Most glycosylation—the posttranslational modification of a peptide or protein with one or more sugar groups—involves forming a link between the sugar and a nitrogen or an oxygen on an amino acid side chain. S-linked glycosylation, in which a sugar is added to the sulfur of cysteine, is rare. Only two enzymes that catalyze S-linked glycosylation have been identified. Wilfred A. van der Donk and coworkers at the University of Illinois, Urbana-Champaign, have now identified a third enzyme that catalyzes S-linked glycosylation. But this one does a little more: It’s the first enzyme capable of catalyzing both S-linked and O-linked glycosylation (J. Am. Chem. Soc. 2013, DOI: 10.1021/ja411159k). The enzyme, ThuS, which was found in a type of Bacillus thuringiensis soil bacterium, adds sugars to both cysteine and serine groups on antimicrobial peptides called thurandacins. In addition to its natural substrate, the researchers show, the enzyme can glycosylate other peptides. Because S-linked glycopeptides are more resistant to enzymatic degradation than are O-linked glycopeptides, van der Donk hopes to use enzymes like ThuS to make S-linked analogs of known O-glycosylated peptides.