Induced circular dichroism spectra reveal binding of the antiinflammatory curcumin to human α1-acid glycoprotein Original Research Article

This paper reports the first experimental evidence on binding of the plant derived curcumin molecule to human α1-acid glycoprotein (AGP), an acute phase protein in blood. Oppositely signed induced circular dichroism (CD) bands measured in the visible spectral region in pH 7.4 phosphate buffer indicate that the protein binds this natural polyphenol molecule in a left-handed chiral conformation. Decreasing of the intrinsic fluorescence of AGP upon addition of curcumin confirmed the binding to take place. Fluorescence quenching titration curve of AGP allowed to calculate the association constant of the ligand (Ka=4×104 M−1). Modification of near UV CD spectrum of the protein suggests that curcumin induces changes in the tertiary structure of AGP, which leads to the decrease of binding affinity. By using rac-warfarin and amitriptyline, selective high affinity ligands of F1–S and A genetic variants of AGP, CD displacement experiments showed that curcumin is able to bind to both variants. Molecular docking calculations performed on curcumin–AGP and warfarin–AGP complexes suggest the existence of two alternative binding sites for curcumin; either at the open end of the central hydrophobic cavity or in a surface cleft of the protein.