Sialic Acid Transfer

Known SiaTs have been classified into six glycosyltransferase (GT) famiUes in the Carbohydrate-Active enZyme (CAZy) database according to their protein sequence 

Several 3D protein structures of bacterial SiaTs have been determined in the presence of GMP-Neu5Ac as a ligand [60]. Based on these crystal structures, from which the active site organization and the catalytic mechanism have been elucidated, SiaT enzymes can be predicted to tolerate rather flexibly structural variations in the sialic acid part. This can be rationalized because only the nucleotide portion and the sialic acid substructure around the anomeric center become buried into the active site upon substrate binding and orientation toward the sialyl acceptor substrate, whereas much of the remainder of the siahc acid portion remains oriented toward the protein surface or even in contact with bulk solvent. This hypothesis has been verified by a number of preparative studies [33,47, 61). 

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A unique, membrane-associated, sialyltransferase system in E. coli has been described that catalyzes the synthesis of sialic acid polymers with the aid of sialyl-monophosphonoundecaprenol.293 In mammalian systems, no evidence was obtained lor the involvement of lipid intermediates in sialic acid transfer, or in the fonnation of sialyl-(2— -8) link-... 

B. Zingales, C. Camiol, R. M. de Lederkremer, and W. Colli, Direct sialic acid transfer from a protein donor to glycolipids of trypomastigote forms of Trypanosoma cruzi, Mol. Biochem. Parasitol., 26 (1987) 135-144. 

NeuSAc is a weak inhibitor of classical sialidases but does not act at all on the trans-sialidases. Correspondingly, the 2,3-difluoro derivative of NeuSAc, forming a covalent intermediate with sialidases and trans-sialidases (see later) was required at high concentrations (20 mM) to inactivate the enzyme completely Since trans-sialidases possess a lactose-binding site in their active center (as described next), lactose and its derivatives, especially lactitol, inhibit the sialic acid transfer of the enzyme from T. cruzi. Oligosaccharides from the mucins of this trypanosome can also inhibit the transfer of sialic acid to, for example, the substrate A-acetyllactosa-mine. It should be noted that heavy metal ions, especially merciuy, are potent inhibitors of sialidases and trans-sialidases. The latter enzymes do not require calcium ions, in contrast to some siaUdases. " " " ... 

Two trans-sialidases isolated from the animal-pathogenic Afiican trypanosome T. Congo lense show pronounced differences in their capacity for sialic acid transfer as compared with the hydrolytic activity. Partial sequences were obtained from these two enzymes by a PCR-based approach, showing 50% identity with each oflier, but are similar to viral, bacterial, animal siahdases and other trans-sialidases. Most of the critical active-site residues common to other trypanosomal siahdases and TS are conserved This similarity, together with the amino acid difference at the active site, between American and African (trans-)siahdases is depicted in the pubUcation. ... 

E. Tiralongo, S. Schrader, H. Lange, H. Lemke, J. Tiralongo, and R. Schauer, Two trans-sialidase forms with different sialic acid transfer and sialidase activities firom Trypanosoma congolense, J. Biol Chem., 278 (2003) 23301-23310. 

Since the cytoplasmic tail and transmembrane domain proved to govern Golgi localization we carefully screened our ST variants for their ability to locate in the Golgi compartment, in which they can meet protein acceptors trafficking to the cell surface and compete with the endogenous ST3Gal enzyme.Such optimized enzymes can be further adapted to any protein drug family to achieve optimal sialic acid transfer under the best cell culture conditions. ... 

To facilitate accesses to suitably functionalized sialic acid derivatives and complex sialyloligosaccharides for other usehil neoglycoconjugates, phase transfer catalysis (PTC) has been exploited extensively [for reviews see 42]. This process provided a wide range of carbohydrate derivatives under essentially clean Sn2 transformations. In the case of acetochloroneuraminic acid 1, the PTC reactions always provided inverted a-sialic acid derivatives [43]. para-Formylphenyl sialoside 7 [44], together with many other sialoside derivatives such as 8-10 [43], including thioacetate 12 [45] and azide 14 [46], were thus obtained (Scheme 1). Aldehyde 7 and similar glycosides are of particular interest since they could be directly conjugated to protein by reductive amina-tion after suitable deprotection [44]. 

Several CMP conjugates made available by this procedure could be transferred effectively to N-acetyl-lactosamine as an acceptor by using the a-2,6-sialyltrans-ferase from rat liver (Fig. 2.2.5.4). In this manner, several trisaccharides 74-77 could be generated carrying natural as well as non-natural sialic acids in good yields [47], limited so far only by the high cost of the commercial glycosyltransferase. 

Transfer of Sialic Acids onto Complex Carbohydrates.188... 

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