Structural glycobiology of human α₁-acid glycoprotein and its implications for pharmacokinetics and inflammation

Abstract

Human α1-acid glycoprotein (AGP) is an abundant human plasma glycoprotein that may be N-glycosylated at five positions. AGP plays important roles on pharmacokinetics and can rise up to 5-fold in inflammatory events. In such events, the glycan chains attached to Asn54, Asn75 and Asn85 may become fucosylated, originating a sialyl-Lewis X epitope. This epitope, in turn, can bind selectin proteins. Such interplay is important for immunomodulation. While the X-ray structure of unglycosylated AGP has been reported, the absence of the glycan chains hampered the further insights into its structural biology and, ultimately, into its biological function. Thus, the current work intends to contribute in the characterization of the structural glycobiology and function of AGP by building a structural model of its fully glycosylated form, taking into account the different glycoforms that are found in vivo. The obtained data points to the absence of a major influence of glycosylation on AGP's secondary structure, in agreement with crystallography observations. However, the glycan chains seem able to interfere with the protein dynamics, mainly at the AGP-ligand-binding site, indicating a possible role in its complexation to drugs and other bioactive compounds. By examining the influence of fucosylation on AGP structure and binding to selectins, it is proposed that the latter may bind to glycan chains linked to Asn54 and Asn75, and that this binding may involve other glycans, such as the one attached to Asn15. These results point to an increased participation of carbohydrates on the observed AGP roles in pharmacokinetics and inflammation.

Pleiotropic ZIP8 A391T implicates abnormal manganese homeostasis in complex human disease

“…AGP is an acute-phase reactant described as an immunomodulator and a major binding protein for endogenous ligands and drugs ( 51 ). AGP has 5 predicted N-glycosylation sites, with over 150 glycoforms that affect the AGP-ligand-binding site and change protein dynamics ( 52 , 53 ). We speculate that further study of the differential biological function of AGP glycoforms, driven in part by ZIP8 391-Thr genotype and abnormal Mn homeostasis, may reveal mechanistic underpinnings of the pleiotropic disease associations of ZIP8 391-Thr.…”

N-glycan microheterogeneity regulates interactions of plasma proteins

“…From the X-ray crystal structure of recombinant AGP, in which all glycans were removed, it is evident that the N-glycans at Asn-38 and -75 are close to the drug-binding cavity and variable N-glycosylation, especially fucosylation, may modulate access of the cavity to warfarin ( 17 ). A previous molecular-dynamics study of AGP revealed that glycosylation expands the volume of the hydrophobic cavity for drug binding ( 47 ). Although other glycosylation sites are far from the hydrophobic cavity, the occurrence of distal antennary fucosylation and N-glycan branching/elongation may induce conformational changes to the AGP protein backbone, contributing to a weaker protein–drug interaction.…”

Glycoform analysis of alpha1-acid glycoprotein by capillary electrophoresis

“…Alpha 1 -acid glycoprotein (AGP) is an acute phase glycoprotein and serum transport protein with a low isoelectric point (pI, 2.8–3.8) and a high carbohydrate content (40%) [1,2]. The normal concentration of this protein in human plasma is 0.5–1.0 g/L (or 12–24 μM); however, this concentration can increase by up to ten-fold in some diseases [1].…”