Abstract
Saponins are a class of natural products composed of an oxidized triterpene core adorned with glycosylations, ultimately giving rise to medicinally important compounds bearing bioactivity that includes, but is not limited to, anti-inflammatory, antimicrobial, antifungal, antiarrhythmic, and immunostimulatory activities. QS-21 is a prominent immunostimulatory saponin and is a critical adjuvant component of several FDA-approved vaccines. One linchpin modification in the biosynthesis and bioactivity of several saponins, including QS-21, is O-d-fucosylation via an ester linkage. In QS-21, the C28-COOH O-d-fucose residue is part of a linear oligosaccharide that is an integral component of the "core pharmacophore" responsible for its immunomodulatory activity. In this work, we performed in-depth in vitro enzymological characterization of two glycosyltransferases involved in C28-COOH O-d-fucosylation during the maturation of two saponin natural products: QsFucT from QS-21 biosynthesis and SvFucT from vaccaroside biosynthesis. QsFucT was previously shown to be a UDP-4-keto-6-deoxy-d-glucosyltransferase; our data reveal that the taxonomically distant SvFucT also functions as a UDP-4-keto-6-deoxy-d-glucosyltransferase and that both glycosyltransferases act on a triterpene acceptor with low-micromolar affinity. Substrate scope studies demonstrate that both enzymes are highly permissive with regard to both the triterpene acceptor and, unexpectedly, the UDP-sugar donor. These data also reveal that the conserved C3-OH branched trisaccharide of QS-21 and other saponins may serve an unusual biosynthetic role in protecting the C23 aldehyde from spurious reduction during biosynthesis. In addition, we crystallized and solved the structures of QsFucT and SvFucT, providing the first structural characterization of 4-keto-6-deoxy-d-glucosyltranferases in the glycosyltransferase family 1 (GT1) class of enzymes and used these structures to explore the importance of conserved residues in the active site. These data suggest that both QsFucT and SvFucT could be leveraged to rapidly explore saponin chemical space and glycodiversify these important medicinal compounds through engineered biosynthesis or in vitro enzymatic synthesis, possibly leading to novel analogs with enhanced physicochemical or pharmacological properties.