The Sweet Side of Plant-Specialized Metabolism

Abstract

Glycosylation plays a major role in the structural diversification of plant natural products. It influences the properties of molecules by modifying the reactivity and solubility of the corresponding aglycones, so influencing cellular localization and bioactivity. Glycosylation of plant natural products is usually carried out by uridine diphosphate(UDP)-dependent glycosyltransferases (UGTs) belonging to the carbohydrate-active enzyme glycosyltransferase 1 (GT1) family. These enzymes transfer sugars from UDP-activated sugar moieties to small hydrophobic acceptor molecules. Plant GT1s generally show high specificity for their sugar donors and recognize a single UDP sugar as their substrate. In contrast, they are generally promiscuous with regard to acceptors, making them attractive biotechnological tools for small molecule glycodiversification. Although microbial hosts have traditionally been used for heterologous engineering of plant-derived glycosides, transient plant expression technology offers a potentially disruptive platform for rapid characterization of new plant glycosyltransferases and biosynthesis of complex glycosides.

Figure 1.

Examples of glycosylated plant-specialized metabolites. Compounds that are active in their fully glycosylated forms (A) or upon hydrolysis (B) are shown.

Figure 2.

Phylogenetic tree of characterized plant glycosyltransferases 1 (GT1s). Reconstruction of GT1 phylogeny from a collection of 246 biochemically characterized GT1 protein sequences. The groups are delineated as defined by Ross et al. (2001) and Caputi et al. (2012). Discrete clusters of enzymes with similar activities are indicated. Monocot-specific branches are shown in blue. The tree was constructed with Mega 6.06 (Tamura et al. 2013) using the maximum likelihood method from a protein alignment obtained with MUSCLE 3.8 (Edgar 2004). *, Closely related Arabidopsis thaliana family F GT1s with different sugar specificities (see main text). The scale bar indicates 0.2 substitutions per site at the amino acid level.

Figure 3.

Determinants of sugar specificity of plant glycosyltransferases 1 (GT1s). (A) Consensus plant secondary product glycosyltransferase (PSPG) motif generated by weblogo (weblogo.berkeley.edu) from an alignment of characterized GT1s. (B) The sugar donor-binding site for the crystal structure of the GT1 enzyme UGT71G (a flavonoid/triterpenoid O-glucosyltransferase Medicago truncatula) in complex with uridine diphosphate glucose (UDP-Glc) (PDB code 2ACW). The PSPG motif is shown in dark green, the N5 loop in light green, and catalytic residues in dark red. UDP-Glc is shown as a ball and stick model and colored in blue. Proposed hydrogen bonds are shown as dashed lines. (C) Compared structures of most common sugar donors of plant GT1s.

Figure 4.

Proposed catalytic mechanisms of retaining glycosyltransferases 1 (GT1s) (A) and inverting glycosyl hydrolase family 1 (GH1) transglucosidases (TGs) (B). UMP, Uridine monophosphate.

Figure 5.

Examples of plant glycosides produced by metabolic engineering in heterologous hosts. Examples shown are from data in Hsu et al. (2018), Shen et al. (2017), Xue et al. (2016), Chung et al. (2017), Bai et al. (2016), Louveau et al. (2018), Liu et al. (2018), Irmisch et al. (2018), Olsson et al. (2016), and Zhuang et al. (2017).