Interaction of 5,7-dihydroxy-4'-methoxyflavone with a multisubunit protein, carmin: thermodynamics and kinetics of...

Acacetin (5,7-dihydroxy-4'-methoxy flavone) is a flavone intrinsically present in the seeds of Carthamus tinctorius. Carmin is a multimeric, high molecular weight protein from the seeds of Carthamus tinctorius. The association constant of interaction of acacetin and carmin is maximum at 37.2 degrees C with a value of (3.96 +/- 0.61) x 10(4) M-1 as measured by fluorescence quenching. Acacetin has at least two binding sites on carmin. The interaction follows pseudo-first-order kinetics with a reaction rate constant of 3.4 +/- 0.4 s-1. The titration calorimetric data suggest that binding sites for acacetin and its structural analogue, biochanin A, are conserved. The interaction does not affect the association-dissociation equilibrium of the protein. Also, the binding does not induce any significant conformational changes in the protein as monitored by circular dichroic spectra. Biochanin A (5, 7-dihydroxy-4'-methoxyisoflavone), a structural analogue, interacts with carmin with an association constant of (9.33 +/- 1.44) x 10(4) M-1 at 36.9 degrees C. This indicates that the stereochemistry of the ligand plays an important role in the binding process of flavone to protein. Interaction studies of chemically modified lysyl and tryptophanyl groups separately, and lysyl and tryptophanyl groups sequentially, in the protein carmin with the ligands reveal the involvement of tryptophanyl residues in the binding process and show that it is predominantly an entropically driven hydrophobic interaction.