The Stability of γ-N-Methylasparagine in Peptides

Abstract

γ-N-Methylasparagine (NMA) is a post-translationally methylated amino acid found at the β-72 position of many phycobiliproteins. The biological function of this methylation is uncertain. We have examined the premise that amide methylation protects against deamidation in proteins. The stabilities of two natural peptides, β(67-74) Ile-Ala-Pro-Gly-NMA/ Asn-Ala-Tyr, were measured at pH 7.4; the NMA parent peptide was ∼2-fold more stable than the Asn peptide at 60°. However, the degradation products of the NMA peptide involve main chain cleavage rather than side chain deamidation/isomerization. This phenomenon was studied in greater detail using two synthetic peptides, Pro-Gly-Gly-NMA/Asn-Ala-Ala, which established that the major products of peptidyl Asn reaction are internal Asp and isoAsp formation. Contrasting this result, the products of peptidyl NMA reaction represent chain cleavage on the carboxyl side of the NMA residue to yield three peptide products including the released carboxyl terminal Ala-Ala fragment along with two tetrapeptides, Pro-Gly-Gly-NMA/isoNMA, differing at the original NMA position. Thus, while peptidyl NMA is successfully stabilized against deamidation a different chemistry has been accentuated by amide methylation resulting in peptide bond cleavage. This outcome is consistent with model chemistry reported earlier employing Asn and NMA as free amino acids.