Asparagine (Asn, N(

The amino acid asparagine is incorporated into the nascent polypeptide chain during protein biosynthesis in response to six codons—CGU, CGC, CGA, CGG, AGA, and AGG—and represents approximately 4.4% of the residues of the proteins that have been characterized. The asparaginyl residue incorporated has a mass of 114.11 Da, a van der Waals volume of 96 ų, and an accessible surface area of 158 Ų. Asn residues are those most variable during divergent evolution; they are interchanged frequently in homologous proteins with serine, aspartic acid, lysine, and histidine residues.

 The side-chain of Asn residues is the same as that of aspartic acid, except that the carboxyl group has been converted to the amide:

Both amino acids occur naturally and are incorporated directly into proteins during their biosynthesis; Asn residues do not arise from amidation of Asp in proteins. The amide side chain does not ionize and is not very reactive chemically. It is polar, however, because it is both a hydrogen bond donor and acceptor. The amide group is labile at extremes of pH and at high temperatures, and Asn residues can deamidate to Asp. The Asn residue is especially labile at alkaline pH because its side chain is sterically suited to interact with the  NH  group of the following residue in the polypeptide chain, to form transiently a cyclic succinimidyl derivative. This derivative can undergo racemization and hydrolysis to cleave the polypeptide chain or to produce a mixture of D and L isomers of Asp and isoAsp residues. In an isoAsp residue, the normal backbone peptide bond is through the side-chain carboxyl group rather than the usual a-carboxyl. The deamidation reaction of Asn residues occurs 30–50 times more rapidly if the following residue is Gly, because the absence of a side chain there favors succinimide formation, and  Asn Gly  sequences are especially prone to deamidation. The rate depends on the polypeptide conformation, however, because only some conformations permit succinimide formation. If the succinimide ring reacts with hydroxylamine instead of water, peptide cleavage results. Therefore, Asn Gly peptide bonds are readily cleaved by incubation with hydroxylamine.

Asn residues are the site of N-glycosylation of proteins, one of the most prevalent post-translational modifications. The Asn residue that is so glycosylated always occurs in a characteristic sequence: -Asn -Xaa -Ser ,  Asn –Xaa- , Thr- , or  -Asn -Xaa -Cys , where Xaa can be any residue except Pro, which also cannot immediately follow the tripeptide sequence.

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