Synthesis of Neu5Ac Oligosaccharides and Analogues by Transglycosylation and their Binding Properties as Ligands to MAG

trans-Sialylation: a strategy used to incorporate sialic acid into oligosaccharides

Sialic acid, as a component of cell surface glycoconjugates, plays a crucial role in recognition events. Efficient synthetic methods are necessary for the supply of sialosides in enough quantities for biochemical and immunological studies. Enzymatic glycosylations obviate the steps of protection and deprotection of the constituent monosaccharides required in a chemical synthesis. Sialyl transferases with CMP-Neu5Ac as an activated donor were used for the construction of α2-3 or α2-6 linkages to terminal galactose or N-acetylgalactosamine units. trans-Sialidases may transfer sialic acid from a sialyl glycoside to a suitable acceptor and specifically construct a Siaα2-3Galp linkage. The trans-sialidase of Trypanosoma cruzi (TcTS), which fulfills an important role in the pathogenicity of the parasite, is the most studied one. The recombinant enzyme was used for the sialylation of β-galactosyl oligosaccharides. One of the main advantages of trans-sialylation is that it circumvents the use of the high energy nucleotide. Easily available glycoproteins with a high content of sialic acid such as fetuin and bovine κ-casein-derived glycomacropeptide (GMP) have been used as donor substrates. Here we review the trans-sialidase from various microorganisms and describe their application for the synthesis of sialooligosaccharides.

Trypanosoma cruzi trans-sialidase. A tool for the synthesis of sialylated oligosaccharides

Cells are covered by a complex array of carbohydrates. Among them, sialosides are of key importance in intracellular adhesion, recognition and signaling. The need for structurally diverse sialosides impelled the search for efficient synthetic methods since their isolation from natural sources is a difficult task. The enzymatic approach obviates the need of a chemical synthesis for protecting or participating groups in the substrates. The trans-sialidase of Trypanosoma cruzi (TcTS) is highly stereospecific for the transfer of sialic acid from an α-sialylglycoside donor to a terminal β-galactopyranosyl unit in the acceptor substrate to form the α-Neu5Ac-(2 → 3)-β-D-Galp motif. The enzyme was cloned and easily available glycoproteins, e.g. fetuin, may be used as donors of sialic acid, constituting strong points for the scalability of TcTS-catalyzed reactions. This review outlines the preparative use of TcTS for the sialylation of oligosaccharides. A detailed description of the substrates used as sialic acid donors, the acceptor substrates and the methods employed to monitor the reaction is included.

Probing the acceptor substrate binding site of Trypanosoma cruzi trans-sialidase with systematically modified substrates and glycoside libraries

Systematically modified octyl galactosides and octyl N-acetyllactosamines were assessed as inhibitors of, and substrates for, T. cruzi trans-sialidase (TcTS) in the context of exploring its acceptor substrate binding site. These studies show that TcTS, which catalyses the α-(2→3)-sialylation of non-reducing terminal β-galactose residues, is largely intolerant of substitution of the galactose 2 and 4 positions whereas substitution of the galactose 6 position is well tolerated. Further studies show that even the addition of a bulky sugar residue (glucose, galactose) does not impact negatively on TcTS binding and turnover, which highlights the potential of 'internal' 6-substituted galactose residues to serve as TcTS acceptor substrates. Results from screening a 93-membered thiogalactoside library highlight a number of structural features (notably imidazoles and indoles) that are worthy of further investigation in the context of TcTS inhibitor development.

Transsialidase fromTrypanosoma cruzi for Regio- and Stereoselective Synthesis of N-Acyl-Modified Sialylated Oligosaccharides and Measurement of Transfer Rates

Recombinant transsialidase from Trypanosoma cruzi (TcTS) was used for the sialylation with natural and non-natural derivatives of neuraminic acid. Neu5Ac-alpha(2-->3)-Gal-beta(1-->6)-Glc-alphaOMe was prepared in 80 % yield. Correspondingly, the modified trisaccharide derivatives Neu5Prop-alpha(2-->3)-Gal-beta(1-->6)-Glc-alphaOMe (32 %) and Neu5Gc-alpha(2-->3)-Gal-beta(1-->6)-Glc-alphaOMe (Prop=propanoyl, Gc=glycolyl) were obtained in 60 % yield, respectively.

Donor substrate binding to trans-sialidase of Trypanosoma cruzi as studied by STD NMR

Using STD NMR experiments, we have studied the binding epitopes of p-nitrophenyl glycosides of sialic acid and analogs thereof when bound to Trypanosoma cruzi trans-sialidase (TSia). Time-dependent NMR spectra yielded data on the rate of substrate hydrolysis in comparison to sialic acid transfer. Our experiments clearly demonstrate that shortening of the glycerol side chain significantly favors the transfer reaction over hydrolysis. Our results extend the basis on which specific trans-sialidase inhibitors may be designed.