Enzymatic synthesis of three pNP-α-galactobiopyranosides: application of the library of fungal α-galactosidases

Characterization of Properties and Transglycosylation Abilities of Recombinant α-Galactosidase from Cold-Adapted Marine Bacterium Pseudoalteromonas KMM 701 and Its C494N and D451A Mutants

A novel wild-type recombinant cold-active α-d-galactosidase (α-PsGal) from the cold-adapted marine bacterium Pseudoalteromonas sp. KMM 701, and its mutants D451A and C494N, were studied in terms of their structural, physicochemical, and catalytic properties. Homology models of the three-dimensional α-PsGal structure, its active center, and complexes with D-galactose were constructed for identification of functionally important amino acid residues in the active site of the enzyme, using the crystal structure of the α-galactosidase from Lactobacillus acidophilus as a template. The circular dichroism spectra of the wild α-PsGal and mutant C494N were approximately identical. The C494N mutation decreased the efficiency of retaining the affinity of the enzyme to standard p-nitrophenyl-α-galactopiranoside (pNP-α-Gal). Thin-layer chromatography, matrix-assisted laser desorption/ionization mass spectrometry, and nuclear magnetic resonance spectroscopy methods were used to identify transglycosylation products in reaction mixtures. α-PsGal possessed a narrow acceptor specificity. Fructose, xylose, fucose, and glucose were inactive as acceptors in the transglycosylation reaction. α-PsGal synthesized -α(1→6)- and -α(1→4)-linked galactobiosides from melibiose as well as -α(1→6)- and -α(1→3)-linked p-nitrophenyl-digalactosides (Gal₂-pNP) from pNP-α-Gal. The D451A mutation in the active center completely inactivated the enzyme. However, the substitution of C494N discontinued the Gal-α(1→3)-Gal-pNP synthesis and increased the Gal-α(1→4)-Gal yield compared to Gal-α(1→6)-Gal-pNP.

Hooked on α-d-galactosidases: from biomedicine to enzymatic synthesis

α-d-Galactosidases (EC 3.2.1.22) are enzymes employed in a number of useful bio-based applications. We have depicted a comprehensive general survey of α-d-galactosidases from different origin with special emphasis on marine example(s). The structures of natural α-galactosyl containing compounds are described. In addition to 3D structures and mechanisms of action of α-d-galactosidases, different sources, natural function and genetic regulation are also covered. Finally, hydrolytic and synthetic exploitations as free or immobilized biocatalysts are reviewed. Interest in the synthetic aspects during the next years is anticipated for access to important small molecules by green technology with an emphasis on alternative selectivity of this class of enzymes from different sources.

α-Galactobiosyl units: thermodynamics and kinetics of their formation by transglycosylations catalysed by the GH36 α-galactosidase from Thermotoga maritima

Broad regioselectivity of α-galactosidase from Thermotoga maritima (TmGal36A) is a limiting factor for application of the enzyme in the directed synthesis of oligogalactosides. However, this property can be used as a convenient tool in studies of thermodynamics of a glycosidic bond. Here, a novel approach to energy difference estimation is suggested. Both transglycosylation and hydrolysis of three types of galactosidic linkages were investigated using total kinetics of formation and hydrolysis of pNP-galactobiosides catalysed by monomeric glycoside hydrolase family 36 α-galactosidase from T. maritima, a retaining exo-acting glycoside hydrolase. We have estimated transition state free energy differences between the 1,2- and 1,3-linkage (ΔΔG(‡)0 values were equal 5.34 ± 0.85 kJ/mol) and between 1,6-linkage and 1,3-linkage (ΔΔG(‡)0=1.46 ± 0.23 kJ/mol) in pNP-galactobiosides over the course of the reaction catalysed by TmGal36A. Using the free energy difference for formation and hydrolysis of glycosidic linkages (ΔΔG(‡)F-ΔΔG(‡)H), we found that the 1,2-linkage was 2.93 ± 0.47 kJ/mol higher in free energy than the 1,3-linkage, and the 1,6-linkage 4.44 ± 0.71 kJ/mol lower.

Induction and characterization of an unusual alpha-D-galactosidase from Talaromyces flavus

An extracellular alpha-d-galactosidase from Talaromyces flavus CCF 2686 with extremely broad and unusual acceptor specificity is produced exclusively in the presence of the specific inducer--6-deoxy-D-glucose (quinovose). The procedure for the preparation of this very expensive substance has been modified and optimized. Surprisingly, any of other common alpha-D-galactosidase inducers or substrates, e.g., D-galactose, melibiose and raffinose, did not stimulate its production. The crude alpha-D-galactosidase preparation was purified by anion-exchange chromatography and three isoenzymes with different substrate specificities were identified. The main isoenzyme (alphaGal1) was further purified by cation-exchange chromatography and fully characterized. When compared with other alpha-galactosidases and also with other isoenzymes produced by T. flavus, it showed a markedly different regioselectivity and also negligible hydrolytic activity towards melibiose. Moreover, it was active on polymeric substrates (locust bean gum, guar gum) and significantly inhibited by alpha-D-galactopyranosyl azide, D-galactose, D-xylose, melibiose, methyl alpha- and beta-D-galactopyranoside and lactose.

Generation of an ?-L-rhamnosidase library and its application for the selective derhamnosylation of natural products

A screening of 16 different fungal strains was performed under different cultivation conditions, using L-rhamnose or L-rhamnose-containing flavonoid glycosides (rutin, hesperidin, and naringin) as specific inducers. No significant constitutive production of alpha-L-rhamnosidases was detected in noninduced cultures, while high levels of these glycosidase activities were obtained using different inducers. New species, so far unknown for the production of alpha-L-rhamnosidases, were identified. More than 30 different alpha-L-rhamnosidase samples were prepared by ammonium sulfate precipitation. Substrate specificity of this alpha-L-rhamnosidase library was tested with various L-rhamnose-containing natural compounds (flavonoids, terpenoids, and saponins). Most of the enzymatic preparations showed broad substrate specificity, and some of them were also acting on sterically hindered substrates (e.g., quercitrin). The screening of the library under different reaction conditions showed the coexistence, in the same preparation, of more than one alpha-L-rhamnosidase activities with different substrate specificity and different stability towards organic cosolvents. To exploit this enzymatic library for synthetic applications, the presence of contaminating alpha-L-arabinosidases and beta-D-glucosidases was investigated. The latter enzymes were observed in several preparations, while alpha-L-arabinosidase content was generally quite low. The selective derhamnosylation of the saponin desglucoruscin was performed on a preparative scale. The enzyme obtained by rhamnose induction of the Aspergillus niger K2 CCIM strain showed high activity towards this substrate and negligible alpha-L-arabinosidase contamination. Therefore, it was chosen as a catalyst for the selective derhamnosylation reaction, which provided the desired product in 70% yield.

Application of selectively acylated glycosides for the alpha-galactosidase-catalyzed synthesis of disaccharides

4-Nitrophenyl alpha-D-galactopyranosyl-(1-->3)-6-O-acetyl-alpha-D-galactopyranoside was prepared in a transglycosylation reaction catalyzed by alpha-D-galactosidase from Talaromyces flavus using 4-nitrophenyl alpha-D-galactopyranoside as a glycosyl donor and 4-nitrophenyl 6-O-acetyl-alpha-D-galactopyranoside as an acceptor. 4-Nitrophenyl 6-O-acetyl-alpha-D-galactopyranoside and 4-nitrophenyl 6-O-acetyl-beta-D-galactopyranoside were prepared in a regioselective enzymic transesterification in pyridine-acetone catalyzed by the lipase PS from Burkholderia cepacia. A series of water-miscible organic solvents (acetone, acetonitrile, dimethylformamide, dimethyl sulfoxide, 1,4-dioxane, 2-methoxyethanol, pyridine, 2-methylpropan-2-ol, tetrahydrofuran, propargyl alcohol) were used as co-solvents in this enzymic reaction. Their influence on the activity and stability of the alpha-galactosidase from T. flavus was established. 2-Methylpropan-2-ol and acetone (increasing the solubility of the modified substrate acceptors and displaying the minimum impairment of the activity and stability of the enzyme) were used as co-solvents in transglycosylation reactions.

Exploitation of a library of alpha-galactosidases for the synthesis of building blocks for glycopolymers

After screening a library of fungal alpha-galactosidases for the synthesis of functionalized alkyl alpha-D-galactopyranosides, four enzymes (isolated from Aspergillus terreus CCM55, Aspergillus commune CCM 2969, Penicillium vinaceum CCM 2384, or Penicillium brasilianum 2155) proved to be suitable for these biotransformations. The effect of different concentrations of alcohol on activity and stability of these enzymes was investigated. After optimization of the reaction conditions, three galactose derivatives (allyl, 2-nitroethyl and 2-(2',2',2'-trifluoroacetamido)-ethyl alpha-D-galactopyranoside, 1a, 3a, and 4a, respectively), suitable for subsequent chemical polymerization, were synthesized using either the "reverse hydrolysis" or the "transglycosylation" protocols.

Compounds isolated at the Institute of Microbiology in 1989-2001 and future trends

A total of 307 new compounds, natural, semisynthetic or synthetic, were isolated at the Institute of microbiology during the last twelve years. Due to the development of separation (chromatographic) methods and of analytical methods used to determine the chemical structure of these compounds, i.e. NMR, MS and X-ray diffraction, many new metabolites could be described.