Structural Characterization of Donkey Lactoferrin: Amino Acid Sequence and Glycan Compositions

Lactoferrin, a protein showing an array of biochemical properties, including immuno-modulation, iron-binding ability, as well as antioxidant, antibacterial and antiviral activities, was isolated from donkey milk by ion exchange chromatography.The characterization of its primary structure, by means of enzymatic digestions, SPITC derivatization of tryptic digest, reversed-phase high performance liquid chromatography, electrospray and matrix-assisted laser desorption/ionization mass spectrometry, is reported. Our results allowed the almost complete characterization of donkey s lactoferrin sequence, that, at least for the covered sequence, differs from the mare s genomic deduced sequence (UniProtKB Acc. Nr. O77811) by five point substitutions located at positions 91 (Arg->His), 328 (Thr->Ile/Leu), 466 (Ala->Gly), 642 (Asn->Ser) and 668 (Ser->Ala). Glycosylation, one of the most common post-translational modifications, can modify the structural conformation of the protein and consequently its biological activity. A comprehensive glycosylation profile by chymotrypsin digestion, TiO2 and HILIC enrichment strategy, reversed-phase high performance liquid chromatography, electrospray mass spectrometry, high collision dissociation fragmentation, is reported. 26 different glycan compositions were identified, linked to the protein backbone via an amide bond to asparagine residues located at the positions 137, 281 and 476