Glycosylation — attaching a sugar molecule to a compound — can dramatically reshape a molecule's fate: boosting solubility, stability, and bioactivity, or even reducing toxicity. Star molecules such as ginsenosides, saikosaponins, and QS-21 (a next-generation vaccine adjuvant) all owe much of their power to sugars. The "tailors" performing this delicate stitching are glycosyltransferases (GTs), which precisely sew a sugar from an activated donor (usually a UDP-sugar) onto a specific site of the acceptor molecule.
Now, a team at Naval Medical University and Shanghai University of Traditional Chinese Medicine has reported a tailor of exceptional talent. In a study published in Chinese Journal of Natural Medicines (2026, 24(8): 999-1005), they identified PamUGT — the first triterpenoid O-glycosyltransferase ever discovered from the genus *Phytolacca* — and showed that it is a rare, broad-spectrum biocatalyst capable of decorating five major classes of natural products.
The Needle in the Haystack: Mining 36 Candidate Genes for One True Enzyme
The medicinal herb Shanglu (derived from *Phytolacca acinosa* and its close relative *P. americana*) contains more than 40 triterpenoid saponins, making it an ideal "enzyme resource bank." Yet a puzzle has persisted: the only two glycosyltransferases previously characterized from American pokeweed, PaGT2 and PaGT3, showed interest solely in flavonoids and phenols. Could any enzyme in this genus glycosylate triterpenes? No one knew.
To find out, the team downloaded public transcriptome data from three *Phytolacca* species and, using known oleanane-type triterpenoid GTs as "baits," screened 36 candidate genes. Thirty-one were successfully cloned and expressed in *E. coli*.
Then came the functional audition: testing each recombinant protein against three triterpene substrates (phytolaccagenin, jaligonic acid, and bayogenin) with UDP-glucose. Only one enzyme passed: PamUGT, converting all three substrates at 30%–40%. Mass spectrometry confirmed a mass increase of exactly 162 Da — the weight of one glucose — and NMR pinpointed where the sugar was attached.
Further characterization revealed a full-length gene of 1,443 bp encoding 480 amino acids (~60 kDa), with optimal activity at 50 °C and pH 7.4, mildly enhanced by Mg²⁺. Phylogenetic analysis showed that although PamUGT clusters with GTs that catalyze 28-COOH glycosylation, its sequence similarity to known enzymes is remarkably low (at most 33.06%) — a genuinely novel, independently evolving branch.
Four Discoveries: One Enzyme, Many Talents
1. The first triterpenoid GT from *Phytolacca* — with a taste for the 30-position carboxyl
PamUGT glycosylates the 28- or 30-position carboxyl groups of pentacyclic triterpenes. Intriguingly, when a molecule carries both a 28-COOH and a 30-COOH, PamUGT selects only the 30-COOH, yielding a single monoglycoside — a regioselectivity rarely seen in previously reported enzymes.
2. A diet spanning five classes of natural products
Across 42 structurally diverse test compounds, PamUGT's "menu" proved astonishingly wide:
- Pentacyclic triterpenes: every substrate tested was catalyzed (while tetracyclic triterpenes showed no activity — the conformational flexibility of their C17 single bond apparently blocks the substrate from adopting the catalytic orientation);
- Flavonoids: conversion rates mostly above 50% — higher than for triterpenes — with strong regioselectivity at 7-OH and 3'-OH, generating mono- and even diglucosides;
- Diterpenes: an impressive 68% conversion for steviol — 1.7 times that of the triterpene substrate;
- Phenols, alkaloids, and phenylpropanoids: low but detectable activity (6%–15%), despite these compounds not being native to American pokeweed at all.
3. Sugar donor preference with flexibility
For substrate 1, PamUGT strictly insists on UDP-glucose. But for substrate 9 (glycyrrhetic acid), it flexibly uses all five UDP-sugars tested — glucose, xylose, galactose, rhamnose, and glucuronic acid — combining innate preference with rare donor promiscuity.
4. A fondness for "naked" sapogenins
Sapogenins (aglycones without sugar chains) were consistently better substrates than their corresponding saponins, and the longer the sugar chain, the lower the efficiency — pointing to steric hindrance as a key determinant. Kinetic data agree: phytolaccagenin showed the lowest Km (169.56 μmol/L), making it a likely natural substrate and suggesting PamUGT participates in the biosynthesis of esculentosides, the signature saponins of *Phytolacca*.
Why It Matters: From Biosynthesis to "Tailor-Made" Glycosylation
- Filling a gap: PamUGT completes a missing piece of the esculentoside biosynthetic pathway, providing a molecular basis for understanding how this traditional Chinese medicine manufactures its active compounds.
- A rare generalist: While promiscuous GTs (MhGT1, OleD, UGT74AN3) are known, enzymes that efficiently glycosylate flavonoids, diterpenes, *and* multiple triterpenes simultaneously are uncommon. PamUGT offers a unique model for studying how plant UGTs evolve such broad "diets" while keeping their natural substrate preference.
- A practical biocatalytic tool: Chemical glycosylation is often laborious and protection-group-heavy. PamUGT works under mild conditions (50 °C, neutral pH), needs no complex cofactor regeneration, and shows clear regioselectivity — a ready-made "molecular sewing machine" for the green preparation of diverse O-glycosides, whether for drug modification or natural product library construction.
- Looking ahead: The enzyme could be engineered to tailor specific glycoforms, or deployed in microbial chassis (e.g., yeast) to build complete *Phytolacca* saponin biosynthetic routes — following the 2024 landmark full biosynthesis of QS-21 in engineered yeast, where tool enzymes like this are precisely the key to the final step.
The authors note open questions: flavonoid substrates showed no clear structure–activity pattern, and the molecular basis of substrate recognition awaits crystal structures or docking studies; whether PamUGT truly functions in vivo also requires gene knockout or heterologous reconstruction experiments.
