Thermodynamics of interaction of caffeic acid and quinic acid with multisubunit proteins.

Helianthinin is a multisubunit protein from Sunflower seeds. Caffeic acid (CA) and quinic acid (QA) are intrinsic ligands present in sunflower seeds. The mechanism of interaction of these ligands with multisubunit proteins is limited. The present study enables one to understand the mechanism of the interaction of these ligands with the protein helianthinin. From this study, it is shown that CA has two classes of binding sites on helianthinin. The high-affinity class of sites total six from 60+/-10 for both high-affinity and low-affinity sites. Tryptophan, tyrosine and lysine residues of the protein are mainly involved in the interaction with CA. The temperature dependence of the binding in the range 10-45 degrees C can be clearly described by an enthalpy-entropy compensation effect at the low-affinity class of sites, while it is described by positive DeltaC(p)(o) at the high-affinity class of sites. This positive DeltaC(p)(o) has a contribution to the protein stability. The binding strength of CA also has a positive cooperativity at higher protein concentration. QA has two classes of binding sites on the protein based on the strength of the interaction. The interaction of QA with the protein is predominantly described by positive DeltaC(p)(o) for both classes of affinity. This suggests predominance of ionic/hydrogen bonding in the interaction process. Differential scanning calorimetric measurements reveal that the binding of both CA and QA induces destabilisation of the subunit-subunit interaction. Human methaemoglobin (mHb) has two binding sites on the molecule for CA. Both CA and QA decrease the stability of mHb, as indicated by decreased T(m). This destabilisation is also accompanied by dissociation to the monomers with concomitant conformational changes.