Title : Site-specific detection and structural characterization of the glycosylation of human plasma
proteins lecithin:cholesterol acyltransferase and
apolipoprotein D using HPLC/electrospray mass spectrometry and sequential glycosidase digestion
Abstract :
- Site-specific structural characterization of the glycosylation of human lecithin:cholesterol acyltransferase ( LCAT ) was carried out using microbore reversed-phase high performance liquid chromatography coupled with electrospray ionization mass spectrometry (HPLC/ESIMS)
- A recently described mass spectrometric technique involving monitoring of carbohydrate-specific fragment ions during HPLC/ESIMS was employed to locate eight different groups of glycopeptides in a digest of a human LCAT protein preparation
- In addition to the four expected N-linked glycopeptides of LCAT , a di-O-linked glycopeptide was detected, as well as three additional glycopeptides
- Structural information on the oligosaccharides from all eight glycopeptides was obtained by sequential glycosidase digestion of the glycopeptides followed by HPLC/ESIMS
- All four potential N-linked glycosylation sites ( Asn20, Asn84, Asn272, and Asn384 ) of LCAT were determined to contain sialylated triantennary and/or biantennary complex structures
- Two unanticipated O-linked glycosylation sites were identified at Thr407 and Ser409 of the LCAT O-linked glycopeptide , each of which contain sialylated galactose beta 1-->3N-acetylgalactosamine structures
- The three additional glycopeptides were determined to be from a copurifying protein , apolipoprotein D , which contains potential N-linked glycosylation sites at Asn45 and Asn78
- These glycopeptides were determined to bear sialylated triantennary oligosaccharides or fucosylated sialylated biantennary oligosaccharides.
- Previous studies of LCAT indicated that removal of the glycosylation site at Asn272 converts this protein to a phospholipase (Francone OL, Evangelista L, Fielding CJ, 1993, Biochim Biophys Acta 1166:301-304)
- Our results indicate that the carbohydrate structures themselves are not the source of this functional discrimination; rather, it must be mediated by the structural environment around Asn272