The reaction of phenylhydrazine with trimethylamine dehydrogenase and with free flavins

Privileged scaffolds as MAO inhibitors: Retrospect and prospects

This review aims to be a comprehensive, authoritative, critical, and readable review of general interest to the medicinal chemistry community because it focuses on the pharmacological, chemical, structural and computational aspects of diverse chemical categories as monoamine oxidase inhibitors (MAOIs). Monoamine oxidases (MAOs), namely MAO-A and MAO-B represent an enormously valuable class of neuronal enzymes embodying neurobiological origin and functions, serving as potential therapeutic target in neuronal pharmacotherapy, and hence we have coined the term "Neurozymes" which is being introduced for the first time ever. Nowadays, therapeutic attention on MAOIs engrosses two imperative categories; MAO-A inhibitors, in certain mental disorders such as depression and anxiety, and MAO-B inhibitors, in neurodegenerative disorders like Alzheimer's disease (AD) and Parkinson's disease (PD). The use of MAOIs declined due to some potential side effects, food and drug interactions, and introduction of other classes of drugs. However, curiosity in MAOIs is reviving and the recent developments of new generation of highly selective and reversible MAOIs, have renewed the therapeutic prospective of these compounds. The initial section of the review emphasizes on the detailed classification, structural and binding characteristics, therapeutic potential, current status and future challenges of the privileged pharmacophores. However, the chemical prospective of privileged scaffolds such as; aliphatic and aromatic amines, amides, hydrazines, azoles, diazoles, tetrazoles, indoles, azines, diazines, xanthenes, tricyclics, benzopyrones, and more interestingly natural products, along with their conclusive SARs have been discussed in the later segment of review. The last segment of the article encompasses some patents granted in the field of MAOIs, in a simplistic way.

An overview of phenylcyclopropylamine derivatives: biochemical and biological significance and recent developments

trans-2-Phencylcyclopropylamine (2-PCPA), a potent, clinically used antidepressant, affects monoamine neurotransmitter levels by inhibiting the main metabolizing enzymes, monoamine oxidases (MAOs). However, the antidepressant action of this compound was not fully explained by its effects on MAOs due to its wide variety of biological effects. 2-PCPA also affects depression-associated pathophysiological pathways, and linked with increased levels of trace amines in brain, upregulation of GABAB receptors (where GABA is gamma amino butyric acid), modulation of phospholipid metabolism, and interference with various cytochrome P450 (CYP) enzymes. Consequently, despite its adverse effects and limited clinical applicability, 2-PCPA has attracted interest as a structural scaffold for the development of mechanism-based inhibitors of various enzymes, including lysine-specific demethylase 1 (LSD1), which is a possible target for cancer chemotherapy. In the recent years, many reports have appeared in the literature based on 2-PCPA scaffold and their potential medicinal implications. This review mainly focuses on the medicinal chemistry aspects including drug design, structure-activity relationships (SAR), biological and biochemical properties, and mechanism of actions of 2-PCPA and its derivatives. Furthermore, we also highlight recent advance in this area and discuss their future applications for beneficial therapeutic effects.

Mechanism-based inhibitors of cytokinin oxidase/dehydrogenase attack FAD cofactor

Cytokinin oxidases/dehydrogenases (CKOs) mediate catabolic regulation of cytokinin levels in plants. Several substrate analogs containing an unsaturated side chain were studied for their possible inhibitory effect on maize CKO (ZmCKO1) by use of various bioanalytical methods. Two allenic derivatives, N(6)-(buta-2,3-dienyl)adenine (HA-8) and N(6)-(penta-2,3-dienyl)adenine (HA-1), were identified as strong mechanism-based inhibitors of the enzyme. Despite exhaustive dialysis, the enzyme remained inhibited. Conversely, substrate analogs with a triple bond in the side chain were much weaker inactivators. The crystal structures of recombinant ZmCKO1 complexed with HA-1 or HA-8 were solved to 1.95 A resolution. Together with Raman spectra of the inactivated enzyme, it was revealed that reactive imine intermediates generated by oxidation of the allenic inhibitors covalently bind to the flavin adenine dinucleotide (FAD) cofactor. The binding occurs at the C4a atom of the isoalloxazine ring of FAD, the planarity of which is consequently disrupted. All the compounds under study were also analyzed for binding to the Arabidopsis cytokinin receptors AHK3 and AHK4 in a bacterial receptor assay and for cytokinin activity in the Amaranthus bioassay. HA-1 and HA-8 were found to be good receptor ligands with a significant cytokinin activity. Nevertheless, due to their ability to inactivate CKO in the desired time intervals or developmental stages, they both represent attractive compounds for physiological studies, as the inhibition mechanism of HA-1 and HA-8 is mainly FAD dependent.

Flavoenzyme catalysed oxidation of amines: roles for flavin and protein-based radicals

Amines are a carbon source for the growth of a number of bacterial species and they also play key roles in neurotransmission, cell growth and differentiation, and neoplastic cell proliferation. Enzymes have evolved to catalyse these reactions and these oxidoreductases can be grouped into the flavoprotein and quinoprotein families. The mechanism of amine oxidation catalysed by the quinoprotein amine oxidases is understood reasonably well and occurs through the formation of enzyme–substrate covalent adducts with TPQ (topaquinone), TTQ (tryptophan tryptophylquinone), CTQ (cysteine tryptophylquinone) and LTQ (lysine tyrosyl quinone) redox centres. Oxidation of amines by flavoenzymes is less well understood. The role of protein-based radicals and flavin semiquinone radicals in the oxidation of amines is discussed.

The interaction of trimethylamine dehydrogenase and electron-transferring flavoprotein

The interaction between the physiological electron transfer partners trimethylamine dehydrogenase (TMADH) and electron-transferring flavoprotein (ETF) from Methylophilus methylotrophus has been examined with particular regard to the proposal that the former protein "imprints" a conformational change on the latter. The results indicate that the absorbance change previously attributed to changes in the environment of the FAD of ETF upon binding to TMADH is instead caused by electron transfer from partially reduced, as-isolated TMADH to ETF. Prior treatment of the as-isolated enzyme with the oxidant ferricenium essentially abolishes the observed spectral change. Further, when the semiquinone form of ETF is used instead of the oxidized form, the mirror image of the spectral change seen with as-isolated TMADH and oxidized ETF is observed. This is attributable to a small amount of electron transfer in the reverse of the physiological direction. Kinetic determination of the dissociation constant and limiting rate constant for electron transfer within the complex of (reduced) TMADH with (oxidized) ETF is reconfirmed and discussed in the context of a recently proposed model for the interaction between the two proteins that involves "structural imprinting" of ETF.

Optimizing the Michaelis complex of trimethylamine dehydrogenase: identification of interactions that perturb the ionization of substrate and facilitate catalysis with trimethylamine base

Recent evidence from isotope studies supports the view that catalysis by trimethylamine dehydrogenase (TMADH) proceeds from a Michaelis complex involving trimethylamine base and not, as thought previously, trimethylammonium cation. In native TMADH reduction of the flavin by substrate (perdeuterated trimethylamine) is influenced by two ionizations in the Michaelis complex with pK(a) values of 6.5 and 8.4; maximal activity is realized in the alkaline region. The latter ionization has been attributed to residue His-172 and, more recently, the former to the ionization of substrate itself. In the Michaelis complex, the ionization of substrate (pK(a) approximately 6.5 for perdeuterated substrate) is perturbed by approximately -3.3 to -3.6 pH units compared with that of free trimethylamine (pK(a) = 9.8) and free perdeuterated trimethylamine (pK(a) = 10.1), respectively, thus stabilizing trimethylamine base by approximately 2 kJ mol(-1). We show, by targeted mutagenesis and stopped-flow studies that this reduction of the pK(a) is a consequence of electronic interaction with residues Tyr-60 and His-172, thus these two residues are key for optimizing catalysis in the physiological pH range. We also show that residue Tyr-174, the remaining ionizable group in the active site that we have not targeted previously by mutagenesis, is not implicated in the pH dependence of flavin reduction. Formation of a Michaelis complex with trimethylamine base is consistent with a mechanism of amine oxidation that we advanced in our previous computational and kinetic studies which involves nucleophilic attack by the substrate nitrogen atom on the electrophilic C4a atom of the flavin isoalloxazine ring. Stabilization of trimethylamine base in the Michaelis complex over that in free solution is key to optimizing catalysis at physiological pH in TMADH, and may be of general importance in the mechanism of other amine dehydrogenases that require the unprotonated form of the substrate for catalysis.

Deuterium isotope effects during carbon-hydrogen bond cleavage by trimethylamine dehydrogenase. Implications for mechanism and vibrationally assisted hydrogen tunneling in wild-type and mutant enzymes

His-172 and Tyr-169 are components of a triad in the active site of trimethylamine dehydrogenase (TMADH) comprising Asp-267, His-172, and Tyr-169. Stopped-flow kinetic studies with trimethylamine as substrate have indicated that mutation of His-172 to Gln reduces the limiting rate constant for flavin reduction approximately 10-fold (Basran, J., Sutcliffe, M. J., Hille, R., and Scrutton, N. S. (1999) Biochem. J. 341, 307-314). A kinetic isotope effect (KIE = k(H)/k(D)) accompanies flavin reduction by H172Q TMADH, the magnitude of which varies significantly with solution pH. With trimethylamine, flavin reduction by H172Q TMADH is controlled by a single macroscopic ionization (pK(a) = 6.8 +/- 0.1). This ionization is perturbed (pK(a) = 7.4 +/- 0.1) in reactions with perdeuterated trimethylamine and is responsible for the apparent variation in the KIE with solution pH. At pH 9.5, where the functional group controlling flavin reduction is fully ionized, the KIE is independent of temperature in the range 277-297 K, consistent with vibrationally assisted hydrogen tunneling during breakage of the substrate C-H bond. Y169F TMADH is approximately 4-fold more compromised than H172Q TMADH for hydrogen transfer, which occurs non-classically. Studies with Y169F TMADH suggest partial thermal excitation of substrate prior to hydrogen tunneling by a vibrationally assisted mechanism. Our studies illustrate the varied effects of compromising mutations on tunneling regimes in enzyme molecules.

Formation of W(3)A(1) electron-transferring flavoprotein (ETF) hydroquinone in the trimethylamine dehydrogenase x ETF protein complex

The electron-transferring flavoprotein (ETF) from Methylophilus methylotrophus (sp. W(3)A(1)) exhibits unusual oxidation-reduction properties and can only be reduced to the level of the semiquinone under most circumstances (including turnover with its physiological reductant, trimethylamine dehydrogenase (TMADH), or reaction with strong reducing reagents such as sodium dithionite). In the present study, we demonstrate that ETF can be reduced fully to its hydroquinone form both enzymatically and chemically when it is in complex with TMADH. Quantitative titration of the TMADH x ETF protein complex with sodium dithionite shows that a total of five electrons are taken up by the system, indicating that full reduction of ETF occurs within the complex. The results indicate that the oxidation-reduction properties of ETF are perturbed upon binding to TMADH, a conclusion further supported by the observation of a spectral change upon formation of the TMADH x ETF complex that is due to a change in the environment of the FAD of ETF. The results are discussed in the context of ETF undergoing a conformational change during formation of the TMADH x ETF electron transfer complex, which modulates the spectral and oxidation-reduction properties of ETF such that full reduction of the protein can take place.

Reaction of the C30A mutant of trimethylamine dehydrogenase with diethylmethylamine

The role played by the 6-S-cysteinyl-FMN bond of trimethylamine dehydrogenase in the reductive half-reaction of the enzyme has been studied by following the reaction of the slow substrate diethylmethylamine with a C30A mutant of the enzyme lacking the covalent flavin attachment to the polypeptide. Removal of the 6-S-cysteinyl-FMN bond diminishes the limiting rate for the first of the three observed kinetic phases of the reaction by a factor of 6, but has no effect on the rate constants for the two subsequent kinetic phases. The flavin in the C30A enzyme recovered from the reaction of the C30A enzyme with excess substrate is found to have been converted to the 6-hydroxy derivative, rendering the enzyme inactive. The noncovalently bound FMN of the C30A mutant enzyme is also converted to 6-hydroxy-FMN and rendered inactive upon reduction with excess trimethylamine, but not by reduction with dithionite, even at high pH or in the presence of the effector tetramethylammonium chloride. These results suggest that one significant role of the 6-S-cysteinyl-FMN bond is to prevent the inactivation of the enzyme during catalysis. A reaction mechanism is proposed whereby OH- attacks C-6 of a flavin-substrate covalent adduct in the course of steady-state turnover to form 6-hydroxy-FMN.

The reaction of trimethylamine dehydrogenase with electron transferring flavoprotein

The kinetics of electron transfer between trimethylamine dehydrogenase (TMADH) and its physiological acceptor, electron transferring flavoprotein (ETF), has been studied by static and stopped-flow absorbance measurements. The results demonstrate that reducing equivalents are transferred from TMADH to ETF solely through the 4Fe/4S center of the former. The intrinsic limiting rate constant (klim) and dissociation constant (Kd) for electron transfer from the reduced 4Fe/4S center of TMADH to ETF are about 172 s-1 and 10 microM, respectively. The reoxidation of fully reduced TMADH with an excess of ETF is markedly biphasic, indicating that partial oxidation of the iron-sulfur center in 1-electron reduced enzyme significantly reduces the rate of electron transfer out of the enzyme in these forms. The interaction of the two unpaired electron spins of flavin semiquinone and reduced 4Fe/4S center in 2-electron reduced TMADH, on the other hand, does not significantly slow down the electron transfer from the 4Fe/4S center to ETF. From a comparison of the limiting rate constants for the oxidative and reductive half-reactions, we conclude that electron transfer from TMADH to ETF is not rate-limiting during steady-state turnover. The overall kinetics of the oxidative half-reaction are not significantly affected by high salt concentrations, indicating that electrostatic forces are not involved in the formation and decay of reduced TMADH-oxidized ETF complex.

Physico-chemical characterization of a recombinant cytoplasmic form of lysine: N6-hydroxylase

A recombinant cytoplasmic preparation of lysine: N6-hydroxylase, IucD398, with a deletion of 47 amino acids at the N-terminus, was purified to homogeneity. IucD398 is capable of N-hydroxylation of L-lysine upon supplementation with FAD and NADPH. The enzyme is stringently specific with L-lysine and (S)-2-aminoethyl-L-cysteine serving as substrates. Protonophores, FCCP and CCCP, as well as cinnamylidene, have been found to serve as potent inhibitors of lysine: N6-hydroxylation by virtue of their ability to interfere in the reduction of the flavin cofactor.

Oxidation-reduction properties of trimethylamine dehydrogenase: effect of inhibitor binding

The redox potentials of trimethylamine dehydrogenase from the bacterium W3A1 have been determined by means of uv-visible spectroelectrochemistry. In the presence of the inhibitor tetramethylammonium chloride a shift of +0.2 V was observed in the midpoint redox potential for conversion of the oxidized 6-S-cysteinyl-FMN to the flavin radical form. The pH-independent value was +0.23 V vs the standard hydrogen electrode. The pH-dependent conversion of this radical to fully reduced flavin was shifted negative by 0.1 V in the presence of the inhibitor to -0.05 V at pH 7.0 and -0.15 V at pH 8.4. Tetramethylammonium chloride also caused moderate negative shifts (0.03-0.05 V) in the midpoint redox potential for the Fe4S(+2)4/Fe4S(+1)4 couple of trimethylamine dehydrogenase. The midpoint potentials are +0.06 V at pH 7.0 and +0.04 V at pH 8.4. Therefore, in the presence of tetramethylammonium chloride, electron transfer from the flavin radical to the Fe4S(+2)4 group is energetically unfavorable and trimethylamine dehydrogenase is trapped in the flavin radical state. The redox potential changes provide a thermodynamic basis for inhibition by tetramethylammonium chloride. Spectroelectrochemical titrations of trimethylamine dehydrogenase which had been inactivated by phenylhydrazine revealed heterogeneity in the redox behavior which had not been observed in other laboratories. The reason for this heterogeneity was not determined, but the midpoint redox potential for the Fe4S(+2)4/Fe4S(+1)4 couple of the main fraction of the inactivated enzyme was the same as that of active trimethylamine dehydrogenase.

Properties of carboxymethylated cross-linked hemoglobin A

The selective carboxymethylation of the N-terminal amino groups of hemoglobin A with glyoxylic acid and sodium cyanoborohydride has been studied as a function of the state of ligation of hemoglobin. The N-terminal residues have been established as the primary sites of reaction by peptide mapping of the tryptic digest of each chain and subsequent amino acid analysis of the modified peptides. With oxyhemoglobin, the desired derivatives with a carboxymethyl group at the N-terminal of either or both chains amounted to 55% [Di Donato, A., Fantl, W. J., Acharya, A. S., & Manning, J. M. (1983) J. Biol. Chem. 258, 11890-11895]. In the present study it is shown that with deoxyhemoglobin the amount of the desired derivative is increased to 75%. The oxygen equilibrium curve of hemoglobin A carboxymethylated on its four N-terminal residues [0.5 mM as tetramer in 50 mM [bis(2-hydroxyethyl)amino]tris(hydroxymethyl)methane (Bis-Tris), pH 7.5, 37 degrees C] had a P50 value of 30 mmHg (Hill coefficient n = 2.8, alkaline Bohr value = 0.4) compared to a P50 of 9 mmHg for unmodified hemoglobin under the same conditions (n = 2.5, alkaline Bohr value = 0.5). In carboxymethylated oxyhemoglobin A, cross-linked with the mild agent glycolaldehyde for 3.5 h, there was 85% of Mr 64,000 species and 15% of Mr 128,000 or higher species. For the former, the extent of cross-linking between two subunits was 19%. For the latter, there was 29% of two cross-linked subunits and 13% of three cross-linked subunits. Termination of cross-linking, which may be desirable in some circumstances, can be successfully achieved with isonicotinic acid hydrazide.(ABSTRACT TRUNCATED AT 250 WORDS)

Suicide inhibition as a likely cause of variable specific activity in trimethylamine dehydrogenase from bacterium W3A1

Trimethylamine hydrogenase isolated from bacterium W3A1 grown on dimethylamine was of variable, but low specific activity and had modified spectral properties. Chemical analyses for Fe, S and P indicated that the [4Fe-4S] clusters of the modified enzyme are intact and that the covalently bound flavin is probably present, but in modified form. A peptide with absorbance maximum at 358 nm and fluorescence excitation and emission maxima in dimethylformamide at 358 nm and 495 nm, respectively, was isolated by gel chromatography and HPLC of tryptic peptides of acetamidylated, modified trimethylamine dehydrogenase. These spectral properties are similar to those of 4a- or 5a-substituted flavins and suggest that the enzyme had been modified by in vivo reaction with a suicide inhibitor. This inhibitor, or a compound giving rise to it, seems to be present in a commercial source of dimethylamine.

Normal mode analysis of lumiflavin and interpretation of resonance Raman spectra of flavoproteins

The normal modes of lumiflavin (10-methyliso-alloxazine) are analyzed with a valence force field constructed with bond length-stretching force constant correlations and bending and interaction force constants transferred from small ring molecules. Observed resonance Raman (RR) bands of flavin are assigned to calculate modes on the basis of frequency and isotope shift matching. The normal mode patterns confirm previous inferences, based on selective effective of chemical substitutions, of localization to certain regions of the molecule. These results are used to interpret the observed variability of the prominent RR bands among different flavoproteins on the basis of protein-isoalloxazine interactions.

Metal-flavin complexation. A resonance Raman investigation

Several groups have recently shown that high quality resonance Raman spectra can be obtained for flavin species in spite of their intense fluorescence. We are interested in obtaining the resonance Raman spectra of flavins in various chemical environments in order to determine whether the spectra are useful in probing the chemical interaction between flavins and protein in flavoenzymes. We have obtained the resonance Raman spectrum of a nonfluorescent Ag+ complex of FMN. Several large changes occur in the FMN resonance Raman spectrum upon Ag+ complexation; among these are changes in the 1580 cm-1 region of the FMN spectrum (assigned to nu C=N at N-5 and C-4a), the 1410 cm-1 region and the 1260 cm-1 region (associated with a vibration having some delta N-N-H character at N-3). Similar changes are observed in the same region of a Ru2+-FMN complex. Since these spectral changes occur in two metal flavin complexes with very different electronic spectra, they would seem to be due to vibrational changes induced by metal complexation at N-5 and the oxygen at C-4 of flavin rather than the details of the vibronic interactions which give rise to the resonance enhancement of the spectrum. A structure for the Ag+-FMN complex is suggested. This study has potential physiological significance, because it illustrates the possible role of resonance Raman spectroscopy as a tool for the determination of direct flavin metal interaction in dilute aqueous solution of metalloflavoproteins.