Biosynthesis of Legionaminic Acid and Its Incorporation Into Glycoconjugates

Abstract Legionaminic acids are analogs of sialic acidthat occur in cell surface glycoconjugatesof several bacteria. Because legionaminic acids share the same stereochemistry as sialic acidbut differ at C7 and C9, they are interesting analogs to probe the impact of varying exocyclic moieties (C7–C9) on biological activities such as susceptibilities to sialidases, interactions with Siglecsand immunogenicity. There are currently no reports on the bacterial enzymes that transfer legionaminic acids to these cell surface glycoconjugates, but some mammalian and bacterial sialyltransferasesdisplay donor promiscuityand can use CMP-Leg5,7Ac 2 efficiently enough to transfer Leg5,7Ac 2 to their natural acceptor glycans. When the natural activity with CMP-Leg5,7Ac 2 is significant but relatively low, an alternate strategy has been to engineer versions with improved activity to transfer Leg5,7Ac 2 . Importantly, we have found that some bacterial sialyltransferasesare very efficient for transferring Leg5,7Ac 2 to small synthetic glycans with various aglycones. The two mammalian sialyltransferasesthat have been tested so far (porcine ST3Gal1 and human ST6Gal1) were found to be more efficient than the bacterial sialyltransferasesfor the modification of glycoproteins. We provide a review of the sialyltransferasesselected to modify different types of glycoconjugateswith Leg5,7Ac 2 , including small synthetic acceptors, glycolipids, and glycoproteins. In the first part, we also propose an optimized biosynthetic pathway for in vitro preparation of the donor CMP-Leg5,7Ac 2 , based on enzymes selected from two bacteria that naturally produce legionaminic acid.