Alteration in spectral properties on ligand binding reveals flexibility in monoamine oxidase

Crystal structures have opened the door to understanding the mechanism and ligand specificities of MAO A and MAO B. We review here functional properties that suggest a flexibility in MAO that is likely to influence catalysis under different cellular conditions. The flexibility indicated by altered oxidation kinetics and a changed redox potential in the presence of a substrate was confirmed by circular dichroism spectroscopy. Circular dichroism also demonstrated alterations in the conformation of aromatic residues during reduction of MAO A and after covalent modification of the flavin. Visible spectra provide a convenient way to monitor ligand binding in the active site. Different groups near the flavin give different spectral changes. During reduction of MAO A, a distinct 412 nm peak appears after partial reduction. Recent work suggests that this may be a tyrosyl radical in equilibrium with the semiquinone of the flavin. Substrates prevent the appearance of the 412 nm peak but many inhibitors enhance it by preventing further reduction. We propose that steric effects in the active site could be the mechanism of this difference. Flexibility is also important for the transmission of the effects of modifying the surface thiols to the active site. Modification of multiple thiols results in inactivation but mutation of a single thiol, cysteine 374 in MAO A to alanine, decreased the catalytic potency (kcat/Km) by 30%. Thus, surface modification of MAO (for example, by oxidative stress) could reduce its activity.