Halophytic <i>C</i> -Glycosyltransferases Enable <i>C</i> -Glycosylation in Organic Solvents

<i>C</i>-glycosyltransferases from glycosyltransferase family 1 transfer sugar moieties to carbon atoms in the substituted aromatic rings of various small molecules. They are coveted biocatalysts for the synthesis of high-value glycosides since the resulting β-<i>C</i>-glycosidic linkage is usually more stable <i>in vivo</i> and <i>in vitro</i> than its <i>O</i>-glycosidic counterpart. One of the main bottlenecks in the biocatalytic glycosylation processes of small molecules is the low aqueous solubility of the acceptor substrate, drastically limiting product yields. One solution is to conduct the reaction in organic solvents provided the enzyme activity is preserved. Salt-tolerant organisms often have enzymes that are tolerant of organic solvents. In this work, we report the discovery and characterization of three novel <i>C</i>-glycosyltransferases from halophytes (i.e., salt-tolerant plants) through sequence mining. All enzymes converted phloretin to its <i>C</i>-glucosides efficiently with high regioselectivity and surprisingly exhibited significantly enhanced conversion yields in the presence of acetonitrile or methanolup to 1563% for the enzyme from<i>Trifolium fragiferum</i> (TfCGT) in 30% methanol (v/v). The halophytic <i>C</i>-glycosyltransferases had activity maxima at 55-65 °C and pH 8.7-10.0. They exhibited varying chemostability profiles toward their substrate, with the newly described enzyme from <i>Phragmites australis</i> (PaCGT) performing remarkably well under low enzyme and high phloretin conditions. In line with the extreme adaptations of their hosts, halophytic <i>C</i>-glycosyltransferases might have evolved to perform better in water-restricted conditions (e.g., in highly saline or arid habitats), thus holding great potential for industrial glycosylation processes with reduced enzyme and increased aglycon concentrations.