Purification and characterization of cyclic maltosyl-(1-->6)-maltose hydrolase and alpha-glucosidase from an Arthrobacter globiformis strain

Cloning of the cycloisomaltotetraose-forming enzymes using whole genome sequence analyses of Agreia sp. D1110 and Microbacterium trichothecenolyticum D2006

We performed whole genome sequence analyses of Agreia sp. D1110 and Microbacterium trichothecenolyticum D2006 that secrete enzymes to produce cyclo-{→6)-α-d-Glcp-(1→6)-α-d-Glcp-(1→6)-α-d-Glcp-(1→6)-α-d-Glcp-(1→} (CI4) from dextran. Full-length amino acid sequences of CI4-forming enzymes were identified by matching known N-terminal amino acid sequences with products of the draft genome. Domain searches revealed that the CI4-forming enzymes are composed of Glycoside Hydrolase family 66 (GH66) domain, Carbohydrate Binding Module family 35 (CBM35) domain, and CBM13 domain, categorizing the CI4-forming enzymes in the GH66. Furthermore, the amino acid sequences of the two CI4-forming enzymes were 71% similar to each other and up to 51% similar to cycloisomaltooligosaccharide glucanotransferases (CITases) categorized in GH66. Differences in sequence between the CI4-forming enzymes and the CITases suggest mechanisms to produce specific cycloisomaltooligosaccharides, and whole genome sequence analyses identified a gene cluster whose gene products likely work in concert with the CI4-forming enzymes.

Purification and characterization of cycloisomaltotetraose-forming glucanotransferases from Agreia sp. D1110 and Microbacterium trichothecenolyticum D2006

Glucanotransferases that can synthesize cyclo-{→6)-α-d-Glcp-(1→6)-α-d-Glcp-(1→6)-α-d-Glcp-(1→6)-α-d-Glcp-(1→} (CI4) from dextran were purified to homogeneity from the culture supernatant of Agreia sp. D1110 and Microbacterium trichothecenolyticum D2006. The molecular mass of both enzymes was estimated to be 86 kDa by SDS-PAGE. The glucanotransferase, named CI4-forming enzyme, from Agreia sp. exhibited the highest activity at pH 6.0 and 40 °C. The enzyme was stable on the pH range of 4.6-9.9 and up to 40 °C. On the other hand, the enzyme from M. trichothecenolyticum exhibited the highest activity at pH 5.7 and 40 °C. The enzyme was stable on the pH range of 5.0-6.9 and up to 35 °C. Both enzymes catalyzed 4 reactions, namely, intramolecular α-1,6-transglycosylation (cyclization), intermolecular α-1,6-transglycosylation, hydrolysis of CI4, and coupling reaction. Furthermore, the CI4-forming enzyme produced CI4 from α-1,6-linked glucan synthesized from starch by 6-α-glucosyltransferase. These findings will enable the production of CI4 from starch.

Molecular analysis of cyclic α-maltosyl-(1→6)-maltose binding protein in the bacterial metabolic pathway

Cyclic α-maltosyl-(1→6)-maltose (CMM) is a cyclic glucotetrasaccharide with alternating α-1,4 and α-1,6 linkages. Here, we report functional and structural analyses on CMM-binding protein (CMMBP), which is a substrate-binding protein (SBP) of an ABC importer system of the bacteria Arthrobacter globiformis. Isothermal titration calorimetry analysis revealed that CMMBP specifically bound to CMM with a K_d value of 9.6 nM. The crystal structure of CMMBP was determined at a resolution of 1.47 Å, and a panose molecule was bound in a cleft between two domains. To delineate its structural features, the crystal structure of CMMBP was compared with other SBPs specific for carbohydrates, such as cyclic α-nigerosyl-(1→6)-nigerose and cyclodextrins. These results indicate that A. globiformis has a unique metabolic pathway specialized for CMM.

A cyclic tetrasaccharide, cycloisomaltotetraose, was enzymatically produced from dextran and its crystal structure was determined

Two bacterial strains isolated from soil, namely Agreia sp. D1110 and Microbacterium trichothecenolyticum D2006, were found to produce a novel oligosaccharide. The oligosaccharide was enzymatically produced from dextran using the culture supernatant of Agreia sp. D1110 or M. trichothecenolyticum D2006. LC-MS and NMR analysis identified the novel oligosaccharide as cyclo-{→6)-α-d-Glcp-(1→6)-α-d-Glcp-(1→6)-α-d-Glcp-(1→6)-α-d-Glcp-(1→}, which was named cycloisomaltotetraose, and abbreviated as CI4. CI4 was subsequently crystalized and its X-ray crystallographic structure was determined. CI4 crystals were shown to be pentahydrate, with the CI4 molecules in the crystal structure displaying a unique 3D structure, in which two glucosyl residues in the molecule were facing each other. This unique 3D structure was quite different from the 3D structure of known cyclic tetrasaccharides. This is the first report of CI4 molecules and their unique crystal structure.

Structural features of a bacterial cyclic α-maltosyl-(1→6)-maltose (CMM) hydrolase critical for CMM recognition and hydrolysis

Cyclic α-maltosyl-(1→6)-maltose (CMM, cyclo-{→6)-α-d-Glcp-(1→4)-α-d-Glcp-(1→6)-α-d-Glcp-(1→4)-α-d-Glcp-(1→})is a cyclic glucotetrasaccharide with alternating α-1,4 and α-1,6 linkages. CMM is composed of two maltose units and is one of the smallest cyclic glucooligosaccharides. Although CMM is resistant to usual amylases, it is efficiently hydrolyzed by CMM hydrolase (CMMase), belonging to subfamily 20 of glycoside hydrolase family 13 (GH13_20). Here, we determined the ligand-free crystal structure of CMMase from the soil-associated bacterium Arthrobacter globiformis and its structures in complex with maltose, panose, and CMM to elucidate the structural basis of substrate recognition by CMMase. The structures disclosed that although the monomer structure consists of three domains commonly adopted by GH13 and other α-amylase-related enzymes, CMMase forms a unique wing-like dimer structure. The complex structure with CMM revealed four specific subsites, namely -3', -2, -1, and +1'. We also observed that the bound CMM molecule adopts a low-energy conformer compared with the X-ray structure of a single CMM crystal, also determined here. Comparison of the CMMase active site with those in other enzymes of the GH13_20 family revealed that three regions forming the wall of the cleft, denoted PYF (Pro-203/Tyr-204/Phe-205), CS (Cys-163/Ser-164), and Y (Tyr-168), are present only in CMMase and are involved in CMM recognition. Combinations of multiple substitutions in these regions markedly decreased the activity toward CMM, indicating that the specificity for this cyclic tetrasaccharide is supported by the entire shape of the pocket. In summary, our work uncovers the mechanistic basis for the highly specific interactions of CMMase with its substrate CMM.

Heterologous expression and biochemical characterization of alpha-glucosidase from Aspergillus niger by Pichia pastroris

The aglu of Aspergillus niger encodes the pro-protein of alpha-glucosidase, and the mature form of wild-type enzyme is a heterosubunit protein. In the present study, the cDNA of alpha-glucosidase was cloned and expressed in Pichia pastoris strain KM71. The activity of recombinant enzyme in a 3 L fermentor reached 2.07 U/mL after 96 h of induction. The recombinant alpha-glucosidase was able to produce oligoisomaltose. The molecular weight of the recombinant enzyme was estimated to be about 145 kDa by SDS-PAGE, and it reduced to 106 kDa after deglycosylation. The enzymatic activity of recombinant alpha-glucosidase was not significantly affected by a range of metal ions. The optimum temperature of the enzyme was 60 degrees C, and it was stable below 50 degrees C. The enzyme was active over the range of pH 3.0-7.0 with maximal activity at pH 4.5. Using pNPG as substrate, the K(m) and V(max) values were 0.446 mM and 43.48 U/mg, respectively. These studies provided the basis for the application of recombinant alpha-glucosidase in the industry of functional oligosaccharides.