Discovery of α-l-Glucosidase Raises the Possibility of α-l-Glucosides in Nature

Substrate Specificity of GH29 α-L-Glucosidases from Cecembia lonarensis

We recently found two α-L-glucosidases, which can hydrolyze p-nitrophenyl α-L-glucopyranoside (PNP L-Glc) rather than p-nitrophenyl α-L-fucopyranoside, in glycoside hydrolase family 29. This study evaluated their substrate specificity for p-nitrophenyl α-L-rhamnopyranoside (PNP L-Rha), α-L-quinovopyranoside (PNP L-Qui), and α-L-xylopyranoside (PNP L-Xyl), of which structure is similar to PNP L-Glc. The two α-L-glucosidases had little activity toward PNP L-Rha. They exhibited higher k cat/K m values for PNP L-Qui but smaller for PNP L-Xyl than for PNP L-Glc. The molecular docking studies indicated that these specificities were correlated well with the active-site structure of the α-L-glucosidases. The finding that α-L-quinovoside, which has been suggested to occur in nature, is also a substrate for α-L-glucosidases indicates that this enzyme are not solely dedicated to α-L-glucoside hydrolysis.

Structural insights into starch-metabolizing enzymes and their applications

Starch is a polysaccharide produced exclusively through photosynthesis in plants and algae; however, is utilized as an energy source by most organisms, from microorganisms to higher organisms. In mammals and the germinating seeds of plants, starch is metabolized by simple hydrolysis pathways. Moreover, starch metabolic pathways via unique oligosaccharides have been discovered in some bacteria. Each organism has evolved enzymes responsible for starch metabolism that are diverse in their enzymatic properties. This review, focusing on eukaryotic α-glucosidases and bacterial α-glucoside-hydrolyzing enzymes, summarizes the structural aspects of starch-metabolizing enzymes belonging to glycoside hydrolase families 15, 31, and 77 and their application for oligosaccharide production.

Exploring the sequence-function space of microbial fucosidases

Microbial α-L-fucosidases catalyse the hydrolysis of terminal α-L-fucosidic linkages and can perform transglycosylation reactions. Based on sequence identity, α-L-fucosidases are classified in glycoside hydrolases (GHs) families of the carbohydrate-active enzyme database. Here we explored the sequence-function space of GH29 fucosidases. Based on sequence similarity network (SSN) analyses, 15 GH29 α-L-fucosidases were selected for functional characterisation. HPAEC-PAD and LC-FD-MS/MS analyses revealed substrate and linkage specificities for α1,2, α1,3, α1,4 and α1,6 linked fucosylated oligosaccharides and glycoconjugates, consistent with their SSN clustering. The structural basis for the substrate specificity of GH29 fucosidase from Bifidobacterium asteroides towards α1,6 linkages and FA2G2 N-glycan was determined by X-ray crystallography and STD NMR. The capacity of GH29 fucosidases to carry out transfucosylation reactions with GlcNAc and 3FN as acceptors was evaluated by TLC combined with ESI-MS and NMR. These experimental data supported the use of SSN to further explore the GH29 sequence-function space through machine-learning models. Our lightweight protein language models could accurately allocate test sequences in their respective SSN clusters and assign 34,258 non-redundant GH29 sequences into SSN clusters. It is expected that the combination of these computational approaches will be used in the future for the identification of novel GHs with desired specificities.

Evaluation of nonnatural L-iminosugar C,C-glycosides, a new class of C-branched iminosugars, as glycosidase inhibitors

Capitalizing on a previously developed Staudinger/azaWittig/Grignard (SAWG)-ring contraction sequence that furnished protected six-membered L-iminosugar C,C-glycosides bearing an allyl group and various substituents at the pseudoanomeric position, the synthesis and glycosidase inhibition of a small library of six- and seven-membered L-iminosugar C,C-glycosides is reported. Their hydrogenolysis or cyclization by RCM followed by deprotection afforded eleven L-iminosugars including spirocyclic derivatives. All compounds adopt a 1C4 conformation in solution according to NMR data. Compared to previously reported branched L-iminosugars, the L-iminosugar C,C-glycosides reported herein were less potent glycosidase inhibitors. However, some of these compounds showed micromolar inhibition of human lysosome β-glucocerebrosidase suggesting that such iminosugars could be useful to access potent CGase inhibitors by adjusting the structure/length of the pseudoanomeric substituents.

Structure and function of microbial α-l-fucosidases: a mini review

Fucose is a monosaccharide commonly found in mammalian, insect, microbial and plant glycans. The removal of terminal α-l-fucosyl residues from oligosaccharides and glycoconjugates is catalysed by α-l-fucosidases. To date, glycoside hydrolases (GHs) with exo-fucosidase activity on α-l-fucosylated substrates (EC 3.2.1.51, EC 3.2.1.-) have been reported in the GH29, GH95, GH139, GH141 and GH151 families of the Carbohydrate Active Enzymes (CAZy) database. Microbes generally encode several fucosidases in their genomes, often from more than one GH family, reflecting the high diversity of naturally occuring fucosylated structures they encounter. Functionally characterised microbial α-l-fucosidases have been shown to act on a range of substrates with α-1,2, α-1,3, α-1,4 or α-1,6 fucosylated linkages depending on the GH family and microorganism. Fucosidases show a modular organisation with catalytic domains of GH29 and GH151 displaying a (β/α)8-barrel fold while GH95 and GH141 show a (α/α)6 barrel and parallel β-helix fold, respectively. A number of crystal structures have been solved in complex with ligands, providing structural basis for their substrate specificity. Fucosidases can also be used in transglycosylation reactions to synthesise oligosaccharides. This mini review provides an overview of the enzymatic and structural properties of microbial α-l-fucosidases and some insights into their biological function and biotechnological applications.