Charles Tellier

Converting a β-Glycosidase into a β-Transglycosidase by Directed Evolution

Directed evolution was applied to the β-glycosidase of Thermus thermophilus in order to increase its ability to synthesize oligosaccharide by transglycosylation. Wild-type enzyme was able to transfer the glycosyl residue with a yield of 50% by self-condensation and of about 8% by transglycosylation on disaccharides without nitrophenyl at their reducing end. By using a simple screening procedure, we could produce mutant enzymes possessing a high transferase activity. In one step of random mutagenesis and in vitro recombination, the hydrolysis of substrates and of transglycosylation products was considerably reduced. For certain mutants, synthesis by self-condensation of nitrophenyl glycosides became nearly quantitative, whereas synthesis by transglycosylation on maltose and on cellobiose could reach 60 and 75%, respectively. Because the most efficient mutations, F401S and N282T, were located just in front of the subsite (-1), molecular modeling techniques were used to explain their effects on the synthesis reaction; we can suggest that repositioning of the glycone in the (-1) subsite together with a better fit of the acceptor in the (+1) subsite might favor the attack of a glycosyl acceptor in the mutant at the expense of water. Thus these new transglycosidases constitute an interesting alternative for the synthesis of oligosaccharides by using stable and accessible donor substrates.

Towards Zirconium Phosphonate-Based Microarrays for Probing DNA−Protein Interactions: Critical Influence of the Location of the Probe Anchoring Groups

Terminal phosphate groups on double-stranded DNA probes bind strongly to glass substrates coated with a zirconium phosphonate monolayer, and probes immobilized in this way as microarrays can be used to detect protein targets. The sensitivity of the microarray was shown to be enhanced by the use of a polyguanine segment ((G)n , n > or = 5) as a spacer between the phosphate linker and the protein interaction domain. More importantly, the presence of phosphate linkers on both ends of the dsDNA probes leads to significant enhancement of target capture. The relevant characteristics of the different probes when bound to the surface were determined, by the original use of a combination of surface characterization techniques (XPS, AFM, and Sarfus). In this context, the location of the phosphate linkers in the duplex probes was found to result in different probe surface coverage and presentation on the surface, which affect subsequent interactions with the target protein.

A phosphorus magnetic resonance spectroscopy and a differential scanning calorimetry study of the physical properties of N-acylphosphatidylethanolamines in aqueous dispersions

The physical properties of aqueous dispersions of N-acyldipalmitoylphosphatidylethanolamine (N-acyl-DPPE) and N-acyldioleylphosphatidylethanolamine (N-acyl-DOPE) have been investigated by differential scanning calorimetry (DSC), freeze-fracture electron microscopy and 31P-NMR spectroscopy. N-Acylation of DPPE by the shortest fatty chains causes a significant decrease in the gel-to-liquid crystalline phase transition, Tm. However, an increase of the N-acyl chain length induces a regular increase of Tm. 31P-NMR and electron microscopy show that N-acyl-DPPE and N-palmitoyl-DOPE are organized in lamellar phases while N-oleyl-DOPE exhibits a hexagonal phase at room temperature. Above Tm, all phospholipids investigated exhibit an asymmetric line shape with a residual chemical shift anisotropy around 15 ppm. Twenty degrees below Tm, a powder type spectrum governed by the static chemical tensor appears. The shielding components are about 40% lower than those of DPPE. In the case of N-oleyl-DPPE, the longitudinal relaxation time exhibits a minimum around 45°C. These results indicate that the N-acyl chain is embedded in the bilayer leading to a strong perturbation of the head group rotational mobility.

Lipid-protein interactions in wheat gluten: a phosphorus nuclear magnetic resonance spectroscopy and freeze-fracture electron microscopy study

Phosphorus magnetic resonance spectroscopy and freeze-fracture electron microscopy of wheat gluten showed that its lipids are organised in small vesicles (60 to 300 nm in diameter), in which polar lipids exhibit a lamellar liquid crystalline phase. Interactions between phospholipids and proteins are sensitive to heating and to mechanical work. From 50 to 70°C and during cooling from 70 to 25°C these interactions are disrupted due to the expulsion into the aqueous phase of lipid vesicles that are embedded in the protein network. This expulsion can be induced by extensive mechanical work and is prevented in the presence of a reducing agent. Residual starch present in gluten does not play a role. Protein rearrangements during heating and mixing may be responsible for the expulsion of lipid vesicles. These results suggest that the formation of complexes between wheat proteins and lipids such as is found in membranes, does not occur in gluten. It appears that lipid vesicles are embedded in the protein matrix due to the particular properties of wheat proteins, so that gluten may be regarded as a system containing stabilised microemulsions.

Semi-rational approach for converting a GH1 β-glycosidase into a β-transglycosidase

A large number of retaining glycosidases catalyze both hydrolysis and transglycosylation reactions, but little is known about what determines the balance between these two activities (transglycosylation/hydrolysis ratio). We previously obtained by directed evolution the mutants F401S and N282T of Thermus thermophilus β-glycosidase (Ttβ-gly, glycoside hydrolase family 1 (GH1)), which display a higher transglycosylation/hydrolysis ratio than the wild-type enzyme. In order to find the cause of these activity modifications, and thereby set up a generic method for easily obtaining transglycosidases from glycosidases, we determined their X-ray structure. No major structural changes could be observed which could help to rationalize the mutagenesis of glycosidases into transglycosidases. However, as these mutations are highly conserved in GH1 β-glycosidases and are located around the -1 site, we pursued the isolation of new transglycosidases by targeting highly conserved amino acids located around the active site. Thus, by single-point mutagenesis on Ttβ-gly, we created four new mutants that exhibit improved synthetic activity, producing disaccharides in yields of 68-90% against only 36% when native Ttβ-gly was used. As all of the chosen positions were well conserved among GH1 enzymes, this approach is most probably a general route to convert GH1 glycosidases into transglycosidases.

Multinuclear NMR study of the pH dependent water state in skim milk and caseinate solutions

The effects of decreasing pH and micellar calcium concentrations of reconstituted skim milk and caseinate solution were studied by 1 H and 17 O NMR spectroscopy. The proton transverse relaxation rate 1/ T 2 of skim milk decreased as the pH decreased, reaching a minimum at pH 5·3. However, as the pH fell sodium caseinate solution showed a continuous increase in 1/ T 2 , with no minimum. Analysis of proton relaxation as a function of the interpulse time in the CPMG (Carr-Purcell-Meiboom-Gill) sequence demonstrated that both the proton exchange mechanism and ‘bound’ water contributed to proton relaxation in skim milk. The study of 17 O relaxation rate as a function of pH confirmed the change in protein hydration upon acidification. Increasing the amount of EDTA showed that the proton transverse relaxation rate of skim milk decreased until a plateau was reached when the micellar calcium was totally solubilized. With excess EDTA the relaxation rates of skim milk and caseinate solution were identical. A strong correlation was also found between the pH dependent relaxation rate and the solubilization of micellar phosphorus as detected by 31 P NMR. Together, these results suggested that aggregation of caseins by calcium and colloidal calcium phosphate is mainly responsible for the excess hydration in skim milk micelles.

Synthesis of a benzylamidine derived from D-mannose. A potent mannosidase inhibitor

Abstract The synthesis of a substituted mannopyranose-based amidine is described and its potential as glycosidase inhibitor evaluated. This new aminosugar derivative acts as a potent glycosidase inhibitor by virtue of its charge and shape similarities to the mannopyranosyl cation. The benzyl group of this pseudodisaccharide may also contribute to enzyme transition-state interactions.

Correlation between thermostability and stability of glycosidases in ionic liquid

The activity and stability of a β-glycosidase (Thermus thermophilus) and two α-galactosidases (Thermotoga maritima and Bacillus stearothermophilus) were studied in different hydrophilic ionic liquid (IL)/water ratios. For the ILs used, the glycosidases showed the best stability and activity in 1,3-dimethylimidazolium methyl sulfate [MMIM][MeSO4] and 1,2,3-trimethylimidazolium methyl sulfate [TMIM][MeSO4]. A close correlation was observed between the thermostability of the enzymes and their stability in IL media. At high IL concentration (80%), a time-dependent irreversible denaturing effect was observed on glycosidases while, at lower concentration (<30%), a reversible inactivation affecting mainly the kcat was obtained. The results demonstrate that highly thermostable glycosidases are more suitable for biocatalytic reactions in water-miscible ILs.

Bisphosphonate adaptors for specific protein binding on zirconium phosphonate-based microarrays.

Two bisphosphonate adaptors were designed to immobilize histidine-tagged proteins onto glass substrates coated with a zirconium phosphonate monolayer, allowing efficient and oriented immobilization of capture proteins, affitins directed to lysozyme, on a microarray format. These bifunctional adaptors contain two phosphonic acid anchors at one extremity and either one nitrilotriacetic acid (NTA) or two NTA groups at the other. The phosphonate groups provide a stable bond to the zirconium interface by multipoint attachment and allow high density of surface coverage of the linkers as revealed by X-ray photoelectron spectroscopy (XPS). Reversible high-density capture of histidine-tagged proteins is shown by real-time surface plasmon resonance enhanced ellipsometry and in a microarray format using fluorescence detection of AlexaFluor 647-labeled target protein. The detection sensitivity of the microarray for the target protein was below 1 nM, despite the monolayer arrangement of the probes, due to very low background staining, which allows high fluorescent signal-to-noise ratio. The performance of these Ni-NTA-modified zirconium phosphonate coated slides compared favorably to other types of microarray substrates, including slides with a nitrocellulose-based matrix, epoxide slides, and epoxide slides functionalized with Ni-NTA groups. This immobilization strategy has a large potential to fix any histidine-tagged proteins on zirconium or titanium ion surfaces.

Digital screening methodology for the directed evolution of transglycosidases

Engineering of glycosidases with efficient transglycosidases activity is an alternative to glycosyltransferases or glycosynthases for the synthesis of oligosaccharides and glycoconjugates. However, the engineering of transglycosidases by directed evolution methodologies is hampered by the lack of efficient screening systems for sugar-transfer activity. We report here the development of digital imaging-based high-throughput screening methodology for the directed evolution of glycosidases into transgalactosidases. Using this methodology, we detected transglycosidase mutants in intact Escherichia coli cells by digital imaging monitoring of the activation of non- or low-hydrolytic mutants by an acceptor substrate. We screened several libraries of mutants of beta-glycosidase from Thermus thermophilus using this methodology and found variants with up to a 70-fold overall increase in the transglycosidase/hydrolysis activity ratio. Using natural disaccharide acceptors, these transglycosidase mutants were able to synthesise trisaccharides, as a mixture of two regioisomers, with up to 76% yield.