Teichulosonic acid, an anionic polymer of a new class from the cell wall of Actinoplanes utahensis VKM Ac-674(T)

Structural and Biosynthetic Diversity of Nonulosonic Acids (NulOs) That Decorate Surface Structures in Bacteria

Nonulosonic acids (NulOs) are a diverse family of 9-carbon α-keto acid sugars that are involved in a wide range of functions across all branches of life. The family of NulOs includes the sialic acids as well as the prokaryote-specific NulOs. Select bacteria biosynthesize the sialic acid N-acetylneuraminic acid (Neu5Ac), and the ability to produce this sugar and its subsequent incorporation into cell-surface structures is implicated in a variety of bacteria-host interactions. Furthermore, scavenging of sialic acid from the environment for energy has been characterized across a diverse group of bacteria, mainly human commensals and pathogens. In addition to sialic acid, bacteria have the ability to biosynthesize prokaryote-specific NulOs, of which there are several known isomers characterized. These prokaryotic NulOs are similar in structure to Neu5Ac but little is known regarding their role in bacterial physiology. Here, we discuss the diversity in structure, the biosynthesis pathways, and the functions of bacteria-specific NulOs. These carbohydrates are phylogenetically widespread among bacteria, with numerous structurally unique modifications recognized. Despite the diversity in structure, the NulOs are involved in similar functions such as motility, biofilm formation, host colonization, and immune evasion.

Influence of side chain conformation and configuration on glycosyl donor reactivity and selectivity as illustrated by sialic acid donors epimeric at the 7-position

Two N-acetyl 4O,5N-oxazolidinone-protected sialyl thioglycosides epimeric at the 7-position have been synthesized and their reactivity and stereoselectivity in glycosylation reactions have been compared. It is demonstrated that the natural 7S-donor is both more reactive and more α-selective than the unnatural 7R-isomer. The difference in reactivity is attributed to the side chain conformation and specifically to the proximity of O7 to the anomeric center. In the natural 7S-isomer, O7 is closer to the anomeric center than in its unnatural 7R-epimer and, therefore, better able to support incipient positive charge at the locus of reaction. The difference in selectivity is also attributed to the side conformation, which in the unnatural 7R-series is placed perpendicularly above the α-face of the donor and so shields it to a greater extent than in the 7S-series. These observations are consistent with earlier conclusions on the influence of the side chain conformation on reactivity and selectivity derived from conformationally locked models in the glucose and galactose series and corroborate the suggestion that those effects are predominantly stereoelectronic rather than torsional. The possible relevance of side chain conformation as a factor in the influence of glycosylation stereoselectivity by remote protecting groups and as a control element in enzymic processes for glycosidic bond formation and hydrolysis are discussed. Methods for assignment of the anomeric configuration in the sialic acid glycosides are critically surveyed.