Transglycosylases, specificity

Romier, C., Meyer, J.E., Suck, D. Slight sequence variations of a common fold explain the substrate specificities of tRNA-guanine transglycosylases from the three kingdoms. FEBS Lett. 1997, 416, 93-98. 

In a final example specifically devoted to the use of mercury reagents following Wittig olefinations, Qiao, et al.,6 utilized mercuric trifluoroacetate to effect ring closure. This example, shown in Scheme 7.1.6, was utilized in the synthesis of the C-phosphonate disaccharide, shown in Figure 7.1.1, as a potential inhibitor of peptidoglycan polymerization by transglycosylase. 

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. ... 

Stengl, B., Reuter, K., and Klebe, G. (2005) Mechanism and substrate specificity of tRNA-guanine transglycosylases (TGTs) tRNA-modifying enzymes from the... 

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