A-Linked glycans

Fig. 15.5. Structures of A/-linked glycans from several different species of parasitic nematodes, illustrating both similarities with mammalian glycans (compare with Figs 15.1 and 15.2) and features unique to nematodes (e.g. tyvelose and PC capping and novel core fucosylation). The filarial nematode glycans are believed to be substituted with charged residues, which are not yet characterized.

Example The PSD-MALDI-TOF spectrum of the [M+Na]" ion, m/z 1418.9, of high-mannose A-linked glycan (Man)6(GlcNAc)2 from chicken ovalbumin recorded from DHB shows distinct cleavages of the branched carbohydrate skeleton (Fig. 10.16). [134]... 

Misaki, R., Kimura, Y, Palacpac, N.Q., Yoshida, S., Fujiyama, K., and Seki, T. (2003). Plant cultured cells expressing human 31,4-galactosyltransferase secrete glycoproteins with galactose-extended A-linked glycans. Glycobiology 13(3) 199-205. 

S. F. Wheeler and D. J. Harvey, Extension of the in-gel release method for structural analysis of neutral and sialylated A-linked glycans to the analysis of sulfated glycans Application to the glycans from bovine thyroid-stimulating hormone, Anal. Biochem., 296 (2001) 92-100. 

Helenius A, Aebi M. Intracellular functions of A-linked glycans. Science 2001 291 2364-2369. 

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. 

Fucosylation of the Asn-linked GlcNAc is a common modification unique to A-linked glycans. In vertebrates, core FucT adds fucose in an al-6 linkage (59), whereas in plants and invertebrates al-3-linked fucose is observed (60). 

Helenius A, Aebi M. Roles of A-linked glycans in the endoplasmic reticulum. Ann. Rev. Biochem. 2004 73 1019-1049. Tulsiani DRP, Touster O. The purification and characterization of mannosidase la from rat liver Golgi membranes. J. Biol. Chem. 1988 263(11) 5408-5417. 

In order to reduce the complexity of the spectra acquired by precursor-ion analysis (m/z 204 as common product ion) in the analysis of complex glycan mixtures without prior LC separation, precursor-ion analysis was performed with larger ions, e.g., with common product ions like m/z 1935, 1406, and 1396 [70]. This method was applied in the characterization of over thirty complex A-linked glycan stmctures from the scrapie-associated prion protein PrP ". 

The biosynthetic pathway of A-linked glycans has been defined by the extensive research efforts of multiple groups, and summaries of the present understanding of the process are now readily available. The pathway for biosynthesis of A-linked glycans includes three major steps. 

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