**Fiona F. Hager 1, Leander Sützl 2, Cordula Stefanovi´c 1, Markus Blaukopf <sup>3</sup> and Christina Schä**ff**er 1,\***


Received: 13 August 2019; Accepted: 27 September 2019; Published: 5 October 2019

**Abstract:** Glycoconjugates are the most diverse biomolecules of life. Mostly located at the cell surface, they translate into cell-specific "barcodes" and offer a vast repertoire of functions, including support of cellular physiology, lifestyle, and pathogenicity. Functions can be fine-tuned by non-carbohydrate modifications on the constituting monosaccharides. Among these modifications is pyruvylation, which is present either in enol or ketal form. The most commonly best-understood example of pyruvylation is enol-pyruvylation of *N*-acetylglucosamine, which occurs at an early stage in the biosynthesis of the bacterial cell wall component peptidoglycan. Ketal-pyruvylation, in contrast, is present in diverse classes of glycoconjugates, from bacteria to algae to yeast—but not in humans. Mild purification strategies preventing the loss of the acid-labile ketal-pyruvyl group have led to a collection of elucidated pyruvylated glycan structures. However, knowledge of involved pyruvyltransferases creating a ring structure on various monosaccharides is scarce, mainly due to the lack of knowledge of fingerprint motifs of these enzymes and the unavailability of genome sequences of the organisms undergoing pyruvylation. This review compiles the current information on the widespread but under-investigated ketal-pyruvylation of monosaccharides, starting with different classes of pyruvylated glycoconjugates and associated functions, leading to pyruvyltransferases, their specificity and sequence space, and insight into pyruvate analytics.

**Keywords:** pyruvylation; pyruvyltransferase; exopolysaccharides; capsular polysaccharides; cell wall glycopolymers; *N*-glycans; lipopolysaccharides; biosynthesis; sequence space; pyruvate analytics
