Catalytic analysis of the validamycin glycosyltransferase (ValG) and enzymatic production of 4-epi-validamycin A

ValG is a glycosyltransferase (GT) that is responsible for the glucosylation of validoxylamine A to validamycin A. To explore the potential utilization of ValG as a tool for the production of validamycin analogues, a number of nucleotidyldiphosphate-sugars were evaluated as alternative substrates for ValG. The results indicated that in addition to its natural substrate, UDP-glucose, ValG also efficiently utilized UDP-galactose as sugar donor and resulted in the production of an unnatural compound, 4-epi-validamycin A. The new compound demonstrated a moderate growth inhibitory activity against the plant fungal pathogen Rhizoctonia solani (=Pellicularia sasakii). A comparative analysis of ValG with its homologous proteins revealed that ValG contains an unusual DTG motif, in place of the DXD motif proposed for metal ion binding and/or NDP-sugar binding and commonly found in other glycosyltransferases. Site-directed mutagenesis of the DTG motif of ValG to DCD altered its preferences for metal ion binding, but did not seem to affect its substrate specificity.