Potentiation of ligand binding through cooperative effects in monoamine oxidase B

Abstract

Crystallographic and biochemical studies have been employed to identify the binding site and mechanism for potentiation of imidazoline binding in human monoamine oxidase B (MAO B). 2-(2-Benzofuranyl)-2-imidazoline (2-BFI) inhibits recombinant human MAO B with a Ki of 8.3 ± 0.6 μM, whereas tranylcypromine-inhibited MAO B binds 2-BFI with a Kd of 9 ± 2 nM, representing an increase in binding energy Δ(ΔG) of -3.9 kcal/mol. Crystal structures show the imidazoline ligand bound in a site that is distinct from the substrate-binding cavity. Contributions to account for the increase in binding affinity upon tranylcypromine inhibition include a conformational change in the side chain of Gln206 and a "closed conformation" of the side chain of Ile199, forming a hydrophobic "sandwich" with the side chain of Ile316 on each face of the benzofuran ring of 2-BFI. Data with the I199A mutant of human MAO B and failure to observe a similar binding potentiation with rat MAO B, where Ile316 is replaced with a Val residue, support an allosteric mechanism where the increased binding affinity of 2-BFI results from a cooperative increase in H-bond strength through formation of a more hydrophobic milieu. These insights should prove valuable in the design of high affinity and specific reversible MAO B inhibitors. © 2010 by The American Society for Biochemistry and Molecular Biology, Inc.