Sialic acids linkages, types

Efficient, stereoselective sialylations are still a cumbersome challenge for synthetic carbohydrate chemists due to the lack of neighboring group participation of the sialic acid [29]. Transferase-catalyzed sialylations therefore offer a welcome synthetic alternative. To date, eight different sialic acid linkage types have been identified. Out of the more than a dozen different sialyltransferases - SiaT - that have been found and cloned [99] a rat liver a(2-6)SiaT (E.C.2.4.99.1) and porcine a(2-3)SiaT (E.C.2.4.99.4) are commercially available. The synthesis of the natural CMP-sialic acid donor and that of various derivatives have been described [39, 48, 100, 101]. The capability for transfer of donor analogs by a(2-6)SiaT and rat liver a(2-3)SiaT (E.C.2.4.99.6) has recently been compiled [28,48]. 

Analysis of Sialic Acid Linkage Types in Glycoconjugates... 

While all these enzymes hydrolytically cleave a-glycosidic bonds of sialic acids, another type of sialidase has been discovered, which can also hydrolyze these linkages but preferably forms sialic acid linkages under physiological conditions in the presence of suitable acceptors. These are the /rans-sialidases, occurring in parasites, and a... 

This family contains non-enveloped DNA viruses that bind to their receptors via interactions with the distal knob of the penton fibers attached to the vertices of the icosahedral virions. Human adenoviruses mainly cause respiratory and gastrointestinal infections. Several adenoviruses also infect the eye where the most important disease is epidemic keratoconjunctivitis (EKC), caused primarily by Ad8, Adl9, and Ad37. Ad37 binds preferentially to a2-3-linked sialic acid which is the most frequent type of sialic acid linkage in corneal and conjunctival cells [201]. The crystal structure of the Ad37 knob-sialic acid complex has been elucidated [202]. 

Collins BE, Kiso M, Hasegawa A, Tropak MB, Roder JC, et al. Binding specificities of the sialoadhesin family of I-type lectins. Sialic acid linkage and substructure requirements for binding of myelin-associated glycoprotein, Schwann cell myelin protein, and sialoadhesin. J. Biol Chem. 1997 272 16889-16895. 

Siglec Chromosomal localisation (human) Ig-like domains Cellular expression Sialic acid linkage Sialic acid type Proposed functions W... 

Figure 9.11 Structure of an oligosaccharide unit of human transferrin. The linkage between the oligosaccharide and polypeptide chains is of the N-asparaginyl type. Dotted arrows indicate positions in which one sometimes finds NeuNAc-Gal-GlcNAc units. Gal is galactose, GIcNAc is N-acetylglucosamine, NeuNAc is N-acetylneuraminic acid (sialic acid), and Man is mannose. (Reproduced with permission from Bezkorovainy A. Biochemistry of Nonheme Iron. New York Plenum Press, 1980, p. 175.)...

Once the number of antennae is established, further extension is possible through addition of backbone polymers and terminal structures similar to those found on mucin-type glycans. LacNAc polymers can be added to any of the aforementioned GlcNAcs (Fig. 4A, w-x). Similarly, these polymers can be elaborated with fucose, sulfate, and sialic acid added in the same linkages described for mucin-type structures (Fig. 4A, y-z). Most fucosyltransferases and sulfotransferases, as well as the sialyltransferases that cap polyLacNAc, are capable of modifying both O-linked and A-linked glycans some exhibit a preference for one or the other. These biases are likely a result of differences in enzyme localization. 

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