Chemoenzymatic synthesis of a bifunctionalized cellohexaoside as a specific substrate for the sensitive assay of cellulase by fluorescence quenching

Measurement of filter paper activities of cellulase with microplate-based assay

It is always a challenge to determine the total cellulase activity efficiently without reducing accuracy. The most common total cellulase activity assay is the filter paper assay (FPA) established by the International Union of Pure and Applied Chemistry (IUPAC). A new procedure to measure the FPA with microplate-based assay was studied in this work, which followed the main idea of IUPAC to dilute cellulase preparation to get fixed glucose release. FPAs of six cellulase preparations were determined with the microplate-based assay. It is shown that FPAs of cellulase Youtell, RCconc, R-10, Lerkam, Yishui and Sinopharm were 67.9, 46.0, 46.1, 27.4, 7.6 and 8.0 IU/ml respectively. There was no significant difference at the 95% confidence level between the FPA determined with IUPAC and the microplate-based assay. It could be concluded that the FPA could be determined by the microplate-based assay with the same accuracy and much more efficiency compared with that by IUPAC.

Characterization of Glycosynthase Mutants Derived from Glycoside Hydrolase Family 10 Xylanases

Four xylanases belonging to glycoside hydrolase family 10-Thermotoga maritima XylB (TM), Clostridium stercorarium XynB (CS), Bacillus halodurans XynA (BH), and Cellulomonas fimi Cex (CF)-were converted to glycosynthases by substituting the nucleophilic glutamic acid residues with glycine, alanine, and serine. The glycine mutants exhibited the highest levels of glycosynthase activity with all four enzymes. All the glycine mutants formed polymeric beta-1,4-linked xylopyranose as a precipitate during reaction with alpha-xylobiosyl fluoride. Two glycine mutants (TM and CF) recognized X(2) as an effective acceptor molecule to prohibit the formation of the polymer, while the other two (CS and BH) did not. The difference in acceptor specificity is considered to reflect the difference in substrate affinity at their +2 subsites. The results agreed with the structural predictions of the subsite, where TM and CF exhibit high affinity at subsite 2, suggesting that the glycosynthase technique is useful for investigating the affinity of +subsites.

Glycan synthesis, structure, and dynamics: A selection

Glycan structural information is a prerequisite for elucidation of carbohydrate function in biological systems. To this end we employ a tripod approach for investigation of carbohydrate 3D structure and dynamics based on organic synthesis; different experimental spectroscopy techniques, NMR being of prime importance; and molecular simulations using, in particular, molecular dynamics (MD) simulations. The synthesis of oligosaccharides in the form of glucosyl fluorides is described, and their use as substrates for the Lam16A E115S glucosyl synthase is exemplified as well as a conformational analysis of a cyclic β-(1→3)-heptaglucan based on molecular simulations. The flexibility of the N-acetyl group of aminosugars is by MD simulations indicated to function as a gatekeeper for transitions of glycosidic torsion angles to other regions of conformational space. A novel approach to visualize glycoprotein (GP) structures is presented in which the protein is shown by, for example, ribbons, but instead of stick or space-filling models for the carbohydrate portion it is visualized by the colored geometrical figures known as CFG representation in a 3D way, which we denote 3D-CFG, thereby effectively highlighting the sugar residues of the glycan part of the GP and the position(s) on the protein.

Construction of a chimeric thermoacidophilic beta-endoglucanase

Background

The archeaon Sulfolobus solfataricus P2 encodes a thermoacidophilic cellulase which shows an extreme acid and thermal stability with a pH optimum at 1.8 and a temperature optimum at 80°C. This extraordinary enzyme could be useful for biotechnological exploitation but the expression and purification in expression hosts like E. coli is unsatisfactory due to the high aggregation tendency of the recombinant enzyme. The thermophilic cellulase CelA from Thermotoga maritima belongs to the same glycoside hydrolase family (GH12) but has a neutral pH optimum. In contrast to SSO1949 this enzyme is expressed partially soluble in E. coli.

Results

We aimed to constructed a hybrid enzyme based on these two beta-endoglucanases which should successfully combine the advantageous properties of both cellulases, i.e. recombinant expression in E. coli, acidophily and thermophily. We constructed two hybrid proteins after bioinformatic analysis: both hybrids are expressed insoluble in E. coli, but one hybrid enzyme was successfully refolded from washed inclusion bodies.

Conclusions

The refolded active chimeric enzyme shows a temperature optimum of approximately 85°C and a pH optimum of approximately pH 3 thus retaining the advantageous properties of the Sulfolobus parent enzyme. This study suggests that the targeted construction of chimeric enzymes is an alternative to point mutational engineering efforts as long as parent enzymes with the wanted properties are available.

Enzyme assays

Enzyme assays are analytical tools to visualize enzyme activities. In recent years a large variety of enzyme assays have been developed to assist the discovery and optimization of industrial enzymes, in particular for "white biotechnology" where selective enzymes are used with great success for economically viable, mild and environmentally benign production processes. The present article highlights the aspects of fluorogenic and chromogenic substrates, sensors, and enzyme fingerprinting, which are our particular areas of interest.

Glycosynthase activity of hybrid aspen xyloglucan endo-transglycosylase PttXET16-34 nucleophile mutants

Glycosynthases are active-site mutants of glycoside hydrolases that catalyse glycosyl transfer using suitable activated donor substrates without competing product hydrolysis (S. M. Hancock, M. D. Vaughan and S. G. Withers, Curr. Opin. Chem. Biol., 2006, 10, 509-519). Site-directed mutagenesis of the catalytic nucleophile, Glu-85, of a Populus tremula x tremuloides xyloglucan endo-transglycosylase (PttXET16-34, EC 2.4.1.207) into alanine, glycine, and serine yielded enzymes with glycosynthase activity. Product analysis indicated that PttXET16-34 E85A in particular was able to catalyse regio- and stereospecific homo- and hetero-condensations of alpha-xylogluco-oligosaccharyl fluoride donors XXXGalphaF and XLLGalphaF to produce xyloglucans with regular sidechain substitution patterns. This substrate promiscuity contrasts that of the Humicola insolens Cel7B E197A glycosynthase, which was not able to polymerise the di-galactosylated substrate XLLGalphaF. The production of the PttXET16-34 E85A xyloglucosynthase thus expands the repertoire of glycosynthases to include those capable of synthesising structurally homogenenous xyloglucans for applications.

In vitro synthesis of artificial polysaccharides by glycosidases and glycosynthases

Artificial polysaccharides produced by in vitro enzymatic synthesis are new biomaterials with defined structures that either mimic natural polysaccharides or have unnatural structures and functionalities. This review summarizes recent developments in the in vitro polysaccharide synthesis by endo-glycosidases, grouped in two major strategies: (a) native retaining endo-glycosidases under kinetically controlled conditions (transglycosylation with activated glycosyl donors), and (b) glycosynthases, engineered glycosidases devoid of hydrolase activity but with high transglycosylation activity. Polysaccharides are obtained by enzymatic polymerization of simple glycosyl donors by repetitive condensation. This approach not only provides a powerful methodology to produce polysaccharides with defined structures and morphologies as novel biomaterials, but is also a valuable tool to analyze the mechanisms of polymerization and packing to acquire high-order molecular assemblies.

Screening systems

Enzyme screening technology has undergone massive developments in recent years, particularly in the area of high-throughput screening and microarray methods. Screening consists of testing each sample of a sample library individually for the targeted reaction. This requires enzyme assays that accurately test relevant parameters of the reaction, such as catalytic turnover with a given substrate and selectivity parameters such as enantio- and regioselectivity. Enzyme assays also play an important role outside of enzyme screening, in particular for drug screening, medical diagnostics, and in the area of cellular and tissue imaging. In the 1990s, methods for high-throughput screening of enzyme activities were perceived as a critical bottleneck. As illustrated partly in this chapter, a large repertoire of efficient screening strategies are available today that allow testing of almost any reaction with high-throughput.

Unexpected regioselectivity of Humicola insolens Cel7B glycosynthase mutants

Four Humicola insolens Cel7B glycoside hydrolase mutants have been evaluated for the coupling of lactosyl fluoride on O-allyl N(I)-acetyl-2(II)-azido-beta-chitobioside. Double mutants Cel7B E197A H209A and Cel7B E197A H209G preferentially catalyze the formation of a beta-(1-->4) linkage between the two disaccharides, while single mutant Cel7B E197A and triple mutant Cel7B E197A H209A A211T produce predominantly the beta-(1-->3)-linked tetrasaccharide. This result constitutes the first report of the modulation of the regioselectivity through site-directed mutagenesis for an endoglycosynthase.

Synthesis of a Library of Xylogluco-Oligosaccharides for Active-Site Mapping of Xyloglucan endo-Transglycosylase

Complex oligosaccharides containing alpha-D-xylosyl-(1-->6)-beta-D-glucosyl residues and unsubstituted beta-(1-->4)-linked D-glucosyl units were readily synthesized using enzymatic coupling catalyzed by the Cel7B E197A glycosynthase from Humicola insolens. Constituting this library required four key steps: (1) preparing unprotected building blocks by chemical synthesis or enzymatic degradation of xyloglucan polymers; (2) generating the donor synthon in the enzymatic coupling by temporarily introducing a lactosyl motif on the 4-OH of the terminal glucosyl units of the xylogluco-oligosaccharides; (3) synthesizing the corresponding alpha-fluorides, followed by their de-O-acetylation and the glycosynthase-catalyzed condensation of these donors onto various acceptors; and (4) enzymatically releasing lactose or galactose from the reaction product, affording the target molecules in good overall yields. These complex oligosaccharides proved useful for mapping the active site of a key enzyme in plant cell wall biosynthesis and modification: the xyloglucan endo-transglycosylase (XET). We also report some preliminary enzymatic results regarding the efficiency of these compounds.

A highly acid-stable and thermostable endo-beta-glucanase from the thermoacidophilic archaeon Sulfolobus solfataricus

The thermoacidophilic archaeon Sulfolobus solfataricus P2 encodes three hypothetic endo-beta-glucanases, SSO1354, SSO1949 and SSO2534. We cloned and expressed the gene sso1949 encoding the 334 amino acids containing protein SSO1949, which can be classified as a member of glycoside hydrolase family 12. The purified recombinant enzyme hydrolyses carboxymethylcellulose as well as cello-oligomers, with cellobiose and cellotriose as main reaction products. By following the hydrolysis of a fluorescently labelled cellohexaoside under a wide variety of conditions, we show that SSO1949 is a unique extremophilic enzyme. This archaeal enzyme has a pH optimum of approx. pH 1.8 and a temperature optimum of approx. 80 degrees C. Furthermore, the enzyme is thermostable, with a half-life of approx. 8 h at 80 degrees C and pH 1.8. The thermostability is strongly pH-dependent. At neutral pH, the thermal inactivation rate is nearly two orders of magnitude higher than at pH 1.8. Homology modelling suggests that the catalytic domain of SSO1949 has a similar fold to other mesophilic, acidophilic and neutral cellulases. The presence of a signal peptide indicates that SSO1949 is a secreted protein, which enables S. solfataricus to use cellulose as an external carbon source. It appears that SSO1949 is perfectly adapted to the extreme environment in solfataric pools. A cellulolytic enzyme with such a combination of stability and activity at high temperatures and low pH has not been described so far and could be a valuable tool for the large-scale hydrolysis of cellulose under acidic conditions.

Anatomy of glycosynthesis: structure and kinetics of the Humicola insolens Cel7B E197A and E197S glycosynthase mutants

The formation of glycoconjugates and oligosaccharides remains one of the most challenging chemical syntheses. Chemo-enzymatic routes using retaining glycosidases have been successfully harnessed but require tight kinetic or thermodynamic control. "Glycosynthases," specifically engineered glycosidases that catalyze the formation of glycosidic bonds from glycosyl donor and acceptor alcohol, are an emerging range of synthetic tools in which catalytic nucleophile mutants are harnessed together with glycosyl fluoride donors to generate powerful and versatile catalysts. Here we present the structural and kinetic dissection of the Humicola insolens Cel7B glycosynthases in which the nucleophile of the wild-type enzyme is mutated to alanine and serine (E197A and E197S). 3-D structures reveal the acceptor and donor subsites and the basis for substrate inhibition. Kinetic analysis shows that the E197S mutant is considerably more active than the corresponding alanine mutant due to a 40-fold increase in k(cat).