Influence of flavin analogue structure on the catalytic activities and flavinylation reactions of recombinant human liver monoamine oxidases A and B

Nuclear quantum effects in enzymatic reactions: simulation of the kinetic isotope effect of phenylethylamine oxidation catalyzed by monoamine oxidase A

By using computational techniques for quantizing nuclear motion one can accurately reproduce kinetic isotope effect of enzymatic reactions, as demonstrated for phenylethylamine oxidation catalyzed by the monoamine oxidase A enzyme.

Monoamine Oxidases

Monoamine oxidases A and B (MAO A and B) are mammalian flavoenzymes bound to the outer mitochondrial membrane. They were discovered almost a century ago and they have been the subject of many biochemical, structural and pharmacological investigations due to their central role in neurotransmitter metabolism. Currently, the treatment of Parkinson's disease involves the use of selective MAO B inhibitors such as rasagiline and safinamide. MAO inhibition was shown to exert a general neuroprotective effect as a result of the reduction of oxidative stress produced by these enzymes, which seems to be relevant also in non-neuronal contexts. MAOs were successfully expressed as recombinant proteins in Pichia pastoris, which allowed a thorough biochemical and structural characterization. These enzymes are characterized by a globular water-soluble main body that is anchored to the mitochondrial membrane through a C-terminal α-helix, similar to other bitopic membrane proteins. In both MAO A and MAO B the enzyme active site consists of a hydrophobic cavity lined by residues that are conserved in the two isozymes, except for few details that determine substrate and inhibitor specificity. In particular, human MAO B features a dual-cavity active site whose conformation depends on the size of the bound ligand. This article provides a comprehensive and historical review of MAOs and the state-of-the-art of these enzymes as membrane drug targets.

90 years of monoamine oxidase: some progress and some confusion

It would not be practical to attempt to deal with all the advances that have informed our understanding of the behavior and functions of this enzyme over the past 90 years. This account concentrates key advances that explain why the monoamine oxidases remain of pharmacological and biochemical interest and on some areas of continuing uncertainty. Some issues that remain to be understood or are in need of further clarification are highlighted.

What a Difference a Methyl Group Makes: The Selectivity of Monoamine Oxidase B Towards Histamine and N-Methylhistamine

Monoamine oxidase (MAO) enzymes catalyze the degradation of a very broad range of biogenic and dietary amines including many neurotransmitters in the brain, whose imbalance is extensively linked with the biochemical pathology of various neurological disorders. Although sharing around 70 % sequence identity, both MAO A and B isoforms differ in substrate affinities and inhibitor sensitivities. Inhibitors that act on MAO A are used to treat depression, due to their ability to raise serotonin concentrations, whereas MAO B inhibitors decrease dopamine degradation and improve motor control in patients with Parkinson disease. Despite this functional importance, the factors affecting MAO selectivity are poorly understood. Here, we used a combination of molecular dynamics (MD) simulations, molecular mechanics with Poisson-Boltzmann and surface area solvation (MM-PBSA) binding free energy evaluations, and quantum mechanical (QM) cluster calculations to address the unexpected, yet challenging MAO B selectivity for N-methylhistamine (NMH) over histamine (HIS), differing only in a single methyl group distant from the reactive ethylamino center. This study shows that a dominant selectivity contribution is offered by a lower activation free energy for NMH by 2.6 kcal mol^-1 , in excellent agreement with the experimental ΔΔG^≠_EXP =1.4 kcal mol^-1 , together with a more favorable reaction exergonicity and active-site binding. This study also confirms the hydrophobic nature of the MAO B active site and underlines the important role of Ile199, Leu171, and Leu328 in properly orienting substrates for the reaction.

Complete sampling of an enzyme reaction pathway: a lesson from gas phase simulations

With proper sampling strategy, convergence of free energy profiles of biomolecular reactions in the gas phase can be achieved in microseconds of simulation.

ONIOM calculations on serotonin degradation by monoamine oxidase B: insight into the oxidation mechanism and covalent reversible inhibition

Monoamine oxidase (MAO) is an enzyme which catalyzes the oxidation of neurotransmitter amines and regulates their level. There are two forms of the enzyme with 70% similarity, known as MAO-A and MAO-B. MAO inhibitors are used in the treatment of neurological disorders such as depression, Parkinson's and Alzheimer's diseases. Therefore, understanding the chemical steps of MAO catalyzed amine oxidation is crucial for rational drug design. However, despite many experimental studies and recent computational efforts in the literature, the amine oxidation mechanism by MAO enzymes is still controversial. The polar nucleophilic mechanism and hydride transfer mechanisms are under debate in recent QM/MM studies. In this study, the serotonin oxidation mechanism by MAO was explored via the ONIOM (QM : QM) methodology at the M06-2X/6-31+G(d,p):PM6 level. A modified MAO mechanism involving a covalent reversible inhibition step via formation of flavin N5 ylide was proposed. This mechanism can be used to modulate the potency and reversibility of novel mechanism-based covalent inhibitors by intelligent modifications of the structure of the inhibitors. NBO donor-acceptor analysis confirms that the rate-determining αC-H cleavage step is a hybrid of hydride and proton transfer where hydride transfer dominates over the proton transfer. The functional role of covalent FAD was also investigated by calculating the activation energy of noncovalent FAD models where a 22 fold decrease in the rate of catalysis was predicted. Geometrical features imply that the function of the covalent bond in FAD might be to maintain the correct geometry and conformation for a more efficient catalysis.

The Use of Multiscale Molecular Simulations in Understanding a Relationship between the Structure and Function of Biological Systems of the Brain: The Application to Monoamine Oxidase Enzymes

HIGHLIGHTS Computational techniques provide accurate descriptions of the structure and dynamics of biological systems, contributing to their understanding at an atomic level.Classical MD simulations are a precious computational tool for the processes where no chemical reactions take place.QM calculations provide valuable information about the enzyme activity, being able to distinguish among several mechanistic pathways, provided a carefully selected cluster model of the enzyme is considered.Multiscale QM/MM simulation is the method of choice for the computational treatment of enzyme reactions offering quantitative agreement with experimentally determined reaction parameters.Molecular simulation provide insight into the mechanism of both the catalytic activity and inhibition of monoamine oxidases, thus aiding in the rational design of their inhibitors that are all employed and antidepressants and antiparkinsonian drugs. Aging society and therewith associated neurodegenerative and neuropsychiatric diseases, including depression, Alzheimer's disease, obsessive disorders, and Parkinson's disease, urgently require novel drug candidates. Targets include monoamine oxidases A and B (MAOs), acetylcholinesterase (AChE), butyrylcholinesterase (BChE), and various receptors and transporters. For rational drug design it is particularly important to combine experimental synthetic, kinetic, toxicological, and pharmacological information with structural and computational work. This paper describes the application of various modern computational biochemistry methods in order to improve the understanding of a relationship between the structure and function of large biological systems including ion channels, transporters, receptors, and metabolic enzymes. The methods covered stem from classical molecular dynamics simulations to understand the physical basis and the time evolution of the structures, to combined QM, and QM/MM approaches to probe the chemical mechanisms of enzymatic activities and their inhibition. As an illustrative example, the later will focus on the monoamine oxidase family of enzymes, which catalyze the degradation of amine neurotransmitters in various parts of the brain, the imbalance of which is associated with the development and progression of a range of neurodegenerative disorders. Inhibitors that act mainly on MAO A are used in the treatment of depression, due to their ability to raise serotonin concentrations, while MAO B inhibitors decrease dopamine degradation and improve motor control in patients with Parkinson disease. Our results give strong support that both MAO isoforms, A and B, operate through the hydride transfer mechanism. Relevance of MAO catalyzed reactions and MAO inhibition in the context of neurodegeneration will be discussed.

Insights into enzyme point mutation effect by molecular simulation: phenylethylamine oxidation catalyzed by monoamine oxidase A

The I335Y point mutation effect on the kinetics of phenylethylamine decomposition catalyzed by monoamine oxidase A was elucidated by means of molecular simulation. The established empirical valence bond methodology was used in conjunction with the free energy perturbation sampling technique and a classical force field representing the state of reactants and products. The methodology allows for the simulation of chemical reactions, in the present case the breaking of the α-C-H bond in a phenylethylamine substrate and the subsequent hydrogen transfer to the flavin cofactor, resulting in the formation of the N-H bond on flavin. The empirical parameters were calibrated against the experimental data for the simulated reaction in a wild type protein and then used for the calculation of the reaction free energy profile in the I335Y mutant. In very good agreement with the measured kinetic data, mutation increases the free energy barrier for the rate limiting step by slightly more than 1 kcal mol(-1) and consequently decreases the rate constant by about an order of magnitude. The magnitude of the computed effect slightly varies with simulation settings, but always remains in reasonable agreement with the experiment. Analysis of trajectories reveals a major change in the interaction between phenyl rings of the substrate and the neighboring Phe352 residue upon the I335Y mutation due to the increased local polarity, leading to an attenuated quadrupole interaction between the rings and destabilization of the transition state. Additionally, the increased local polarity in the mutant allows for a larger number of water molecules to be present near the active site, effectively shielding the catalytic effect of the enzyme and contributing to the increased barrier.

Truncated FAD synthetase for direct biocatalytic conversion of riboflavin and analogs to their corresponding flavin mononucleotides

The preparation of flavin mononucleotide (FMN) and FMN analogs from their corresponding riboflavin precursors is traditionally performed in a two-step procedure. After initial enzymatic conversion of riboflavin to flavin adenine dinucleotide (FAD) by a bifunctional FAD synthetase, the adenyl moiety of FAD is hydrolyzed with snake venom phosphodiesterase to yield FMN. To simplify the protocol, we have engineered the FAD synthetase from Corynebacterium ammoniagenes by deleting its N-terminal adenylation domain. The newly created biocatalyst is stable and efficient for direct and quantitative phosphorylation of riboflavin and riboflavin analogs to their corresponding FMN cofactors at preparative-scale.

Structure-based redesign of cofactor binding in putrescine oxidase

Putrescine oxidase (PuO) from Rhodococcus erythropolis is a soluble homodimeric flavoprotein, which oxidizes small aliphatic diamines. In this study, we report the crystal structures and cofactor binding properties of wild-type and mutant enzymes. From a structural viewpoint, PuO closely resembles the sequence-related human monoamine oxidases A and B. This similarity is striking in the flavin-binding site even if PuO does not covalently bind the cofactor as do the monoamine oxidases. A remarkable conserved feature is the cis peptide conformation of the Tyr residue whose conformation is important for substrate recognition in the active site cavity. The structure of PuO in complex with the reaction product reveals that Glu324 is crucial in recognizing the terminal amino group of the diamine substrate and explains the narrow substrate specificity of the enzyme. The structural analysis also provides clues for identification of residues that are responsible for the competitive binding of ADP versus FAD (~50% of wild-type PuO monomers isolated are occupied by ADP instead of FAD). By replacing Pro15, which is part of the dinucleotide-binding domain, enzyme preparations were obtained that are almost 100% in the FAD-bound form. Furthermore, mutants have been designed and prepared that form a covalent 8α-S-cysteinyl-FAD linkage. These data provide new insights into the molecular basis for substrate recognition in amine oxidases and demonstrate that engineering of flavoenzymes to introduce covalent linkage with the cofactor is a possible route to develop more stable protein molecules, better suited for biocatalytic purposes.

Involvement of ionizable groups in catalysis of human liver glycolate oxidase

Glycolate oxidase is a flavin-dependent, peroxisomal enzyme that oxidizes alpha-hydroxy acids to the corresponding alpha-keto acids, with reduction of oxygen to H(2)O(2). In plants, the enzyme participates in photorespiration. In humans, it is a potential drug target for treatment of primary hyperoxaluria, a genetic disorder where overproduction of oxalate results in the formation of kidney stones. In this study, steady-state and pre-steady-state kinetic approaches have been used to determine how pH affects the kinetic steps of the catalytic mechanism of human glycolate oxidase. The enzyme showed a Ping-Pong Bi-Bi kinetic mechanism between pH 6.0 and 10.0. Both the overall turnover of the enzyme (k(cat)) and the rate constant for anaerobic substrate reduction of the flavin were pH-independent at pH values above 7.0 and decreased slightly at lower pH, suggesting the involvement of an unprotonated group acting as a base in the chemical step of glycolate oxidation. The second-order rate constant for capture of glycolate (k(cat)/K(glycolate)) and the K(d)((app)) for the formation of the enzyme-substrate complex suggested the presence of a protonated group with apparent pK(a) of 8.5 participating in substrate binding. The k(cat)/K(oxygen) values were an order of magnitude faster when a group with pK(a) of 6.8 was unprotonated. These results are discussed in the context of the available three-dimensional structure of GOX.

Serine 209 resides within a putative p38(MAPK) consensus motif and regulates monoamine oxidase-A activity

The p38 mitogen-activated protein kinase (MAPK) cascade as well as the enzyme monoamine oxidase-A (MAO-A) have both been associated with oxidative stress. We observed that the specific inhibition of the p38(MAPK) protein [using either a chemical inhibitor or a dominant-negative p38(MAPK) clone] selectively induces MAO-A activity and MAO-A-sensitive toxicity in several neuronal cell lines, including primary cortical neurons. Over-expression of a constitutively active p38(MAPK) results in the phosphorylation of the MAO-A protein and inhibition of MAO-A activity. The MAO-A(Ser209Glu) phosphomimic - bearing a targeted substitution within a putative p38(MAPK) consensus motif - is neither active nor neurotoxic. In contrast, the MAO-A(Ser209Ala) variant (mimics dephosphorylation) does not associate with p38(MAPK), and is both very active and very toxic. Substitution of the homologous serine in the MAO-B isoform, i.e. Ser200, with either Glu or Ala does not affect the catalytic activity of the corresponding over-expressed proteins. These combined in vitro data strongly suggest a direct p38(MAPK)-dependent inhibition of MAO-A function. Based on published observations, this endogenous means of selectively regulating MAO-A function could provide for an adaptive response to oxidative stress associated with disorders as diverse as depression, reperfusion/ischemia, and the early stages of Alzheimer's disease.

To be or not to be an oxidase: challenging the oxygen reactivity of flavoenzymes

Flavin-dependent enzymes catalyse a wide range of reactions and, thereby, facilitate a variety of cellular processes. Among the properties that equip flavoenzymes with this chemical versatility is their reactivity towards oxygen, which shows huge variation among flavoproteins. A survey of known 3D structures of flavin-dependent oxidases and dehydrogenases and the correlation with their functional properties indicates that there are no structural rules that enable prediction of whether or how a flavoenzyme reacts with oxygen. Combinations of subtle factors such as dipole pre-organization, charge distribution, dynamics and solvation in the active centre determine the balance of interactions that control oxygen reactivity. The chemical basis of oxygen reactivity remains a puzzling problem and represents one of the challenging questions in modern flavoenzymology.

Biosynthesis of covalently bound flavin: isolation and in vitro flavinylation of the monomeric sarcosine oxidase apoprotein

The covalently bound FAD in native monomeric sarcosine oxidase (MSOX) is attached to the protein by a thioether bond between the 8alpha-methyl group of the flavin and Cys315. Large amounts of soluble apoenzyme are produced by controlled expression in a riboflavin-dependent Escherichia coli strain. A time-dependent increase in catalytic activity is observed upon incubation of apoMSOX with FAD, accompanied by the covalent incorporation of FAD to approximately 80% of the level observed with the native enzyme. The spectral and catalytic properties of the reconstituted enzyme are otherwise indistinguishable from those of native MSOX. The reconstitution reaction exhibits apparent second-order kinetics (k = 139 M(-)(1) min(-)(1) at 23 degrees C) and is accompanied by the formation of a stoichiometric amount of hydrogen peroxide. A time-dependent reduction of FAD is observed when the reconstitution reaction is conducted under anaerobic conditions. The results provide definitive evidence for autoflavinylation in a reaction that proceeds via a reduced flavin intermediate and requires only apoMSOX and FAD. Flavinylation of apoMSOX is not observed with 5-deazaFAD or 1-deazaFAD, an outcome attributed to a decrease in the acidity of the 8alpha-methyl group protons. Covalent flavin attachment is observed with 8-nor-8-chloroFAD in an aromatic nucleophilic displacement reaction that proceeds via a quininoid intermediate but not a reduced flavin intermediate. The reconstituted enzyme contains a modified cysteine-flavin linkage (8-nor-8-S-cysteinyl) as compared with native MSOX (8alpha-S-cysteinyl), a difference that may account for its approximately 10-fold lower catalytic activity.

Analysis of conserved active site residues in monoamine oxidase A and B and their three-dimensional molecular modeling

Monoamine oxidase (MAO) is a key enzyme responsible for the degradation of serotonin, norepinephrine, dopamine, and phenylethylamine. It is an outer membrane mitochondrial enzyme existing in two isoforms, A and B. We have recently generated 14 site-directed mutants of human MAO A and B, and we found that four key amino acids, Lys-305, Trp-397, Tyr-407, and Tyr-444, in MAO A and their corresponding amino acids in MAO B, Lys-296, Trp-388, Tyr-398, and Tyr-435, play important roles in MAO catalytic activity. Based on the polyamine oxidase three-dimensional crystal structure, it is suggested that Lys-305, Trp-397, and Tyr-407 in MAO A and Lys-296, Trp-388, and Tyr-398 in MAO B may be involved in the non-covalent binding to FAD. Tyr-407 and Tyr-444 in MAO A (Tyr-398 and Tyr-435 in MAO B) may form an aromatic sandwich that stabilizes the substrate binding. Asp-132 in MAO A (Asp-123 in MAO B) located at the entrance of the U-shaped substrate-binding site has no effect on MAO A nor MAO B catalytic activity. The similar impact of analogous mutants in MAO A and MAO B suggests that these amino acids have the same function in both isoenzymes. Three-dimensional modeling of MAO A and B using polyamine oxidase as template suggests that the overall tertiary structure and the active sites of MAO A and B may be similar.

High-level expression of human liver monoamine oxidase A in Pichia pastoris: comparison with the enzyme expressed in Saccharomyces cerevisiae

The high-level expression, purification, and characterization of recombinant membrane-bound human liver monoamine oxidase A (MAO-A) in Pichia pastoris is described. Two liters of fermentation culture produces 1170 units (660 mg) of MAO-A. The enzyme is purified in a 35% yield, is homogeneous on denaturing gel electrophoresis, and exhibits a single species (60,512 +/- 6 Da) on electrospray mass spectrometry. It contains 1 mol of 8alpha-S-cysteinyl FAD/mole of enzyme and exhibits >95% functionality. In contrast, the Saccharomyces cerevisiae-expressed enzyme is partially processed by C-terminal serine removal as demonstrated by mass spectra. The amino termini of both P. pastoris- and S. cerevisiae-expressed MAO-A are acetylated on the N-terminal methionine. The steady-state kinetic properties of P. pastoris-expressed MAO-A are similar to those of S. cerevisiae-expressed MAO-A using the following substrates: phenethylamine, p-CF(3)-benzylamine, dopamine, serotonin, and kynuramine. Reductive titrations demonstrate that the recombinant enzyme is reduced by 1 mol of substrate or dithionite as expected for the two electron equivalents required for flavin reduction. Absorption and EPR spectra show no radical species in the resting enzyme while the anionic flavin radical is formed in 50% yield during the reductive titration with dithionite. These data demonstrate significant advantages in the heterologous expression of human MAO-A in P. pastoris compared with the published S. cerevisiae system in higher expression level (329 mg/L) and in a higher level of homogeneity of the isolated enzyme.

Loss of serotonin oxidation as a component of the altered substrate specificity in the Y444F mutant of recombinant human liver MAO A

To investigate the roles of tyrosyl residues located near the covalent 8alpha-S-cysteinyl FAD in monoamine oxidase A (MAO A) and to test the suggestion that MAO A and plant polyamine oxidase may have structural homology, tyrosyl to phenylalanyl mutants of MAO A at positions 377, 402, 407, 410, 419, and 444 were constructed and expressed in Saccharomyces cerevisiae. All mutant enzymes were expressed and exhibited lower specific activities as compared to WT MAO A using kynuramine as substrate. The lowest specific activities in this assay are exhibited by the Y407F and Y444F mutant enzymes. On purification and further characterization, these two mutants were found to each contain covalent FAD. Both mutant enzymes are irreversibly inhibited by the MAO A inhibitor clorgyline and exhibit binding stoichiometries of 0.54 (Y407F) and 0.95 (Y444F) as compared to 1.05 for WT MAO A. Y444F MAO A oxidizes kynuramine with a k(cat) <2% of WT enzyme and is greater than 100-fold slower in catalyzing the oxidation of phenylethylamine or of serotonin. In contrast, Y444F MAO A oxidizes p-CF(3)-benzylamine at a rate 25% that of WT enzyme. Steady state and reductive half-reaction stopped-flow data using a series of para-substituted benzylamine analogues show Y444F MAO A exhibits quantitative structure activity relationships (QSAR) properties on analogue binding and rates of substrate oxidation very similar to that exhibited by the WT enzyme (Miller and Edmondson (1999) Biochemistry 38, 13670): log K(d) = -(0.37 +/- ()()0.07)V(W)(x0.1) - 4.5 +/- 0.1; log k(red) = +(2.43 +/- 0.19)sigma + 0.17 +/- 0.05. The Y444F MAO A mutant also exhibits similar QSAR properties on the binding of phenylalkyl side chain amine analogues as WT enzyme: log K(i) = (4.37 +/- 0.51)E(S) + 1.21 +/- 0.77. These data show that mutation of Y444F in MAO A results in a mutant that has lost its ability to efficiently oxidize serotonin (its physiological substrate) but, however, exhibits unaltered quantitative structure-activity parameters in the binding and rate of benzylamine analogues. The mechanism of C-H abstraction is therefore unaltered. The suggestion that polyamine oxidase and monoamine oxidase may have structural homology appears to be valid as regards Y444 in MAO A and Y439 in plant polyamine oxidase.

The covalent FAD of monoamine oxidase: structural and functional role and mechanism of the flavinylation reaction

The family of flavoenzymes in which the flavin coenzyme redox cofactor is covalently attached to the protein through an amino acid side chain is covered in this review. Flavin-protein covalent linkages have been shown to exist through each of five known linkages: (a) 8alpha-N(3)-histidyl, (b) 8alpha-N(1)-histidyl, (c) 8alpha-S-cysteinyl, (d) 8alpha-O-tyrosyl, or (e) 6-S-cysteinyl with the flavin existing at either the flavin mononucleotide or flavin adenine dinucleotide (FAD) levels. This class of enzymes is widely distributed in diverse biological systems and catalyzes a variety of enzymatic reactions. Current knowledge on the mechanism of covalent flavin attachment is discussed based on studies on the 8alpha-S-cysteinylFAD of monoamine oxidases A and B, as well as studies on other flavoenzymes. The evidence supports an autocatalytic quinone-methide mechanism of protein flavinylation. Proposals to explain the structural and mechanistic advantages of a covalent flavin linkage in flavoenzymes are presented. It is concluded that multiple factors are involved and include: (a) stabilization of the apoenzyme structure, (b) steric alignment of the cofactor in the active site to facilitate catalysis, and (c) modulation of the redox potential of the covalent flavin through electronic effects of 8alpha-substitution.

High-level expression of human liver monoamine oxidase B in Pichia pastoris

The high-level heterologous expression, purification, and characterization of the mitochondrial outer membrane enzyme human liver monoamine oxidase B (MAO B) using the methylotrophic yeast Pichia pastoris expression system are described. A 2-L culture of P. pastoris expresses approximately 1700 U of MAO B activity, with the recombinant enzyme associated tightly with the membrane fraction of the cell lysate. By a modification of the published procedure for purification of bovine liver MAO B [Salach, J. I. (1979) Arch. Biochem. Biophys. 192, 128-137], recombinant human liver MAO B is purified in a 34% yield ( approximately 200 mg from 2 L of cell culture). The isolated enzyme exhibits an M(r) of approximately 60, 000 on SDS-PAGE and 59,474 from electrospray mass spectrometry measurements, which is in good agreement with the mass predicted from the gene sequence and inclusion of the covalent FAD. One mole of covalent FAD per mole of MAO B is present in the purified enzyme and is bound by an 8alpha-S-cysteinyl(397) linkage, as identified by electrospray mass spectrometry of the isolated tryptic/chymotryptic flavin peptide. Recombinant human liver MAO B and bovine liver MAO B are shown to be acetylated at the seryl residues at their respective amino termini. The benzylamine oxidase activity of recombinant MAO B ranges from 3.0 to 3.4 U/mg and steady-state kinetic parameters for this enzyme preparation compare well with those published for the bovine liver enzyme: k(cat) = 600 min(-1), K(m)(benzylamine) = 0.50 mM, and K(m)(O(2)) = 0.33 mM. Kinetic isotope effect parameters using [alpha,alpha-(2)H(2)]benzylamine are also similar to those found for the bovine enzyme. Recombinant MAO B exhibits a (D)k(cat) = 4.7, a (D)[k(cat)/K(m)(benzylamine)] = 4.5, and a (D)[k(cat)/K(m)(O(2))] = 1.0. In contrast to bovine liver MAO B, no evidence was found for the presence of any anionic flavin radical either by UV-vis or by EPR spectroscopy in the resting form of the enzyme. These data demonstrate the successful heterologous expression of a functional, membrane-bound MAO B, which will permit a number of mutagenesis studies as structural and mechanistic probes not previously possible.

Influence of FAD structure on its binding and activity with the C406A mutant of recombinant human liver monoamine oxidase A

The FAD binding site of human liver monoamine oxidase A (MAO A) has been investigated by mutagenesis of the amino acid site of covalent FAD attachment (Cys-406) to an alanyl residue. Expression of the C406A mutant in Saccharomyces cerevisiae results in the formation of an active enzyme, as found previously with the rat liver enzyme. The activity of this mutant enzyme is labile to solubilization, thus requiring all experiments to be done with membrane preparations. C406A MAO A was expressed in a rib 5(-) strain of S. cerevisiae in the presence of 16 different riboflavin analogues. Inactive apoC406A MAO A is formed by induction of the enzyme in the absence of riboflavin. FAD but not FMN or riboflavin restores catalytic activity with an apparent K(d) of 62 +/- 5 nm. The results from both in vivo and in vitro reconstitution experiments show increased activity levels (up to approximately 7-fold higher) with those analogues exhibiting higher oxidation-reduction potentials than normal flavin and decreased activity levels with analogues exhibiting lower potentials. Analogues with substituents on the pyrimidine ring bind to C406A MAO A more weakly than normal FAD, suggesting specific interactions with the N(3) and N(1) positions. Analogues with substituents in the 7 and 8 positions bind to C406A MAO A with affinities comparable with that of normal FAD. These results are discussed in regard to functional significance of 8alpha-covalent binding of flavins to proteins.

Asp-170 is crucial for the redox properties of vanillyl-alcohol oxidase

Vanillyl-alcohol oxidase is a flavoprotein containing a covalent flavin that catalyzes the oxidation of 4-(methoxymethyl)phenol to 4-hydroxybenzaldehyde. The reaction proceeds through the formation of a p-quinone methide intermediate, after which, water addition takes place. Asp-170, located near the N5-atom of the flavin, has been proposed to act as an active site base. To test this hypothesis, we have addressed the properties of D170E, D170S, D170A, and D170N variants. Spectral and fluorescence analysis, together with the crystal structure of D170S, suggests that the Asp-170 replacements do not induce major structural changes. However, in D170A and D170N, 50 and 100%, respectively, of the flavin is non-covalently bound. Kinetic characterization of the vanillyl-alcohol oxidase variants revealed that Asp-170 is required for catalysis. D170E is 50-fold less active, and the other Asp-170 variants are about 10(3)-fold less active than wild type enzyme. Impaired catalysis of the Asp-170 variants is caused by slow flavin reduction. Furthermore, the mutant proteins have lost the capability of forming a stable complex between reduced enzyme and the p-quinone methide intermediate. The redox midpoint potentials in D170E (+6 mV) and D170S (-91 mV) are considerably decreased compared with wild type vanillyl-alcohol oxidase (+55 mV). This supports the idea that Asp-170 interacts with the protonated N5-atom of the reduced cofactor, thus increasing the FAD redox potential. Taken together, we conclude that Asp-170 is involved in the process of autocatalytic flavinylation and is crucial for efficient redox catalysis.

The FAD binding sites of human liver monoamine oxidases A and B: investigation of the role of flavin ribityl side chain hydroxyl groups in the covalent flavinylation reaction and catalytic activities

The role of ribityl side chain hydroxyl groups of the flavin moiety in the covalent flavinylation reaction and catalytic activities of recombinant human liver monoamine oxidases (MAO) A and B have been investigated using the riboflavin analogue: N(10)-omega-hydroxypentyl-isoalloxazine. Using a rib5 disrupted strain of Saccharomyces cerevisiae which is auxotrophic for riboflavin, MAO A and MAO B were expressed separately under control of a galactose inducible GAL10/CYC1 promoter in the presence of N(10)-omega-hydroxypentyl-isoalloxazine as the only available riboflavin analogue. Analysis of mitochondrial membrane proteins shows both enzymes to be expressed at levels comparable to those cultures grown on riboflavin and to contain covalently bound flavin. Catalytic activities, as monitored by kynuramine oxidation, are equivalent to (MAO A) or 2-fold greater (MAO B) than control preparations expressed in the presence of riboflavin. Although N(10)-omega-hydroxypentyl-isoalloxazine is unable to support growth of riboflavin auxotrophic S. cerevisiae, it is converted to the FMN level by yeast cell free extracts. The FMN form of the analogue is converted to the FAD level by the yeast FAD synthetase, as shown by expression of the recombinant enzyme in Escherichia coli. These data show that the ribityl hydroxyl groups of the FAD moiety are not required for covalent flavinylation or catalytic activities of monoamine oxidases A and B. This is in contrast to the suggestion based on mutagenesis studies that an interaction between the 3'-hydroxyl group of the flavin and the beta-carbonyl of Asp(227) is required for the covalent flavinylation reaction of MAO B (Zhou et al., J. Biol. Chem. 273 (1998) 14862-14868).