Effects of Desialylation on Human α1-Acid Glycoprotein–Ligand Interactions

Human α1-acid glycoprotein (AGP), an acute-phase glycoprotein, exists predominantly in blood. With its ability to bind basic, lipophilic, and acidic drugs, AGP has served as a drug carrier. It has been shown that the carbohydrate composition of AGP changes in response to tissue injury, inflammation, or infection and can have a great impact on AGP’s drug binding activities. The molecular-level details of the effects of desialylation on the AGP conformation and AGP–ligand interactions, however, are unknown. Here we report the use of hydrogen–deuterium exchange coupled with mass spectrometry (HDX–MS) to reveal the changes in AGP conformational dynamics induced by the removal of terminal sialic acid. HDX–MS also reveals the changes in the conformational dynamics of sialylated and unsialylated AGP upon formation of complexes of holo-AGP with progesterone or propranolol. Our HDX–MS results demonstrate that desialylation stabilizes two loop regions that are exterior to the β-sheet barrel in AGP, and this stabilization minimizes the conformational changes of AGP upon binding with progesterone or propranolol.