Binding of ochratoxin a to human serum albumin stabilized by a protein-ligand ion pair
Ochratoxin A (OTA), a fungal metabolite of strains of Penicillium and Aspergillus, binds in its dianion form to Sudlow site I of human serum albumin (HSA) with high affinity. In this study, isothermal calorimetry (ITC) is used to study the binding of OTA and its O-methyl derivative (MOA). Calculations of the equilibrium geometry of the monoanion and dianion of OTA reveal only small structural changes among the lowest energy conformers. The ITC data show the binding of MOA, which lacks the phenolic proton of OTA, is accompanied by the uptake of a proton from the surrounding solvent. At pH 7.13, the binding of OTA is accompanied by uptake of 0.43 ± 0.15 protons from the solvent. At this pH, the monoanion (0.54) and dianion (0.46) forms of OTA are both present in solution. However, the pKa of the phenolic group of OTA decreases by more than three units upon protein binding, and so all available OTA is bound to the protein as the dianion. To account for the ITC data, a model is proposed in which the proton is provided by the phenolic moiety of OTA in the case of initial binding of the monoanion, and a proton is taken up from the surrounding solvent for initial binding of the dianion. The binding constant of MOA is 2 orders of magnitude smaller than that of OTA, indicating the ion pair between the phenoxide group of OTA and the protonated amino acid is a major contributor to the high binding affinity of OTA to HSA. To identify the specific amino acid involved, the binding of OTA to bovine, rat, and porcine serum albumins was examined. Deprotonation of the monoanion of OTA occurred upon binding to all species. Assuming the amino acid is conserved between species and taking into account crystal structures of ligands bound to site I of HSA and their ability to displace OTA from HSA, either R218 or R257 is involved in the ion pairing with OTA. These two amino acids sit across the binding cavity from one other in site I.
Perry, JL; Christensen, T; Goldsmith, MR; Toone, EJ; Beratan, DN; Simon, JD
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