Cloning and nucleotide sequence of the mycodextranase gene from Streptomyces sp. J-13-3

Glycoside hydrolases active on microbial exopolysaccharide α-glucans: structures and function

Glucose is the most abundant monosaccharide in nature and is an important energy source for living organisms. Glucose exists primarily as oligomers or polymers and organisms break it down and consume it. Starch is an important plant-derived α-glucan in the human diet. The enzymes that degrade this α-glucan have been well studied as they are ubiquitous throughout nature. Some bacteria and fungi produce α-glucans with different glucosidic linkages compared with that of starch, and their structures are quite complex and not fully understood. Compared with enzymes that degrade the α-(1→4) and α-(1→6) linkages in starch, biochemical and structural studies of the enzymes that catabolize α-glucans from these microorganisms are limited. This review focuses on glycoside hydrolases that act on microbial exopolysaccharide α-glucans containing α-(1→6), α-(1→3), and α-(1→2) linkages. Recently acquired information regarding microbial genomes has contributed to the discovery of enzymes with new substrate specificities compared with that of previously studied enzymes. The discovery of new microbial α-glucan-hydrolyzing enzymes suggests previously unknown carbohydrate utilization pathways and reveals strategies for microorganisms to obtain energy from external sources. In addition, structural analysis of α-glucan degrading enzymes has revealed their substrate recognition mechanisms and expanded their potential use as tools for understanding complex carbohydrate structures. In this review, the author summarizes the recent progress in the structural biology of microbial α-glucan degrading enzymes, touching on previous studies of microbial α-glucan degrading enzymes.

A novel thermostable α-1,3-glucanase from Streptomyces thermodiastaticus HF 3-3

Thermally stable α-1,3-glucanase HF65 was purified from culture filtrate of Streptomyces thermodiastaticus HF3-3. The molecular mass of this enzyme was estimated to be 65 kDa and 45.7 kDa by using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and size exclusion chromatography, respectively. The purified enzyme retained more than 50% of maximum activity even after incubation at 65°C more than 2 h. Moreover, α-1,3-glucanase HF65 was stable in the presence of chemicals like SDS, benzethonium chloride, and sodium fluoride at a concentration of 1%. The enzyme also exhibited salt tolerance at a concentration up to 20%. The observed stability of α-1,3-glucanase HF65 to salt and surfactants is a great advantage for its addition to commercial oral care products. Interestingly, the N-terminal amino acid sequence did not show any similarity to those of known α-1,3-glucanases, while the sequence of internal eight amino acid residues of this enzyme was homologous with those of mycodextranases. Nevertheless, the enzyme exhibited high specificity against α-1,3-glucan. According to these results, the enzyme purified from S. thermodiastaticus HF3-3 was classified as α-1,3-glucanase which was highly homologous to mycodextranase in amino acid sequence.

Discovery of nigerose phosphorylase from Clostridium phytofermentans

A novel phosphorylase from Clostridium phytofermentans belonging to the glycoside hydrolase family (GH) 65 (Cphy1874) was characterized. The recombinant Cphy1874 protein produced in Escherichia coli showed phosphorolytic activity on nigerose in the presence of inorganic phosphate, resulting in the release of D-glucose and β-D-glucose 1-phosphate (β-G1P) with the inversion of the anomeric configuration. Kinetic parameters of the phosphorolytic activity on nigerose were k(cat) = 67 s(-1) and K(m) = 1.7 mM. This enzyme did not phosphorolyze substrates for the typical GH65 enzymes such as trehalose, maltose, and trehalose 6-phosphate except for a weak phosphorolytic activity on kojibiose. It showed the highest reverse phosphorolytic activity in the reverse reaction using D-glucose as the acceptor and β-G1P as the donor, and the product was mostly nigerose at the early stage of the reaction. The enzyme also showed reverse phosphorolytic activity, in a decreasing order, on D-xylose, 1,5-anhydro-D-glucitol, D-galactose, and methyl-α-D-glucoside. All major products were α-1,3-glucosyl disaccharides, although the reaction with D-xylose and methyl-α-D-glucoside produced significant amounts of α-1,2-glucosides as by-products. We propose 3-α-D-glucosyl-D-glucose:phosphate β-D-glucosyltransferase as the systematic name and nigerose phosphorylase as the short name for this Cphy1874 protein.

Cloning and characterization of the gene encoding endo-beta-1,3-glucanase from Arthrobacter sp. NHB-10

The gluA gene, encoding an endo-beta-1,3-glucanase from Arthrobacter sp. (strain NHB-10), was cloned and analyzed. The deduced endo-beta-1,3-glucanase amino acid sequence was 750 amino acids long and contained a 42 amino acid signal peptide with a mature protein of 708 amino acids. There was no similarity to known endo-beta-1,3-glucanases, but GluA was partially similar to two fungal exo-beta-1,3-glucanases in glycoside hydrolase (GH) family 55. Of five possible residues for catalysis and two motifs in two beta-helix heads of GH family 55, three residues and one motif were conserved in GluA, suggesting that GluA is the first bacterial endo-beta-1,3-glucanase in GH family 55. Significant similarity was also found to two proteins of unknown function from Streptomyces coelicolor A3(2) and S. avermitilis.

Beta-helical catalytic domains in glycoside hydrolase families 49, 55 and 87: domain architecture, modelling and assignment of catalytic residues

X-ray crystallography and bioinformatics studies reveal a tendency for the right-handed beta-helix domain architecture to be associated with carbohydrate binding proteins. Here we demonstrate the presence of catalytic beta-helix domains in glycoside hydrolase (GH) families 49, 55 and 87 and provide evidence for their sharing a common evolutionary ancestor with two structurally characterized GH families, numbers 28 and 82. This domain assignment helps assign catalytic residues to each family. Further analysis of domain architecture reveals the association of carbohydrate binding modules with catalytic GH beta-helices, as well as an unexpected pair of beta-helix domains in GH family 55.