Nigerotriose

Informative dissociation of carbohydrates using an infrared (IR) irradiation system is demonstrated under ambient conditions without the instrumentation of a mass spectrometer. Structural identification of carbohydrates and associated conjugates is essential for understanding their biological functions, but identification remains challenging. Herein, an easy and rugged method is reported for the structural identification of model carbohydrates, including Globo-H, three trisaccharide isomers (nigerotriose/laminaritriose/cellotriose), and two hexasaccharide isomers (laminarihexaose/isomaltohexaose). For Globo-H, the numbers of cross-ring cleavages increased by factors of 4.4 and 3.4 upon ambient IR exposure, compared to an untreated control and a collision-induced dissociation (CID) sample. Moreover, 25-82% enhancement in the numbers of glycosidic bond cleavages upon ambient IR exposure was also obtained compared to untreated and CID samples. Unique features of first-generation fragments produced by ambient IR facilitated the differentiation of three trisaccharide isomers. Semi-quantitative analysis was achieved (coefficient of determination (R²) of 0.982) in a mixture of two hexasaccharide isomers via unique features generated upon ambient IR. Photothermal and radical migration effects induced by ambient IR were postulated as responsible for promoting carbohydrate fragmentation. This easy and rugged method could be a universally applicable protocol and complementary to other techniques for detailed structural characterization of carbohydrates.

Carbohydrate-active enzymes are involved in the degradation, biosynthesis, and modification of carbohydrates and vary with the diversity of carbohydrates. The glycoside hydrolase (GH) family 31 is one of the most diverse families of carbohydrate-active enzymes, containing various enzymes that act on α-glycosides. However, the function of some GH31 groups remains unknown, as their enzymatic activity is difficult to estimate due to the low amino acid sequence similarity between characterized and uncharacterized members. Here, we performed a phylogenetic analysis and discovered a protein cluster (GH31_u1) sharing low sequence similarity with the reported GH31 enzymes. Within this cluster, we showed that a GH31_u1 protein from Lactococcus lactis (LlGH31_u1) and its fungal homolog demonstrated hydrolytic activities against nigerose [α-D-Glcp-(1→3)-D-Glc]. The k_cat/K_m values of LlGH31_u1 against kojibiose and maltose were 13% and 2.1% of that against nigerose, indicating that LlGH31_u1 has a higher specificity to the α-1,3 linkage of nigerose than other characterized GH31 enzymes, including eukaryotic enzymes. Furthermore, the three-dimensional structures of LlGH31_u1 determined using X-ray crystallography and cryogenic electron microscopy revealed that LlGH31_u1 forms a hexamer and has a C-terminal domain comprising four α-helices, suggesting that it contributes to hexamerization. Finally, crystal structures in complex with nigerooligosaccharides and kojibiose along with mutational analysis revealed the active site residues involved in substrate recognition in this enzyme. This study reports the first structure of a bacterial GH31 α-1,3-glucosidase and provides new insight into the substrate specificity of GH31 enzymes and the physiological functions of bacterial and fungal GH31_u1 members.