The binding site of ochratoxin A (OTA) within domain 2A of human serum albumin (HSA) is examined by theoretical simulations and site-directed mutagenesis experiments. The calculated binding constant, based on docking experiments and theoretical affinity constants derived from the empirical free energy of binding as implemented in AutoDock 3.0, for the OTA dianion (3.7 x 10(6) M-1) is in good agreement with experimental value of 5.2 x 10(6) M-1. The carboxy terminus of OTA associates with R218 and R222 of the protein. Binding is reduced by over an order of magnitude for the mutant R218A in both experiments and theoretical simulations. The carbonyl of the lactone and the phenolic group of OTA are in close proximity to R257. The experimental binding constant of OTA to the R257A mutant is 1.6 x 10(5) M-1, over an order of magnitude smaller than for the wild-type protein. The predicted binding constant based on a comparison of the lowest-energy conformer from docking studies performed in AutoDock 3.0 of OTA to the R257A mutant (8.3 x 10(4) M-1) is also in good agreement with the experimental result. R257 clearly plays an important role in the binding of the isocoumarin ring of OTA by serving as a proton acceptor and stabilizing the binding through the creation of an ion pair with the phenoxide group on OTA.