Bacterial Sialate O-Acetyltransferases

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Several pathogenic bacteria decorate their cell surface with sialoglycoconjugates that in many cases mimic host structures and serve as important virulence factors. In addition to N-acetyl neuraminic acid, the prevalent sialic acid in the humans, O-acetylated sialic acids are observed in bacteria that carry acetyl groups at position C-7, C-8 and/or C-9. The ability to modify cell surface sialo-glycoconjugates by O-acetylation depends on the presence of sialate O-acetyltransferases, an enzyme class that catalyzes the transfer of acetyl groups from acetyl Coenzyme A to hydroxyl groups of either free or CMP-activated sialic acid or particularly sialylated carbohydrate structures. On the genetic level, distinct mechanisms were observed which lead to an 'on/off' switch of sialate O-acetyltransferase expression and/or modification of the enzymatic activity. The resulting changes in the degree of surface O-acetylation of these bacteria can lead to a huge structural variety that make them difficult targets for the immune system. Structural and biochemical analyzes demonstrated that bacterial sialate O-acetyltransferases evolved independently on two distinct structural frameworks, the left-handed β-helix fold and the α/β-hydrolase fold.