Identification by FT-ICR-MS of Camelus dromedarius α-lactalbumin variants as the result of nonenzymatic deamidation of Asn-16 and Asn-45

Nonenzymatic deamidation of asparaginyl residues can occur spontaneously under physiological conditions principally when a glycyl residue is at the carboxyl side of Asn and leads to formation of aspartyl and isoaspartyl residues. This modification can change the biological activity of proteins or peptides and trigger an auto-immune response. The α-lactalbumins of members of the Camelidae family are the only of described α-lactalbumins that carry two AsnGly sequences. In the present study, high-resolution mass spectrometry, which enables accurate mass measurement has shown that Asn¹⁶ and Asn⁴⁵ underwent a nonenzymatic deamidation, the sequence Asn⁴⁵-Gly⁴⁶ being deamidated spontaneously at near-neutral and basic pH and Asn¹⁶-Gly¹⁷ rather at basic pH. The 16-17 sequence was probably stabilized at near-neutral pH by hydrogen bonds according to the molecular modelisation performed with the camel protein.