Haloperidol binding to monoclonal antibodies. Predictions of three-dimensional combining site structure via computer modeling

The amino acid sequences of five monoclonal antibodies (designated mAbs A-E) which bind to the dopaminergic D-2 antagonist, haloperidol, with a variety of affinities (Kd = 4-810 nM), have been used to build theoretical, three-dimensional, computer models of the variable region combining sites. Physiocochemical interactions which have been previously determined from in vitro binding data have been used to orient the drug molecule within the combining site model. The results indicate that hydrophobic, aromatic, and ionic amino acids are involved in specific interactions with the antagonist molecule. For example, fluorescence quenching data suggests that a tryptophan residue is intimately involved in the binding of haloperidol by mAb A. Examination of the modeled structure reveals five tryptophans within the variable fragment, only one of which (H-50) is within the classical beta-barrel binding pocket and is readily accessible to the antigen. Haloperidol's relatively electron poor fluorophenyl ring system stacks with the electron-rich tryptophan ring system at a distance of 3.3 A and in so doing, places haloperidol's positively charged piperidinyl nitrogen atom within hydrogen bond distance of the negatively charged Glu-95 and Asp-100A residues of the H3 loop (Glu-H-95 and Asp-H-100A). This type of analysis for each antibody provides an interesting profile of changes in amino acid composition and hypervariable loop length which markedly effect binding affinity and specificity for a series of proteins which have similar combining site.