| Abstract: | ![]()
Aspartate and asparagine residues in polypeptides are subject to nonenzymatic reactions that lead to deamidation, isomerization, peptide bond cleavage and racemization. Much of this reactivity is due to the propensity for the initial formation of a cyclic succinimide intermediate. We have been interested in determining the effect of the side chains of neighboring histidine and cysteine residues in facilitating these reactions, particularly in the possibility that they can act as general acids and bases. In this study, we found little or no effect of histidine residues preceding an asparagine residue in hexapeptides derived from the sequence of adrenocorticotropic hormone, while a histidine residue preceding an aspartic acid residue was found to increase the rate of succinimide formation 8- to 11-fold. The presence of a histidine residue following either an asparagine or aspartic acid residue did not effect the rate of succinimide formation by peptide-bond nitrogen attack, but did increase the rate of the competing side-chain nitrogen attack leading to cleavage in the asparaginyl-containing peptide. We found that the effect of a cysteine residue following an asparagine or aspartic acid residue was in general similar to that of a serine residue, although the cleavage reaction appeared to be enhanced. These results suggest that His-Asp sequences may be particularly labile to spontaneous degradation in proteins and peptides, possibly owing to the ability of the histidine residue to facilitate succinimide formation by protonating the OH? leaving group on the side chain carboxylic acid of the aspartic acid residue. Finally, we have also utilized these results, along with previously accumulated data on succinimide formation in related peptides, to correlate the rate of succinimide formation with the predicted acidity of the peptide bond nitrogen atom that is involved in the initial nucleophilic attack. © Munksgaard 1995. |
