Glycosidases inactivation

M. K. Tong and B. Ganem, A potent new class of active-site-directed glycosidase inactivators, J. Am. 

Electrophilic fluorination of glycals is a key step in the syntheses of Withers glycosidase inactivators and related potential pharmaceuticals. 

Only recently, a very interesting protein family, namely RIPs, has been isolated from S. nigra [57-60]. RIPs are RNA N-glycosidases inactivating ribosomes through a site-specific deadenylation of the large ribosomal RNA [61]. In addition to this, some RIPs have been reported to have superoxide dismutase [62,63] and phospholipase [64] type of activities. It is supposed that RIPs are defense-related... 

Withers et al. have reported the development of a new class of fluorosugar glycosidase inactivators (32) bearing tunable phosphorus-based leaving groups that reacted with lysosomal p glucocerebrosidase over 4000 times faster than 2-deoxy-2-fluoro-p-Z)-glucosyl fluoride. ... 

Kinetic Constants for the Inactivation of Glycosidases by Conduritol Epoxides Corresponding to their Respective Glycon Specificities (see Text)... 

The principle of active-site-directed inactivation of glycosidases by gly-con-related epoxides can be extended to compounds having an exocyclic oxirane ring, either directly attached to the six-membered ring (32) or at some distance (33,34). Studies with -o-glucosidase from sweet almonds and intestinal sucrase-isomaltase revealed that, in spite of the higher intrinsic reactivity of these epoxides, this shift of the position of the epoxide function causes a 10- to 30-fold decrease of kj(max)/Ki, an effect which probably reflects the limited flexibility of the catalytic groups involved in the epoxide reaction. 

Shiga toxin produced by Shigella dysenteriae has similar structural features. The toxin binds to a glycolipid (Gb3), undergoes endocytosis, and the enzymatie Ai fragment, which is a specific N-glycosidase, removes adenine from one particular adenosine residue in the 28S RNA of the 60S ribosomal subunit. Removal of the adenine inactivates the 60S ribosome, blocking protein synthesis. Ricin, abrin, and a number of related plant proteins inhibit eukaryotic protein synthesis in a similar manner (Chapter 25). 

Barbieri, L., Valbonesi, P., Bonora, E., Gorini, P., Bolognesi, A. and Stirpe, F. (1997) Polynucleotide adenosine glycosidase activity of ribosome-inactivating proteins effect on DNA, RNA and poly(A). Nucleic Acids Res, 25, 518-522. 

Endo, Y., Tsurugi, K. and Lamberf, J.M. (1988a) The site of action of six different ribosome-inactivating proteins from plants on eukaryotic ribosomes the RNA A-glycosidase activity of the proteins. Biochem Biophys Res Commun, 150, 1032-1036. 

Ren, J., Wang, Y., Dong, Y. and Stuart, D.I. (1994) The yV-glycosidase mechanism of rihosome-inactivating proteins implied by crystal structures of alpha-momorcharin. Structure, 2, 7 16. 

Figure 5.36 Some inactivators of retaining glycosidases/transglycosylases on the Withers principle. Top possible 2-OH-nucleophile interaction in retaining P-glycosidases which largely accounts for the success of a single F-for-OH substitution. ...

The A-chain is effectively an enz)une which interferes with protein synthesis by inactivating the 60S ribosomal subunit. The A-chain depurinates a specific adenine residue of the ribosomal RNA the adenine ring is hydrolysed, by the N-glycosidase action of the A-chain, when it becomes situated between two tyrosine rings in the enzyme s active site. As the ribosome is modified it can no longer act as a site of protein synthesis and this leads to the eventual death of the cell. The A-chain enzyme then moves on to deactivate another ribosome. Although only a very small proportion of the ricin molecules that enter the cell are actually moved into the cytosol, one A-chain is sufficient to destroy it. A single ricin molecule is able to deactivate more than 1500 ribosomes per minute. 

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