Tyrosyl Radical Formation and Propagation in Flavin Dependent Monoamine Oxidases

Abstract

Understanding the mechanism of biological amine oxidation by flavoproteins remains a controversial area of research. This is particularly true for the mammalian monoamine oxidases (MAO), which are major pharmaceutical targets for the development of antidepressants and neuroprotective agents.[1] Despite intensive research efforts, the detailed reaction mechanism, which will have major implications for drug discovery with this class of enzyme, has yet to be discovered. Mechanistic debates have centred on the potential involvement of radical species in the activity of these enzymes. We now provide strong evidence that supports a role for radical catalysis in flavoprotein monoamine oxidases.The mammalian monoamine oxidases (EC 1.4.3.4) are localised to the outer mitochondrial membrane. In these enzymes, the flavin cofactor, FAD, is covalently linked through the 8a-methyl group to an active site cysteine residue.[2] A number of mechanisms for MAO-catalysed amine oxidation have been proposed over the years, and several reviews are available. [3][4][5][6] There are currently three main mechanistic proposals for MAO catalysis. These comprise: 1) the concerted polar nucleophilic mechanism; 2) the direct hydride transfer mechanism; and 3) the single electron transfer mechanism. Recent support for the concerted polar nucleophilic mechanism has come from kinetic and structural studies on tyrosine mutants of MAO B, [7] and also from computational studies. [8,9] However, analysis of the nitrogen isotope effects conducted on a related amine oxidase, Nmethyltryptophan oxidase, supported either a direct hydride transfer mechanism or, possibly, a discrete electron transfer mechanism.[10] The single electron transfer (SET) mechanism initially proposed by Silverman and colleagues involves single electron transfer from the substrate nitrogen lone pair to yield the substrate radical and flavin semiquinone.[11] The SET mechanism is consistent with electronic effects observed in quantitative structure activity relationships studies with a series of substituted benzylamines, but the identity of the one-electron oxidant required for the formation of the aminyl radical cation was a major concern. [12] Recent spectroscopic evidence for the presence of a stable tyrosyl radical in partially reduced human MAO A provided support for the SET mechanism and led to the proposal of a modified SET mechanism (Scheme 1).[13] After the initial singleelectron transfer from the amine to the flavin to form the aminyl radical cation and flavin semiquinone, a redox equilibrium was proposed between the semiquinone and a neutral tyrosyl radical(s) during the catalytic cycle. On initial inspection, it might seem thermodynamically unfeasible that a ground state flavin (E m~À 0.2 to 0 V) is capable of oxidising a tyrosine residue (E m~0 .9 V). However, large apparent barriers to electron transfer might not prevent the reaction in an enzyme active site. For example, endergonic tunnelling over a relatively short distance followed by a thermodynamically favourable s...

Substrate-Induced Radical Formation in 4-Hydroxybutyryl Coenzyme A Dehydratase from Clostridium aminobutyricum

“…4 and 7, as well as the results of the preliminary stopped-flow experiments, indicate that the electron transfer occurs before the actual dehydration. Therefore, we speculate that the conserved tyrosine 296 could be the electron source and that it is converted to a radical, as observed in flavin monoamine oxidases (60). This would fit well with the observed inactivity of Y296F in the electron transfer ( Fig.…”

Kinetic Solvent Effects on Hydrogen Abstraction from Phenol by the Cumyloxyl Radical. Toward an Understanding of the Role of Protic Solvents

“…= − α β k k log( ) log( ) 8 11,14 and the reaction of phenol with a π,π* ketone triplet. 23 It has also been applied to the self-combination reaction of the hydroperoxyl radical (HOO • ) 24 and proton transfer reactions from phenols to the anthracene radical anion.…”

Catalytic Amine Oxidation under Ambient Aerobic Conditions: Mimicry of Monoamine Oxidase B

“…[34] This mechanistic suggestion centers upon ac harge-transfer event promoted by the free-base substrate interacting with an electron-rich phenol of Y398 near the flavin acceptor,asdemonstrated by Scrutton and coworkers. [35] This acceptor is itself activated by the H 2 O-K296 hydrogen-bonding motif.T he neutral semiquinone thus formed can mediate hydrogen-atom transfer from the substrate,with the tyrosinyl radical cation now able to accept the second substrate electron, in direct analogy to the role played by alloxan in the currently discussed model. Indeed, both components can be viewed as redox-active hydroxylated units.…”