title: Computational Study of the pKa Values of Potential Catalytic Residues in the Active Site of Monoamine Oxidase B.
abstract: Monoamine oxidase (MAO), which exists in two isozymic forms, MAO A and MAO B, is an important flavoenzyme responsible for the metabolism of amine neurotransmitters such as dopamine, serotonin, and norepinephrine. Despite extensive research effort, neither the catalytic nor the inhibition mechanisms of MAO have been completely understood. There has also been dispute with regard to the protonation state of the substrate upon entering the active site, as well as the identity of residues that are important for the initial deprotonation of irreversible acetylenic inhibitors, in accordance with the recently proposed mechanism. Therefore, in order to investigate features essential for the modes of action of MAO, we have calculated pKa values of three relevant tyrosine residues in the MAO B active site, with and without dopamine bound as the substrate (as well as the pKa of the dopamine itself in the active site). The calculated pKa values for Tyr188, Tyr398, and Tyr435 in the complex are found to be shifted upward to 13.0, 13.7, and 14.7, respectively, relative to 10.1 in aqueous solution, ruling out the likelihood that they are viable proton acceptors. The altered tyrosine pKa values could be rationalized as an interplay of two opposing effects: insertion of positively charged bulky dopamine that lowers tyrosine pKa values, and subsequent removal of water molecules from the active site that elevates tyrosine pKa values, in which the latter prevails. Additionally, the pKa value of the bound dopamine (8.8) is practically unchanged compared to the corresponding value in aqueous solution (8.9), as would be expected from a charged amine placed in a hydrophobic active site consisting of aromatic moieties. We also observed potentially favorable cation-π interactions between the -NH3(+) group on dopamine and aromatic moieties, which provide a stabilizing effect to the charged fragment. Thus, we offer here theoretical evidence that the amine is most likely to be present in the active site in its protonated form, which is similar to the conclusion from experimental studies of MAO A (Jones et al. J. Neural Trans.2007, 114, 707-712). However, the free energy cost of transferring the proton from the substrate to the bulk solvent is only 1.9 kcal mol(-1), leaving open the possibility that the amine enters the chemical step in its neutral form. In conjunction with additional experimental and computational work, the data presented here should lead toward a deeper understanding of mechanisms of the catalytic activity and irreversible inhibition of MAO B, which can allow for the design of novel and improved MAO B inhibitors.
authors.affiliation: Laboratory for Biocomputing and Bioinformatics, National Institute of Chemistry , Hajdrihova 19, SI-1000 Ljubljana, Slovenia.
authors.forename: Rok
authors.initials: R
authors.lastname: Borštnar
authors.affiliation: Laboratory for Biocomputing and Bioinformatics, National Institute of Chemistry , Hajdrihova 19, SI-1000 Ljubljana, Slovenia.
authors.forename: Matej
authors.initials: M
authors.lastname: Repič
authors.affiliation: Department of Cell and Molecular Biology, Uppsala University , Uppsala Biomedical Centre, Box 596, SE-751 24 Uppsala, Sweden.
authors.forename: Shina Caroline Lynn
authors.initials: SC
authors.lastname: Kamerlin
authors.affiliation: Laboratory for Biocomputing and Bioinformatics, National Institute of Chemistry , Hajdrihova 19, SI-1000 Ljubljana, Slovenia.|Quantum Organic Chemistry Group, Ruđer Bošković Institute , Bijenička cesta 54, HR-10000 Zagreb, Croatia.
authors.forename: Robert
authors.initials: R
authors.lastname: Vianello
authors.affiliation: Laboratory for Biocomputing and Bioinformatics, National Institute of Chemistry , Hajdrihova 19, SI-1000 Ljubljana, Slovenia.|EN-FIST Centre of Excellence , Dunajska 156, SI-1000 Ljubljana, Slovenia.
authors.forename: Janez
authors.initials: J
authors.lastname: Mavri