Structural mechanism of a dual-functional enzyme DgpA/B/C as both a C-glycoside cleaving enzyme and an O- to C-glycoside isomerase

The C-glycosidic bond that connects the sugar moiety with aglycone is difficult to be broken or made due to its inert nature. The knowledge of C-glycoside breakdown and synthesis is very limited. Recently, the enzyme DgpA/B/C cascade from a human intestinal bacterium PUE was identified to specifically cleave the C-glycosidic bond of puerarin (daidzein-8-C-glucoside). Here we investigated how puerarin is recognized and oxidized by DgpA based on crystal structures of DgpA with or without substrate and biochemical characterization. More strikingly, we found that apart from being a C-glycoside cleaving enzyme, DgpA/B/C is capable of efficiently converting O- to C-glycoside showing the activity as a structure isomerase. A possible mechanistic model was proposed dependently of the simulated complex structure of DgpB/C with 3″-oxo-daidzin and structure-based mutagenesis. Our findings not only shed light on understanding the enzyme-mediated C-glycosidic bond breakage and formation, but also may help to facilitate stereospecific C-glycoside synthesis in pharmaceutical industry.

Discovery and functional characterization of flavone O-glycosyltransferases in Scutellaria baicalensis / 药学学报

Huang-Qin is a traditional Chinese medicine with antiviral, antioxidant, and anti-inflammatory activities. Its major bioactive compounds are diverse flavone O-glucuronides and glucosides. Although three flavonoid O-glycosyltransferases have been identified from S. baicalensis, this information is not sufficient to elucidate the structural diversity of flavonoid glycosides. In this study, nine glycosyltransferase candidate genes were discovered from S. baicalensis by BLAST analysis and their functions were characterized after heterologous expression. Three new flavone O-glycosyltransferases were able to catalyze the formation of major compounds in S. baicalensis, including baicalin and wogonoside. These enzymes could also utilize exogenous flavones as sugar acceptors. This work further elucidates biosynthetic pathways for Scutellaria flavonoid O-glycosides.

Chemical constituents from fruits of Rhododendron molle / 中草药

Objective: To investigate the chemical constituents from the fruits of Rhododendron molle. Methods: Compounds were isolated and purified by a series of methods including silica gel, ODS column, Sephadex LH-20, semi-preparative HPLC, recrystallization, and so on. Their structures were identified by analysis of physico-chemical propertiesand spectroscopic technique. Results: Fifteen known compounds were isolated and their structures were elucidated as myricetin (1), dihydromyricetin (2), gallocatechin (3), quercetin-3′-O-glycoside (4), catechin (5), epicatechin (6), taxifolin 3′-O-glucopyranoside (7), proanthocyanidin A-2 (8), dehydroicatechin A (9), quercetin-3-O-α-arabinoside (10), phlorizin (11), lyoniresinol 3-O-rhamnopyranoside (12), 2,6- dimethoxy-4-hydroxyphenol-1-O-glucopyranoside (13), 2,4,6-trihydroxy acetophenone-2-O-glucopyranoside (14), and 5′-β- glucopyranosyloxy-jasmonic acid (15). Conclusion: Compounds 1—4, 7—9, and 13—15 are obtained from R. molle for the first time. Compounds 5, 6, and 10—12 are obtained from the fruits of this plant for the first time, of which compounds 1—11 are flavonoids.

Elucidation of the fragmentation pathways of a complex 3,7-O-glycosyl flavonol by CID, HCD, and PQD on an LTQ-Orbitrap Velos Pro hybrid mass spectrometer / 中国天然药物

The present study was designed to systematically investigate the ESI-MS(n) behavior of a complex 3, 7-O-glycosyl flavonol, kaempferol 3-O-α-L-[2,3-di-O-β-D-(6-E-p-coumaroyl)glucopyranosyl]-rhamnopyranosyl-7-O-α-L-rhamnopyranoside (KO) isolated from Epimedium wushanense, and to address the elimination priority among different glycosylation sites and different sugars/substituents. The direct-infusion ESI-MS(n) experiment of KO was performed on a hybrid LTQ-Orbitrap Velos Pro mass spectrometer in both negative and positive ion modes by three different fragmentation mechanisms (CID, HCD, and PQD). The CID, HCD, and PQD analyses of KO exhibited remarkable discrimination in respect of the scan range, richness, and distribution of product ions through the entire spectra. KO experienced different fragmentation pathways between two ionization modes: the negative mode CID of KO eliminated the glycosyl portions (priority: 7-sugar > 3-substituent and terminal substituents > inner sugar) and produced aglycone product ions at m/z 284.03/285.04; however, abundant sodium-adduct B(3)2 together with subsequent (i,j)X(3)0 cleavages were found characteristic for the positive mode CID-MS(n). The fragmentation pathways by CID for KO were proposed by analyzing the high accuracy ESI-MS(n) data. Complementary structural information of KO regarding the aglycone and glycosyl portions was obtained by analyzing the ESI-MS(n) data in both ionization modes. In conclusion, the LTQ-Orbitrap method facilitates highly reliable qualitative analysis of bioactive flavonoids with three alternative fragmentation modes.

Preliminary Discussion on the Structure of a Novel Housefly Pupae Lectin / 中国生物工程杂志

In order to provide plenty of information about the relationship between its structure and function,the structure of a novel housefly pupae D-galactose binding lectin with the molecular weight 55kDa and immune acitivity was analyzed preliminarily.In the first place,oligosaccharide chain was confirmed to be existed in this kind of novel housefly pupae lectin by the method of gel staining,and then its structure was analyzed with the help of protein sequencing instrument,spectrophotometer color contrast,?-elimination reaction,infrared spectroscopy and atomic force microscopy.This kind lectin was a global-shaped monomer with the diameter 75 nm or so and the protein and oligosaccharide content 97.36% and 2.1% respectively.Peptide chain and oligosaccharide chain was linked by O-glycoside bond with the N-terminal blocked and the sugar ring alpinum type.All above was the reliable theory for further analysis of structure.