multiple sequence alignment on 73 proteins belonging to glycosyl hydrolase family 1 reveals the occurrence of a segment (83-124) in the enzyme sequences from hyperthermophilic archaea bacteria, which is absent in all the mesophilic members of the family. The alignment of the known three-dimensional structures of hyperthermophilic glycosidases with the known ones from mesophilic organisms shows a similar spatial organizations of beta-glycosidases except for this sequence segment whose structure is located on the external surface of each of four identical subunits, where it overlaps two alpha-helices. Site-directed mutagenesis substituting N97 or S101 with a cysteine residue in the sequence of beta-glycosidase from hyperthermophilic archaeon Sulfolobus solfataricus caused some changes in the structural and dynamic properties as observed by circular dichroism in far- and near-UV light, as well as by frequency domain fluorometry, with a simultaneous loss of thermostability. The results led us to hypothesize an important role of the sequence segment present only in hyperthermophilic beta-glycosidases, in the thermal adaptation of archaea beta-glycosidases. The thermostabilization mechanism could occur as a consequence of numerous favorable ionic interactions of the 83-124 sequence with the other part of protein matrix that becomes more rigid and less accessible to the insult of thermal-activated solvent molecules.