NAD+ hydrolysis

Figure 5.49 NAD" hydrolysis and cyclisation with retention. The appropriate Withers inactivator is shown.

Bacterial ADP-ribosylating toxins and non-enzymatic NAD hydrolysis 291 Nucleoside and AMP hydrolases 294 Phosphorylase and pyrophosphorylase reactions 297 DNA and RNA 301 Conclusions and future directions 306 The immediate future 306 Crucial questions 307 Acknowledgments 308 References 308... 

Information on the nature of the oxocarbenium ion from a variety of experimental and computational approaches has indicated that the oxonium-like resonance form lb predominates.Evidence emerging from TS analysis supports this model. In TS structures derived from both ad hoc and structure interpolation techniques, and supported by electronic structure calculations, essentially all the loss in bond order upon leaving group departure is compensated by increases in the Cff-04 and Cff-C2 bond orders. In NAD hydrolysis reactions, the total bond... 

Further evidence comes from a model of the DTA-TS complex for NAD hydrolysis based on the X-ray crystal structure of DTA-NAD. In the complex, the positive charge that would accumulate on the anomeric carbon at the transition state was located directly adjacent to the carboxylate of a glutamate residue that is completely conserved in the bacterial toxin sequences. 

Beyond these general trends, however, the detailed interpretation of a-secondary hydron KIEs in glycosidic reactions has proven problematic. As a specific example, the E- H KIEs for pertussis toxin-catalyzed reactions were 1.21 0.01 for NAD hydrolysis,and 1.20 0.01 for the ADP-ribosylation of protein The... 

Large hydron KIEs have been observed in enzymatic reactions at sites far removed from where chemistry is occurring. In enzymatic hydrolysis and transfer reactions of A-ribosides, it has become a common pattern to observe an inverse 4 - H KIE and a normal 5 - H KIE. These KIEs are not intrinsic to the reaction as the KIEs at these positions are negligible in the non-enzymatic reactions. Examples of such enzymatic KIEs include inosine hydrolysis by lU-NH from Crithidia fasciculata, where 4 - H KIE = 0.992, 5 - H KIE = 1.051 and NAD+ hydrolysis by diphtheria toxin A-chain (DTA), where 4 - H KIE = 0.990, 5 - H KIE = 1.032. ... 

Figure 7.5 Adenosine diphosphate (ADP)-ribosylation biochemical reactions. Mono-ADP-ribosyltransferases (ARTs) and poly-ADP-ribose pol5mierases Poly (PARPs) catalyse the ADP-ribose moiety of NAD transfer to amino acid residues. ADP-ribosyl cyclases generate cyclic ADP-ribose and 2-phospho-cyclic ADP-ribose from NAD and NADP, respectively. Both molecules trigger cyclic ADP-ribose cytosolic Ca " elevation, presumably by activating the ryanodine receptor in the endoplasmic/sarcoplasmic reticulum (RER). SIRTl catalyses a reaction that couples protein deacetylation to NAD hydrolysis.

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