Abstract
Ligand binding of neutral progesterone, basic propranolol, and acidic warfarin to human α(1)-acid glycoprotein (AGP) was investigated by Raman spectroscopy. The binding itself is characterized by a uniform conformational shift in which a tryptophan residue is involved.
Slight differences corresponding to different contacts of the individual ligands inside the β-barrel are described. Results are compared with in silico ligand docking into the available crystal structure of deglycosylated AGP using quantum/molecular mechanics.
Calculated binding energies are -18.2, -14.5, and -11.5 kcal/mol for warfarin, propranolol, and progesterone, respectively. These calculations are consistent with Raman difference spectroscopy; nevertheless, minor discrepancies in the precise positions of the ligands point to structural differences between deglycosylated and native AGP.
Thermal dynamics of AGP with/without bounded warfarin was followed by Raman spectroscopy in a temperature range of 10-95 oC and analyzed by principal component analysis. With increasing temperature, a slight decrease of α-helical content is observed that coincides with an increase in β-sheet content.
Above 45 oC, also β-strands tend to unfold, and the observed decrease in β-sheet coincides with an increase of β-turns accompanied by a conformational shift of the nearby disulfide bridge from high-energy trans-gauche-trans to more relaxed gauche-gauche-trans. This major rearrangement in the vicinity of the bridge is not only characterized by unfolding of the β-sheet but also by subsequent ligand release.
Hereby, ligand binding alters the protein dynamics, and the more rigid protein-ligand complex shows an improved thermal stability, a finding that contributes to the reported chaperone-like function of AGP.