Transglycosylase reactions

Figure 24 Transglycosylase reactions. The four known transglycosylation reactions involved in RNA modification generating in archaea, archaeosine (G+) in eubacteria and eukarya, queuosine (Q) and across the three kingdoms, pseudouridine ( ).

Synthesis. The synthases are present at the endomembrane system of the cell and have been isolated on membrane fractions prepared from the cells (5,6). The nucleoside diphosphate sugars which are used by the synthases are formed in the cytoplasm, and usually the epimerases and the other enzymes (e.g., dehydrogenases and decarboxylases) which interconvert them are also soluble and probably occur in the cytoplasm (14). Nevertheless some epimerases are membrane bound and this may be important for the regulation of the synthases which use the different epimers in a heteropolysaccharide. This is especially significant because the availability of the donor compounds at the site of the transglycosylases (the synthases) is of obvious importance for control of the synthesis. The synthases are located at the lumen side of the membrane and the nucleoside diphosphate sugars must therefore cross the membrane in order to take part in the reaction. Modulation of this transport mechanism is an obvious point for the control not only for the rate of synthesis but for the type of synthesis which occurs in the particular lumen of the membrane system. Obviously the synthase cannot function unless the donor molecule is transported to its active site and the transporters may only be present at certain regions within the endomembrane system. It has been observed that when intact cells are fed radioactive monosaccharides which will form and label polysaccharides, these cannot always be found at all the membrane sites within the cell where the synthase activities are known to occur (15). A possible reason for this difference may be the selection of precursors by the transport mechanism. 

The enzyme tRNA-guanine transglycosylase (TGT) catalyzes the complete exchange of a base in tRNA [25, 26]. Upon reaction, guanine in the wobble position of tRNAs with the anticodon sequence GUN is replaced by the modified bases... 

Brenk, R., Stubbs, M.T., Heine, A., Reuter, K., Klebe, G. Flexible adaptations in the structure of the tRNA modifying enzyme tRNA-guanine transglycosylase and their implications for substrate selectivity, reaction mechanism and structure-based drug design. ChemBiochem 2003, 4, 1066-1077. 

They postulated that UDP-D-glucose may have an important role in the initial step of transformation of sucrose into starch, whereas synthesis of starch is primarily catalyzed by ADP-D-glucose-starch transglycosylase. This concept has been based on the smaller constant of the sucrose synthetase toward UDP as compared with that for ADP, and by the fact that the ADP D-sucrose transglycosylase is specifically inhibited by UDP. From these results, de Fekete and Cardini postulated the following two sequences of reaction for the incorporation of D-glucose into the granule. 

It has now been found that the ADP-ribose moiety of nicotinamide adenine dinucleotide is also transferred onto some pro-teins. " When histone serves as an acceptor, several ADP-ribose units are transferred in succession, so that a short chain of oligo-(ADP-ribose), linked covalently to the protein, is formed. In another reaction, transferase II, a soluble enzyme involved in protein synthesis in mammalian cells, acts as an acceptor of a single ADP-ribose unit in the presence of diphtheria toxin. - Treatment of the product with venom pyrophosphatase releases adenosine 5 -monophosphate, but the D-ribose 5-phosphate portion still remains attached to the protein it is, therefore, assumed that the linkage involves C-1 of D-ribose. The transferase II that carries the ADP-ribose unit is completely inactive, but it can be reactivated by incubating with nicotinamide and diphtheria toxin. Under these conditions, the reaction is reversed, generating free transferase II protein and nicotinamide adenine dinucleotide. Thus, diphtheria toxin was shown to have a very specific transglycosylase activity the mechanism of this reaction has been studied in detail. ... 

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