Trans-sialidases reaction

Fig. 10. Trans-sialidase reaction. Sialic acid is transferred fiom one galactose with Ri as penultimate glycan to another galactose bound to R2 in a reversible reaction. From Schrader et al. with the permission of the pubhshers.

Most information on how the trans-sialidase reaction proceeds was obtained from binding studies with either active T cruzi TS or with inactive natural mutants using surface plasmon resonance technology and NMR spectroscopy. While, for example, sialyllactose was readily bound to the enzyme protein, the acceptor substrate lactose or other asialo glycoconjugates did not interact with the enzyme at all unless it had been pre-incubated with the sialylated donor substrate. All of these studies clearly show that sialic acid modulates the afiSnity for die asialo acceptor substrate. The association of the asialo receptor with the active site is an... 

S. Schenkman, M. A. J. Ferguson, N. Heise, M. L. de Almeida, R. A. Mortara, andN. Yoshida, Mucinlike glycoproteins linked to the membrane by glycosylphosphatidylinositol anchor are the major acceptors of sialic acid in a reaction catalyzed by trans-sialidase in metacyclic forms of Trypanosoma cruzi, Mol. Biochem. Parasitol., 59 (1993) 293-303. 

M. Ribeirao, V. L. Pereira-Chioccola, D. Eichinger, M. M. Rodrigues, and S. Schenkman, Temperature differences for frans-glycosylation and hydrolysis reaction reveal an acceptor binding site in the catalytic mechanism of Trypanosoma cruzi trans-sialidase, Glycobiology, 1 (1997) 1237-1246. 

Remarkably, the tran -sialidases from T. cruzi and T. brucei are not inhibited by Neu2en5Ac [550,809,813]. As this is in contrast to most other sialidases described in the groups 1-3 of the previous section, Neu2en5Ac is probably not an intermediate of the tra 5-sialidase reaction. Furthermore, A-(4-nitrophenyl)oxamic acid and related A-acylanilines do not inhibit T. cruzi or T. brucei trans-sialidases, which is also in contrast to the influence of this substance on the activity of sialidases from V. cholerae and other bacteria [813]. 

Another unusual glycosidase mechanism classified is encountered in family 4, which requires a unique NAD" and Mn " " assisted catalysis. Those enzymes do not follow the classical double-displacement mechanism but act through multiple catalytic steps involving oxidation-elimination-addition and reduction reactions. Other interesting examples can also be found in the case of trans-sialidases and a-glucan lyases. ... 

Abstract In contrast to the general hydrolases, trans-sialidase from Trypanosoma cruzi (TcTS) shows excellent regio- and stereoselectivity as well as high yields in transfer reactions. Discussed are the occurrence of trans-sialidases and studies on the transfer mechanism. In detail, the preparative use by chemoenzymatic syntheses with TcTS are outlined with emphasis on the design of modified donor and acceptor substrates. Another section focuses on attempts to develop inhibitors for TcTS, and these endeavors are based on donor- and acceptor-inspired modifications as well as on some completely different structures. 

Fig. 8. Alternative catalytic nucleophiles, (a) Tyrosine functions as the catalytic nucleophile in Clan GH-E sialidases and trans-silalidases, with the assistance of a proximal carboxylate amino acid side chain, (b) Anchimeric assistance in hexosaminidases involving reaction of the A -acetyl group of the substrate with the anomeric carbon to form an oxazoline intermediate in the enzyme active site, (c) Sugar thiazolines are noncleavable S-linked inhibitors of hexosaminidases that use an oxazoline intermediate. Enz5mie selectivity can be further tuned by varying i . ...

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