Characterization of a buried neutral histidine residue in Bacillus circulans xylanase: NMR assignments, pH titration, and hydrogen exchange.

Bacillus circulans xylanase contains two histidines, one of which (His 156) is solvent exposed, whereas the other (His 149) is buried within its hydrophobic core. His 149 is involved in a network of hydrogen bonds with an internal water and Ser 130, as well as a potential weak aromatic-aromatic interaction with Tyr 105. These three residues, and their network of interactions with the bound water, are conserved in four homologous xylanases. To probe the structural role played by His 149, NMR spectroscopy was used to characterize the histidines in BCX. Complete assignments of the 1H, 13C, and 15N resonances and tautomeric forms of the imidazole rings were obtained from two-dimensional heteronuclear correlation experiments. An unusual spectroscopic feature of BCX is a peak near 12 ppm arising from the nitrogen bonded 1H epsilon 2 of His 149. Due to its solvent inaccessibility and hydrogen bonding to an internal water molecule, the exchange rate of this proton (4.0 x 10(-5) s-1 at pH*7.04 and 30 degrees C) is retarded by > 10(6)-fold relative to an exposed histidine. The pKa of His 156 is unperturbed at approximately 6.5, as measured from the pH dependence of the 15N- and 1H-NMR spectra of BCX. In contrast, His 149 has a pKa < 2.3, existing in the neutral N epsilon 2H tautomeric state under all conditions examined. BCX unfolds at low pH and 30 degrees C, and thus His 149 is never protonated significantly in the context of the native enzyme. The structural importance of this buried histidine is confirmed by the destablizing effect of substituting a phenylalanine or glutamine at position 149 in BCX.     

Analysis of the dynamic properties of Bacillus circulans xylanase upon formation of a covalent glycosyl-enzyme intermediate

NMR spectroscopy was used to search for mechanistically significant differences in the local mobility of the main-chain amides of Bacillus circulans xylanase (BCX) in its native and catalytically competent covalent glycosyl-enzyme intermediate states. 15N T1, T2, and 15N[1H] NOE values were measured for approximately 120 out of 178 peptide groups in both the apo form of the protein and in BCX covalently modified at position Glu78 with a mechanism-based 2-deoxy-2-fluoro-beta-xylobioside inactivator. Employing the model-free formalism of Lipari and Szabo, the measured relaxation parameters were used to calculate a global correlation time (tau(m)) for the protein in each form (9.2 +/- 0.2 ns for apo-BCX; 9.8 +/- 0.3 ns for the modified protein), as well as individual order parameters for the main-chain NH bond vectors. Average values of the order parameters for the protein in the apo and complexed forms were S2 = 0.86 +/- 0.04 and S2 = 0.91 +/- 0.04, respectively. No correlation is observed between these order parameters and the secondary structure, solvent accessibility, or hydrogen bonding patterns of amides in either form of the protein. These results demonstrate that the backbone of BCX is well ordered in both states and that formation of the glycosyl-enzyme intermediate leads to little change, in any, in the dynamic properties of BCX on the time scales sampled by 15N-NMR relaxation measurements.     

Complete measurement of the pKa values of the carboxyl and imidazole groups in Bacillus circulans xylanase.

Electrostatic interactions in proteins can be dissected experimentally by determining the pKa values of their constituent ionizable amino acids. To complement previous studies of the glutamic acid and histidine residues in Bacillus circulans xylanase (BCX), we have used NMR methods to measure the pKa s of the seven aspartic acids and the C-terminus of this protein. The pKa s of these carboxyls are all less than the corresponding values observed with random coil polypeptides, indicating that their ionization contributes favorably to the stability of the folded enzyme. In general, the aspartic acids with the most reduced pKa s are those with limited exposure to the solvent and a high degree of conservation among homologous xylanases. Most dramatically, Asp 83 and Asp 101 have pKa s < 2 and thus remain deprotonated in native BCX under all conditions examined. Asp 83 is completely buried, forming a strong salt bridge with Arg 136. In contrast, Asp 101 is located on the surface of the protein, stabilized in the deprotonated form by an extensive network of hydrogen bonds involving an internal water molecule and the neutral side-chain and main-chain atoms of Ser 100 and Thr 145. These data provide a complete experimental database for theoretical studies of the ionization behavior of BCX under acidic conditions.     

Dissecting the electrostatic interactions and pH-dependent activity of a family 11 glycosidase

Previous studies of the low molecular mass family 11 xylanase from Bacillus circulans show that the ionization state of the nucleophile (Glu78, pK(a) 4.6) and the acid/base catalyst (Glu172, pK(a) 6.7) gives rise to its pH-dependent activity profile. Inspection of the crystal structure of BCX reveals that Glu78 and Glu172 are in very similar environments and are surrounded by several chemically equivalent and highly conserved active site residues. Hence, there are no obvious reasons why their apparent pK(a) values are different. To address this question, a mutagenic approach was implemented to determine what features establish the pK(a) values (measured directly by (13)C NMR and indirectly by pH-dependent activity profiles) of these two catalytic carboxylic acids. Analysis of several BCX variants indicates that the ionized form of Glu78 is preferentially stabilized over that of Glu172 in part by stronger hydrogen bonds contributed by two well-ordered residues, namely, Tyr69 and Gln127. In addition, theoretical pK(a) calculations show that Glu78 has a lower pK(a) value than Glu172 due to a smaller desolvation energy and more favorable background interactions with permanent partial charges and ionizable groups within the protein. The pK(a) value of Glu172 is in turn elevated due to electrostatic repulsion from the negatively charged glutamate at position 78. The results also indicate that all of the conserved active site residues act concertedly in establishing the pK(a) values of Glu78 and Glu172, with no particular residue being singly more important than any of the others. In general, residues that contribute positive charges and hydrogen bonds serve to lower the pK(a) values of Glu78 and Glu172. The degree to which a hydrogen bond lowers a pK(a) value is largely dependent on the length of the hydrogen bond (shorter bonds lower pK(a) values more) and the chemical nature of the donor (COOH > OH > CONH(2)). In contrast, neighboring carboxyl groups can either lower or raise the pK(a) values of the catalytic glutamic acids depending upon the electrostatic linkage of the ionization constants of the residues involved in the interaction. While the pH optimum of BCX can be shifted from -1.1 to +0.6 pH units by mutating neighboring residues within the active site, activity is usually compromised due to the loss of important ground and/or transition state interactions. These results suggest that the pH optima of an enzyme might be best engineered by making strategic amino acid substitutions, at positions outside of the "core" active site, that electrostatically influence catalytic residues without perturbing their immediate structural environment.     

Hydrogen bonding and catalysis: a novel explanation for how a single amino acid substitution can change the pH optimum of a glycosidase

The pH optima of family 11 xylanases are well correlated with the nature of the residue adjacent to the acid/base catalyst. In xylanases that function optimally under acidic conditions, this residue is aspartic acid, whereas it is asparagine in those that function under more alkaline conditions. Previous studies of wild-type (WT) Bacillus circulans xylanase (BCX), with an asparagine residue at position 35, demonstrated that its pH-dependent activity follows the ionization states of the nucleophile Glu78 (pKa 4.6) and the acid/base catalyst Glu172 (pKa 6.7). As predicted from sequence comparisons, substitution of this asparagine residue with an aspartic acid residue (N35D BCX) shifts its pH optimum from 5.7 to 4.6, with an approximately 20% increase in activity. The bell-shaped pH-activity profile of this mutant enzyme follows apparent pKa values of 3.5 and 5.8. Based on 13C-NMR titrations, the predominant pKa values of its active-site carboxyl groups are 3.7 (Asp35), 5.7 (Glu78) and 8.4 (Glu172). Thus, in contrast to the WT enzyme, the pH-activity profile of N35D BCX appears to be set by Asp35 and Glu78. Mutational, kinetic, and structural studies of N35D BCX, both in its native and covalently modified 2-fluoro-xylobiosyl glycosyl-enzyme intermediate states, reveal that the xylanase still follows a double-displacement mechanism with Glu78 serving as the nucleophile. We therefore propose that Asp35 and Glu172 function together as the general acid/base catalyst, and that N35D BCX exhibits a "reverse protonation" mechanism in which it is catalytically active when Asp35, with the lower pKa, is protonated, while Glu78, with the higher pKa, is deprotonated. This implies that the mutant enzyme must have an inherent catalytic efficiency at least 100-fold higher than that of the parental WT, because only approximately 1% of its population is in the correct ionization state for catalysis at its pH optimum. The increased efficiency of N35D BCX, and by inference all "acidic" family 11 xylanases, is attributed to the formation of a short (2.7 A) hydrogen bond between Asp35 and Glu172, observed in the crystal structure of the glycosyl-enzyme intermediate of this enzyme, that will substantially stabilize the transition state for glycosyl transfer. Such a mechanism may be much more commonly employed than is generally realized, necessitating careful analysis of the pH-dependence of enzymatic catalysis.     

Xylanase homology modeling using the inverse protein folding approach.

Xylanase has been used in wood pulp bleaching in an effort to reduce chlorine release into the environment and pollution associated with paper production. The three-dimensional structure of xylanase is important to enable better understanding of the enzyme mechanism and to help design a more thermostable xylanase mutant. At the time this work was begun, there was no sequence homologous protein available for traditional sequence-based homology modeling. In order to circumvent this problem, the inverse protein folding approach was undertaken to find a suitable template structure. Model structures of Bacillus circulans xylanase were built based on the data-base search results of related proteins. The model structures were refined and compared to the recently solved xylanase X-ray crystal structure. The overall structural similarity between the theoretical model and experimental structure demonstrate the usefulness of this approach. Disagreement in folding topology, however, warrants further research into the inverse protein folding approach.     

MOTOR: Model assisted software for NMR structure determination

Eukaryotic proteins with important biological function can be partially unstructured, conformational flexible, or heterogenic. Crystallization trials often fail for such proteins. In NMR spectroscopy, parts of the polypeptide chain undergoing dynamics in unfavorable time regimes cannot be observed. De novo NMR structure determination is seriously hampered when missing signals lead to an incomplete chemical shift assignment resulting in an information content of the NOE data insufficient to determine the structure ab initio. We developed a new protein structure determination strategy for such cases based on a novel NOE assignment strategy utilizing a number of model structures but no explicit reference structure as it is used for bootstrapping like algorithms. The software distinguishes in detail between consistent and mutually exclusive pairs of possible NOE assignments on the basis of different precision levels of measured chemical shifts searching for a set of maximum number of consistent NOE assignments in agreement with 3D space. Validation of the method using the structure of the low molecular‐weight‐protein tyrosine phosphatase A (MptpA) showed robust results utilizing protein structures with 30–45% sequence identity and 70% of the chemical shift assignments. About 60% of the resonance assignments are sufficient to identify those structural models with highest conformational similarity to the real structure. The software was benchmarked by de novo solution structures of fibroblast growth factor 21 (FGF21) and the extracellular fibroblast growth factor receptor domain FGFR4 D2, which both failed in crystallization trials and in classical NMR structure determination. Proteins 2013; 81:2007–2022. © 2013 Wiley Periodicals, Inc.     

Hyperexpression of a Bacillus circulans xylanase gene in Escherichia coli and characterization of the gene product

A 4.0-kilobase (kb) fragment of Bacillus circulans genomic DNA inserted into pUC19 and encoding endoxylanase activity was subjected to a series of subclonings. A 1.0-kb HindIII-HincII subfragment was found to code for xylanase activity. Maximum expression levels were observed with a subclone that contained an additional 0.3-kb sequence upstream from the coding region. Enhancer sequences in the upstream region are thought to be responsible for these high expression levels. Southern hybridization analyses revealed that the cloned gene hybridized with genomic DNA from Bacillus subtilis and Bacillus polymyxa. Xylanase activity expressed by Escherichia coli harboring the cloned gene was located primarily in the intracellular fraction. Levels of up to 7 U/ml or 35 mg/liter were obtained. The protein product was purified by ion exchange and gel permeation chromatography. The xylanase had a molecular weight of 20,500 and an isoelectric point of 9.0.     

Hyperexpression of a Bacillus circulans xylanase gene in Escherichia coli and characterization of the gene product.

A 4.0-kilobase (kb) fragment of Bacillus circulans genomic DNA inserted into pUC19 and encoding endoxylanase activity was subjected to a series of subclonings. A 1.0-kb HindIII-HincII subfragment was found to code for xylanase activity. Maximum expression levels were observed with a subclone that contained an additional 0.3-kb sequence upstream from the coding region. Enhancer sequences in the upstream region are thought to be responsible for these high expression levels. Southern hybridization analyses revealed that the cloned gene hybridized with genomic DNA from Bacillus subtilis and Bacillus polymyxa. Xylanase activity expressed by Escherichia coli harboring the cloned gene was located primarily in the intracellular fraction. Levels of up to 7 U/ml or 35 mg/liter were obtained. The protein product was purified by ion exchange and gel permeation chromatography. The xylanase had a molecular weight of 20,500 and an isoelectric point of 9.0.     

Structure of the xylanase from Penicillium simplicissimum.

Despite its relatively low pH and temperature optimum, the xylanase from Penicillium simplicissimum performs exceedingly well under conditions of paper bleaching. We have purified and characterized this enzyme, which belongs to family 10 of glycosyl hydrolases. Its gene was cloned, and the sequence of the protein was deduced from the nucleotide sequence. The xylanase was crystallized from ammonium sulfate at pH 8.4, and X-ray data were collected at cryo-temperature to a crystallographic resolution of 1.75 A. The crystal structure was solved by molecular replacement using the catalytic domain of the Clostridium thermocellum xylanase as a search model, and refined to a residual of R = 20% (R(free) = 23%) for data between 10 and 1.75 A. The xylanase folds in an (alpha/beta)8 barrel (TIM-barrel), with additional helices and loops arranged at the "top" forming the active site cleft. In its overall shape, the P. simplicissimum xylanase structure is similar to other family 10 xylanases, but its active site cleft is much shallower and wider. This probably accounts for the differences in catalysis and in the mode of action of this enzyme. Three glycerol molecules were observed to bind within the active site groove, one of which interacts directly with the catalytic glutamate residues. It appears that they occupy putative xylose binding subsites.     

Solution structure of murine epidermal growth factor determined by NMR spectroscopy and refined by energy minimization with restraints.

The solution structure of murine epidermal growth factor (mEGF) at pH 3.1 and a temperature of 28 degrees C has been determined from NMR data, using distance geometry calculations and restrained energy minimization. The structure determination is based on 730 conformational constraints derived from NMR data, including 644 NOE-derived upper bound distance constraints, constraints on the ranges of 32 dihedral angles based on measurements of vicinal coupling constants, and 54 upper and lower bound constraints associated with nine hydrogen bonds and the three disulfide bonds. The distance geometry interpretation of the NMR data is based on previously published sequence-specific 1H resonance assignments [Montelione et al. (1988) Biochemistry 27, 2235-2243], supplemented here with individual assignments for some side-chain amide, methylene, and isopropyl methyl protons. The molecular architecture of mEGF is the same as that described previously [Montelione et al. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 5226-5230], but the structure is overall more precisely determined by a more extensive set of NMR constraints. Analysis of proton NMR line widths, amide proton exchange rates, and side-chain 3J(H alpha-H beta) coupling constants provides evidence for internal motion in several regions of the mEGF molecule. Because mEGF is one member of a large family of homologous growth factors and protein domains for which X-ray crystal structures are not yet available, the atomic coordinates resulting from the present structure refinement (which we have deposited in the Brookhaven Protein Data Bank) are important data for understanding the structures of EGF-like proteins and for further detailed comparisons of these structures with mEGF.     

Expression of Bacillus circulans Teri-42 xylanase gene in Bacillus subtilis

The xylanase gene of Bacillus circulans Teri-42 was cloned in both B. subtilis and Escherichia coli. The enzyme activity was almost 87% higher in B. subtilis (pBA7) than in E. coli (pAQ4). No cellulase activity was detected in the clones, B. subtilis (pBA7) and E. coli (pAQ4). Approximately 1120 U (80%) of the xylanase was secreted extracellularly by the clone B. subtilis (pBA7) as compared to 79 U (88%) excreted in E. coli (pAQ4). In B. subtilis (pBA7) the optimal xylanase activity was at pH 7.0 and 50 degrees C, which was the same as that of the parent B. circulans Teri-42. The recombinant xylanase in B. subtilis was more stable at higher temperatures than the parent B. circulans Teri-42. Purification of xylanase from the clone B. subtilis (pBA7) showed a 71 kDa polypeptide similar to that observed in B. circulans Teri-42.     

Continuum secondary structure captures protein flexibility

The DSSP program assigns protein secondary structure to one of eight states. This discrete assignment cannot describe the continuum of thermal fluctuations. Hence, a continuous assignment is proposed. Technically, the continuum results from averaging over ten discrete DSSP assignments with different hydrogen bond thresholds. The final continuous assignment for a single NMR model successfully reflected the structural variations observed between all NMR models in the ensemble. The structural variations between NMR models were verified to correlate with thermal motion; these variations were captured by the continuous assignments. Because the continuous assignment reproduces the structural variation between many NMR models from one single model, functionally important variation can be extracted from a single X-ray structure. Thus, continuous assignments of secondary structure may affect future protein structure analysis, comparison, and prediction.     

A systematic mutagenesis-driven strategy for site-resolved NMR studies of supramolecular assemblies

Obtaining sequence-specific assignments remains a major bottleneck in solution NMR investigations of supramolecular structure, dynamics and interactions. Here we demonstrate that resonance assignment of methyl probes in high molecular weight protein assemblies can be efficiently achieved by combining fast NMR experiments, residue-type-specific isotope-labeling and automated site-directed mutagenesis. The utility of this general and straightforward strategy is demonstrated through the characterization of intermolecular interactions involving a 468-kDa multimeric aminopeptidase, PhTET2.     

Secondary structure of a calcium binding protein (CaBP) from Entamoeba histolytica.

A calcium binding protein from Entamoeba histolytica, (EhCaBP, M(r) approximately 15 kDa) is the causative agent for amoebiosis and has a very low sequence homology (approximately 30%) with other known CaBPs. Almost complete sequence specific resonance assignments for (1)H, (13)C and (15)N spins in EhCaBP were obtained using double and triple resonance NMR experiments. Qualitative interpretation of the nuclear Overhauser enhancements, chemical shift indices and of hydrogen exchange rates threw valuable light upon the secondary structure of this protein. CaBP is found to have two globular domains each of which consists of two pairs of helix-loop-helix motifs. Though this protein has a very small sequence homology with calmodulins, the topological arrangement of the alpha-helices and beta-strands in EhCaBP resemble them.     

In silico studies on bacterial xylanase enzyme: Structural and functional insight

Xylans are the second most abundant form of hemicelluloses and are the second most abundant polysaccharide in nature after cellulose. To degrade xylan, microbes produce mainly xylanase enzyme. Wide range of microorganisms like fungi, bacteria, yeast, marine algae etc. are capable of producing xylanase. Main source of xylanase is fungi but industrial production of bacterial xylanase is low cost, easy downstream process and high production rate. To understand primary, secondary and tertiary structure of xylanase, in silico composition of amino acids, basic physiological characteristics; viz., pI, molecular weight, instability index, GRAVY, molar extinction coefficient, secondary structure, presence of functional domain and motifs, phylogenetic tree, salt bridge compositions are determined. In silico study of xylanase focused on 36 different bacterial sources are performed by retrieving FASTA and PDB sequences using RCSB PDB. FASTA and PDB files are proceed further in ExPASy-ProtParam, RAMPAGE, QMEAN, MEME, PSIPRED, InterProScan, MOTIF scan, ERRAT, Peptide cutter, ESBRI and MEGA 7. The instability index range (16.90–38.78) clearly indicates that the protein is highly stable. α-helix mean value (27.11%) infers the protein is dominated by α-helix region. The aliphatic index (39.80–90.68) gives information that the protein is highly thermostable, prevalence by alanine amino acid in aliphatic side chain. No transmembrane domain was found in the protein which confirms the enzyme is extracellular in nature. Ancestor chart analysis confirmed that it is a part of carbohydrate metabolic process and more specifically a member of glycoside hydrolase super family.     

A novel family 8 xylanase,functional and physicochemical characterization

Xylanases are generally classified into glycosyl hydrolase families 10 and 11 and are found to frequently have an inverse relationship between their pI and molecular mass values. However, we have isolated a psychrophilic xylanase that belongs to family 8 and which has both a high pI and high molecular mass. This novel xylanase, isolated from the Antarctic bacterium Pseudoalteromonas haloplanktis, is not homologous to family 10 or 11 enzymes but has 20-30% identity with family 8 members. NMR analysis shows that this enzyme hydrolyzes with inversion of anomeric configuration, in contrast to other known xylanases which are retaining. No cellulase, chitosanase or lichenase activity was detected. It appears to be functionally similar to family 11 xylanases. It hydrolyzes xylan to principally xylotriose and xylotetraose and is most active on long chain xylo-oligosaccharides. Kinetic studies indicate that it has a large substrate binding cleft, containing at least six xylose-binding subsites. Typical psychrophilic characteristics of a high catalytic activity at low temperatures and low thermal stability are observed. An evolutionary tree of family 8 enzymes revealed the presence of six distinct clusters. Indeed classification in family 8 would suggest an (alpha/alpha)(6) fold, distinct from that of other currently known xylanases.     

1H NMR assignments of apo calcyclin and comparative structural analysis with calbindin D9k and S100 beta.

The homodimeric S100 protein calcyclin has been studied in the apo state by two-dimensional 1H NMR spectroscopy. Using a combination of scalar correlation and NOE experiments, sequence-specific 1H NMR assignments were obtained for all but one backbone and > 90% of the side-chain resonances. To our knowledge, the 2 x 90 residue (20 kDa) calcyclin dimer is the largest protein system for which such complete assignments have been made by purely homonuclear methods. Sequential and medium-range NOEs and slowly exchanging backbone amide protons identified directly the four helices and the short antiparallel beta-type interaction between the two binding loops that comprise each subunit of the dimer. Further analysis of NOEs enabled the unambiguous assignment of 556 intrasubunit distance constraints, 24 intrasubunit hydrogen bonding constraints, and 2 x 26 intersubunit distance constraints. The conformation of the monomer subunit was refined by distance geometry and restrained molecular dynamics calculations using the intrasubunit constraints only. Calculation of the dimer structure starting from this conformational ensemble has been reported elsewhere. The extent of structural homology among the apo calcyclin subunit, the monomer subunit of apo S100 beta, and monomeric apo calbindin D9k has been examined in detail by comparing 1H NMR chemical shifts and secondary structures. This analysis was extended to a comprehensive comparison of the three-dimensional structures of the calcyclin monomer subunit and calbindin D9k, which revealed greater similarity in the packing of their hydrophobic cores than was anticipated previously. Together, these results support the hypothesis that all members of the S100 family have similar core structures and similar modes of dimerization. Analysis of the amphiphilicity of Helix IV is used to explain why calbindin D9k is monomeric, but full-length S100 proteins form homodimers.     

New insights on the structure of algaenan from Botryoccocus braunii race A and its hexane insoluble botryals based on multidimensional NMR spectroscopy and electrospray-mass spectrometry techniques

Through solution state NMR spectroscopy and quadrupole-time-of-flight mass spectrometry (Q-TOF MS) studies of the hexane insoluble botryal extract of the algae Botryococcus braunii race A, coupled with high-resolution magic-angle-spinning (HR-MAS) NMR spectroscopy of the algaenan from this alga, it has been possible to advance the structural understanding of this geochemically important biopolymer. It was found that the hexane insoluble botryals in this study constitute a mixture of low molecular weight unsaturated aliphatic aldehydes and unsaturated hydrocarbons with an average chain length of about 40 carbons. Exact assignments were provided for many of the structural units present and describe the average constitution of the mixture as a whole. Reticulation of the low molecular weight chains via acetal and ester links explains the primary make up of algaenan. In this study, it is concluded that the algaenan results from the reticulation of low molecular weight hexane insoluble botryal species rather than the polyaldehyde as previously observed in studies of algae at alternate stages of their growth cycle.     

The crystal structure of beta-glucosidase from Bacillus circulans sp. alkalophilus: ability to form long polymeric assemblies

Family 1 of glycosyl hydrolases is a large and biologically important group of enzymes. A new three-dimensional structure of this family, beta-glucosidase from Bacillus circulans sp. alkalophilus is reported here. This is the first structure of beta-glucosidase from an alkaliphilic organism. The model was determined by the molecular replacement method and refined to a resolution of 2.7 A. The quaternary structure of B. circulans sp. alkalophilus beta-glucosidase is an octamer and subunits of the octamer show a similar (beta/alpha)(8) barrel fold to that previously reported for other family 1 enzymes. The crystal structure suggested that Cys169 in the active site is substituted. The Cys169 is located near the putative acid/base catalyst Glu166 and it may contribute to the high pH optimum of the enzyme. The crystal structure also revealed that the asymmetric unit contains two octamers which have a clear binding interaction with each other. The ability of the octamers to link with each other suggested that beta-glucosidase from Bacillus circulans sp. alkalophilus is able to form long polymeric assemblies, at least in the crystalline state.