Sudlow Site I of human serum albumin (HSA) is located in subdomain IIA of the protein and serves as a binding cavity for a variety of ligands. In this study, the binding of warfarin (W) is examined using computational techniques and isothermal titration calorimetry (ITC). The structure of the docked warfarin anion (W−) to Site I is similar to that revealed by X-ray crystallography, with a calculated binding constant of 5.8 × 105 M−1. ITC experiments (pH 7.13 and I = 0.1) carried out in three different buffers (MOPs, phosphate and Tris) reveal binding of W− is accompanied by uptake of 0.30 ± 0.02 protons from the solvent. This measurement suggests that the binding of W− is stabilized by an ion-pair interaction between protonated H242 and the phenoxide group of W−.
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Vol. 82 • No. 5