Biochemical and Structural Characterization of Thermostable GH159 Glycoside Hydrolases Exhibiting α-L-Arabinofuranosidase Activity

Functional, biochemical, and preliminary structural properties are reported for three glycoside hydrolases of the recently described glycoside hydrolase (GH) family 159. The genes were cloned from the genomic sequences of different Caldicellulosiruptor strains. This study extends the spectrum of functions of GH159 enzymes. The only activity previously reported for GH159 was hydrolytic activity on β-galactofuranosides. Activity screening using a set of para-nitrophenyl (pNP) glycosides suggested additional arabinosidase activity on substrates with arabinosyl residues, which has not been previously reported for members of GH159. Even though the thermophilic enzymes investigated—Cs_Gaf159A, Ch_Gaf159A, and Ck_Gaf159A—cleaved pNP-α-l-arabinofuranoside, they were only weakly active on arabinogalactan, and they did not cleave arabinose from arabinan, arabinoxylan, or gum arabic. However, the enzymes were able to hydrolyze the α-1,3-linkage in different arabinoxylan-derived oligosaccharides (AXOS) with arabinosylated xylose at the non-reducing end (A³X, A^2,3XX), suggesting their role in the intracellular hydrolysis of oligosaccharides. Crystallization and structural analysis of the apo form of one of the Caldicellulosiruptor enzymes, Ch_Gaf159A, enabled the elucidation of the first 3D structure of a GH159 member. This work revealed a five-bladed β-propeller structure for GH159 enzymes. The 3D structure and its substrate-binding pocket also provides an explanation at the molecular level for the observed exo-activity of the enzyme. Furthermore, the structural data enabled the prediction of the catalytic amino acids. This was supported by the complete inactivation by mutation of residues D19, D142, and E190 of Ch_Gaf159A.

Modulation of the Activity and Regioselectivity of a Glycosidase: Development of a Convenient Tool for the Synthesis of Specific Disaccharides

The synthesis of disaccharides, particularly those containing hexofuranoside rings, requires a large number of steps by classical chemical means. The use of glycosidases can be an alternative to limit the number of steps, as they catalyze the formation of controlled glycosidic bonds starting from simple and easy to access building blocks; the main drawbacks are the yields, due to the balance between the hydrolysis and transglycosylation of these enzymes, and the enzyme-dependent regioselectivity. To improve the yield of the synthesis of β-d-galactofuranosyl-(1→X)-d-mannopyranosides catalyzed by an arabinofuranosidase, in this study we developed a strategy to mutate, then screen the catalyst, followed by a tailored molecular modeling methodology to rationalize the effects of the identified mutations. Two mutants with a 2.3 to 3.8-fold increase in transglycosylation yield were obtained, and in addition their accumulated regioisomer kinetic profiles were very different from the wild-type enzyme. Those differences were studied in silico by docking and molecular dynamics, and the methodology revealed a good predictive quality in regards with the regioisomer profiles, which is in good agreement with the experimental transglycosylation kinetics. So, by engineering CtAraf51, new biocatalysts were enabled to obtain the attractive central motif from the Leishmania lipophosphoglycan core with a higher yield and regioselectivity.

Identification and characterization of β-d-galactofuranosidases from Aspergillus nidulans and Aspergillus fumigatus

Although β-d-galactofuranosidases (Galf-ases) that hydrolyze β-d-galactofuranose (Galf)-containing oligosaccharides have been characterized in various organisms, to date no Galf-specific Galf-ase-encoding genes have been reported in Aspergillus fungi. Based on the amino acid sequences of previously identified bacterial Galf-ases, here we found two candidate Galf-specific Galf-ase genes AN2395 (gfgA) and AN3200 (gfgB) in the genome of Aspergillus nidulans. Indeed, recombinant GfgA and GfgB proteins exhibited Galf-specific Galf-ase activity, but no detectable α-l-arabinofuranosidase (Araf-ase) activity. Phylogenetic analysis of GfgA and GfgB orthologs indicated that there are two types of Aspergillus species: those containing one ortholog each for GfgA and GfgB; and those containing only one ortholog in total, among which Aspergillus fumigatus there is a representative with a single ortholog Galf-ase Afu2g14520. Unlike GfgA and GfgB, the recombinant Afu2g14520 protein showed higher Araf-ase activity than Galf-ase activity. An assay of substrate specificity revealed that although GfgA and GfgB are both exo-type Galf-ases and hydrolyze β-(1,5) and β-(1,6) linkages, GfgA hydrolyzes β-(1,6)-linked Galf-oligosaccharide more effectively as compared with GfgB. Collectively, our findings indicate that Galf-ases in Aspergillus species may have a role in cooperatively degrading Galf-containing oligosaccharides depending on environmental conditions.