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Human alpha(1)-acid glycoprotein (AGP), which is comprised of 183 amino acid residues and 5 carbohydrate chains, is a major plasma protein that binds to basic and neutral drugs as well as to steroid hormones. It has a beta-sheet-rich structure in aqueous solution. Our previous findings suggest that AGP forms an alpha-helix structure through an interaction with biomembranes. We report herein on a study of the mechanism of alpha-helix formation in AGP using various modified AGPs. The disulfide reduced AGP (R-AGP) was extensively unfolded, whereas asialylated AGP (A-AGP) maintained the native structure. Intriguingly, reduced and asialylated AGP (RA-AGP) increased the alpha-helix content as observed in the presence of biomembrane models, and showed a significant decrease in ligand binding capacity. This suggests that AGP has an innate tendency to form an alpha-helix structure, and disulfide bonds are a key factor in the conformational transition between the beta-sheet and alpha-helix structures. However, RA-AGP with all histidine residues chemically modified (HRA-AGP) was found to lose the intrinsic ability to form an alpha-helix structure. Furthermore, disulfide reduction of the H172A mutant expressed in Pichia pastoris also caused a similar loss of folding ability. The present results indicate that disulfide bonds and the C-terminal region, including H172 of AGP, play important roles in alpha-helix formation in the interaction of the protein with biomembranes.

a 1 -Acid glycoprotein (AGP) is a serum glycoprotein that mainly binds basic drugs. Previous reports have shown that AGP converts from a b-sheet to an a-helix upon interaction with biomembranes. In the current studies, we found that alkanols, diols, and halogenols all induce this conformational change. Increased length and bulkiness of the hydrocarbon group and the presence of a halogen atom promoted this conversion, whereas the presence of a hydroxyl group inhibited it. Moreover, the effect was dependent on the hydrophobic and electrostatic properties of the alcohols. These results indicate that, in a membrane environment, hydrophobic and electrostatic factors cooperatively induce the transition of AGP from a b-sheet to an a-helix.

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