Asparagine residues, reactivity

Before doing so, we briefly examine the influence of conformation and flexibility. Indeed, formation of succinimide is limited in proteins due to conformational constraints, such that the optimal value of the and ip angles (Sect. 6.1.2) around the aspartic acid and asparagine residues should be +120° and -120°, respectively [99], These constraints often interfere with the reactivity of aspartic acid residues in proteins, but they can be alleviated to some extent by local backbone flexibility when it allows the reacting groups to approach each other and, so, favors the intramolecular reactions depicted in Fig. 6.27. When compared to the same sequence in more-flexible random coils, elements of well-formed secondary structure, especially a-helices and 13-turns, markedly reduce the rate of succinimide formation and other intramolecular reactions [90][100],... 

Another factor that influences partitioning between Pathways e and / (Fig. 6.29) is substitution at the y-amido group of asparagine. The reactivities of two octapeptide analogues, one containing an asparagine residue, the other a y-AT-methylasparagine residue (6.78, R"=H or Me, respectively, Fig. 6.31) [117], were compared. When R"=H, the ratio of Pathways elf was 45 1 at pH 7.4. However, y-A-methylalion of Asn dramatically modified the ratio of Pathways elf which, in this case, was 1 3. 

The influence of secondary structure on reactions of deamidation has been confirmed in a number of studies. Thus, deamidation was inversely proportional to the extent of a-helicity in model peptides [120], Similarly, a-hel-ices and /3-turns were found to stabilize asparagine residues against deamidation, whereas the effect of /3-sheets was unclear [114], The tertiary structure of proteins is also a major determinant of chemical stability, in particular against deamidation [121], on the basis of several factors such as the stabilization of elements of secondary structure and restrictions to local flexibility, as also discussed for the reactivity of aspartic acid residues (Sect. 6.3.3). Furthermore, deamidation is markedly decreased in regions of low polarity in the interior of proteins because the formation of cyclic imides (Fig. 6.29, Pathway e) is favored by deprotonation of the nucleophilic backbone N-atom, which is markedly reduced in solvents of low polarity [100][112],... 

A systematic study with two series of pentapeptides has afforded much information on the influence of flanking residues on asparagine reactivity [126]. In these two series, the central asparagine residue occurred in the sequences Val-Xaa-Asn-Ser-Val and Val-Ser-Asn-Xaa-Val, where Xaais one of ten different residues. In acidic solutions, the Asp peptide was the only product found, and its rate of formation was independent of the nature of the... 

Ichiyama, S., Kurihara, T., Kogure, Y., Tsunasawa, S., Kawasaki, H., Esaki, N. (2004). Reactivity of asparagine residue at the active site of the D105N mutant of fluoroacetate dehalogenase from Moraxella sp. B. Biochim. Biophys. Acta 1698 27-36. 

As in the case of degradation at aspartic acid residues, the major structural factors that influence the reactivity of asparagine and glutamine residues... 

In the case of the influence of adjacent residues, there are clear mechanistic analogies between activation of aspartic acid (Sect. 6.3.3.2) and asparagine sites. The presence of a C-flanking glycine residue consistently increases deamidation of peptides, for the reasons discussed in Sect. 6.3.3.2 [6], Replacement of glycine with a more bulky residue such as valine, leucine, or proline can decrease reactivity more than tenfold [99]. 

In addition to the amino acids described above, several other amino acid residues are also reactive toward compounds containing heavy atoms. These are the side chains of arginine, asparagine, glutamine, lysine, tryptophan, and tyrosine. Those that are not reactive are alanine, glycine, isoleucine. 

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