Asx-Tum

This evidence demonstrates that the conformation of the peptide substrate is important for its recognition by the glycosyltransferase and that the acceptor peptides prefer the Asx-tum conformation. However, the solution conformation of a peptide may not reflect the conformation in which it actually binds to the enzyme. One approach to determining the bound conformation is to impose conformational constraints upon the peptide substrates, restricting them to a limited number of conformations or perhaps even a single conformation. 

Evaluation of Previous Proposals. The original Marshall model does not adequately model the observed behavior. This model predicts deprotonation of the asparagine carboxyamido nitrogen to yield a potent nucleophile which can attack the oligosaccharide (27). Such a deprotonation would place anionic character on the carboxyamido nitrogen, which has already been ruled out. This model also does not reflect the hydrogen bonding interactions within the Asx-tum. 

Figure 5.58 Conformation and activation of asparagines, (a) P-Tum and ASX turn (b) activated forms of the amide.

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