Structural, functional, and mutagenesis studies of UDP-glycosyltransferases

Structure of a fungal 1,3-β-glucan synthase

1,3-β-Glucan serves as the primary component of the fungal cell wall and is produced by 1,3-β-glucan synthase located in the plasma membrane. This synthase is a molecular target for antifungal drugs such as echinocandins and the triterpenoid ibrexafungerp. In this study, we present the cryo-electron microscopy structure of Saccharomyces cerevisiae 1,3-β-glucan synthase (Fks1) at 2.47-Å resolution. The structure reveals a central catalytic region adopting a cellulose synthase fold with a cytosolic conserved GT-A-type glycosyltransferase domain and a closed transmembrane channel responsible for glucan transportation. Two extracellular disulfide bonds are found to be crucial for Fks1 enzymatic activity. Through structural comparative analysis with cellulose synthases and structure-guided mutagenesis studies, we gain previously unknown insights into the molecular mechanisms of fungal 1,3-β-glucan synthase.

Promiscuity Characteristics of Versatile Plant Glycosyltransferases for Natural Product Glycodiversification

Glycodiversification can optimize the properties of pharmaceutical compounds, and versatile glycosyltransferases (GTs) are the key enzymatic toolkits to achieve this goal. Plant GTs in the GT1 family (GT1-pGTs) have attracted much attention due to their promising substrate promiscuity, but previous investigations on GT1-pGTs were mainly conducted sporadically and without systematic phylogenetic comparisons. In this study, we exemplified the phylogeny-guided characterization of highly promiscuous GT1-pGTs from the contemporary surge of genomic information. All the available GT1-pGT sequences in the database were analyzed to explore the relationships between the substrate promiscuity and the phylogeny of GT1-pGTs. This systematic phylogenetic analysis directed us to choose 29 anonymous GT sequences from different evolutionary branches to probe their substrate promiscuity toward 10 aromatic compounds differing in chemical scaffolds. We found that promiscuous plant GTs (PPGTs) active toward ≥3 substrates were widely distributed in different clades but particularly enriched in the one containing the known promiscuous enzyme GuGT10. Ten highly promiscuous plant GTs were found to tolerate a wide spectrum (≥8) of substrates and inclusively catalyze the formation of O-, N-, and S-glycosidic bonds. The promiscuity of these 10 PPGTs was further tested using 15 sugar donors. Finally, we characterized FiGT2 that simultaneously exhibited pronounced promiscuity in terms of both the sugar acceptor and sugar donor. All in all, this study paves the way to unearth many more PPGTs and thus strengthen the enzymatic toolkit for the sustainable production of valuable glycosides through a synthetic biological approach.

UDP-glycosyltransferase family in Haemonchus contortus: Phylogenetic analysis, constitutive expression, sex-differences and resistance-related differences

UDP-glycosyltransferases (UGT), catalysing conjugation of UDP-activated sugar donors to small lipophilic chemicals, are widespread in living organisms from bacteria to fungi, plant, or animals. The progress of genome sequencing has enabled an assessment of the UGT multigene family in Haemonchus contortus (family Trichostrongylidae, Nematoda), a hematophagous gastrointestinal parasite of small ruminants. Here we report 32 putative UGT genes divided into 15 UGT families. Phylogenetic analysis in comparison with UGTs from Caenorhabditis elegans, a free-living model nematode, revealed several single member homologues, a lack of the dramatic gene expansion seen in C. elegans, but also several families (UGT365, UGT366, UGT368) expanded in H. contortus only. The assessment of constitutive UGT mRNA expression in H. contortus adults identified significant differences between females and males. In addition, we compared the expression of selected UGTs in the drug-sensitive ISE strain to two benzimidazole-resistant strains, IRE and WR, with different genetic backgrounds. Constitutive expression of UGT368B2 was significantly higher in both resistant strains than in the sensitive strain. As resistant strains were able to deactivate benzimidazole anthelmintics via glycosylation more effectively then the sensitive strain, UGT368B2 enhanced constitutive expression might contribute to drug resistance in H. contortus.

Aldo-Keto Reductase Regulation by the Nrf2 System: Implications for Stress Response, Chemotherapy Drug Resistance, and Carcinogenesis

Human aldo-keto reductases (AKRs) are NAD(P)H-dependent oxidoreductases that convert aldehydes and ketones to primary and secondary alcohols for subsequent conjugation reactions and can be referred to as "phase 1" enzymes. Among all the human genes regulated by the Keap1/Nrf2 pathway, they are consistently the most overexpressed in response to Nrf2 activators. Although these enzymes play clear cytoprotective roles and deal effectively with carbonyl stress, their upregulation by the Keap1/Nrf2 pathway also has a potential dark-side, which can lead to chemotherapeutic drug resistance and the metabolic activation of lung carcinogens (e.g., polycyclic aromatic hydrocarbons). They also play determinant roles in 4-(methylnitrosoamino)-1-(3-pyridyl)-1-butanone metabolism to R- and S-4-(methylnitrosoamino)-1-(3-pyridyl)-1-butanol. The overexpression of AKR genes as components of the "smoking gene" battery raises the issue as to whether this is part of a smoking stress response or acquired susceptibility to lung cancer. Human AKR genes also regulate retinoid, prostaglandin, and steroid hormone metabolism and can regulate the local concentrations of ligands available for nuclear receptors (NRs). The prospect exists that signaling through the Keap1/Nrf2 system can also effect NR signaling, but this has remained largely unexplored. We present the case that chemoprevention through the Keap1/Nrf2 system may be context dependent and that the Nrf2 "dose-response curve" for electrophilic and redox balance may not be monotonic.

Glycomimetics Targeting Glycosyltransferases: Synthetic, Computational and Structural Studies of Less-Polar Conjugates

The Leloir donors are nucleotide sugars essential for a variety of glycosyltransferases (GTs) involved in the transfer of a carbohydrate to an acceptor substrate, typically a protein or an oligosaccharide. A series of less-polar nucleotide sugar analogues derived from uridine have been prepared by replacing one phosphate unit with an alkyl chain. The methodology is based on the radical hydrophosphonylation of alkenes, which allows coupling of allyl glycosyl compounds with a phosphate unit suitable for conjugation to uridine. Two of these compounds, the GalNAc and galactose derivatives, were further tested on a model GT, such as GalNAc-T2 (an important GT widely distributed in human tissues), to probe that both compounds bound in the medium-high micromolar range. The crystal structure of GalNAc-T2 with the galactose derivative traps the enzyme in an inactive form; this suggests that compounds only containing the β-phosphate could be efficient ligands for the enzyme. Computational studies with GalNAc-T2 corroborate these findings and provide further insights into the mechanism of the catalytic cycle of this family of enzymes.

Recent developments in the enzymatic O-glycosylation of flavonoids

The glycosylation of bioactive compounds, such as flavonoids, is of particular relevance, as it modulates many of their pharmacokinetic parameters. This article reviews the literature between 2010 and the end of 2015 that deals with the enzymatic O-glycosylation of this class of compounds. Enzymes of glycosyltransferase family 1 remain the biocatalysts of choice for glycodiversification of flavonoids, in spite of relatively low yields. Transfers of 14 different sugars, in addition to glucose, were reported. Several Escherichia coli strains were metabolically engineered to enable a (more efficient) synthesis of the required donor during in vivo glycosylations. For the transfer of glucose, enzymes of glycoside hydrolase families 13 and 70 were successfully assayed with several flavonoids. The number of acceptor substrates and of regiospecificities characterized so far is smaller than for glycosyltransferases. However, their glycosyl donors are much cheaper and yields are considerably higher. A few success stories of enzyme engineering were reported. These improved the catalytic efficiency as well as donor, acceptor, or product ranges. Currently, the development of appropriate high-throughput screening systems appears to be the major bottleneck for this powerful technology.

Recent biotechnological progress in enzymatic synthesis of glycosides

Glycosylation is one of the most important post-modification processes of small molecules and enables the parent molecule to have increased solubility, stability, and bioactivity. Enzyme-based glycosylation has achieved significant progress due to advances in protein engineering, DNA recombinant techniques, exploitation of biosynthetic gene clusters of natural products, and computer-based modeling programs. Our report summarizes glycosylation data that have been published within the past five years to provide an overall review of current progress. We also present the future trends and perspectives for glycosylation.

In situ UDP-glucose regeneration unravels diverse functions of plant secondary product glycosyltransferases

The catalytic function of plant secondary product glycosyltransferases (PSPGs) was investigated by coupling the activities of recombinant flavonoid glucosyltransferases having different regiospecificities with sucrose synthase from Arabidopsis thaliana. In the present system, UDP, a product inhibitor of PSPGs, was removed from the reaction mixture and used for regeneration of UDP-glucose by AtSUS1. The in situ UDP-glucose regeneration system not only enhanced the glucosylation efficiency but also unraveled the novel regioselectivity of PSPGs. The effect of the system was shown to be because of the removal of UDP from the reaction system and not because of the additional supply of UDP-glucose.