Neurotransmitter selection by monoamine oxidase isoforms, dissected in terms of functional groups by mixed double mutant cycles

Double mutant cycles were constructed using neurotransmitters and synthetic substrates that measure their selective binding to one monoamine oxidase (MAO) enzyme isoform over another as a function of structural change. This work measures a reduction in selectivity for the MAOB isoform of 3 to 9.5 kJ mol−1 upon the addition of hydroxy functional groups to a phenethylamine scaffold. Replacement of hydroxy functional groups on the phenethylamine scaffold by hydrophobic substituents measures an increase in selectivity for MAOB of −1.1 to −6.9 kJ mol−1. The strategies presented here can be applied to the development of competitive reversible inhibitors of MAO enzymes and other targets with structurally related isoforms.