Studies on Monooxygenases

Mechanistic insights into the dual activities of the single active site of l-lysine oxidase/monooxygenase from Pseudomonas sp. AIU 813

l-Lysine oxidase/monooxygenase (l-LOX/MOG) from Pseudomonas sp. AIU 813 catalyzes the mixed bioconversion of l-amino acids, particularly l-lysine, yielding an amide and carbon dioxide by an oxidative decarboxylation (i.e. apparent monooxygenation), as well as oxidative deamination (hydrolysis of oxidized product), resulting in α-keto acid, hydrogen peroxide (H₂O₂), and ammonia. Here, using high-resolution MS and monitoring transient reaction kinetics with stopped-flow spectrophotometry, we identified the products from the reactions of l-lysine and l-ornithine, indicating that besides decarboxylating imino acids (i.e. 5-aminopentanamide from l-lysine), l-LOX/MOG also decarboxylates keto acids (5-aminopentanoic acid from l-lysine and 4-aminobutanoic acid from l-ornithine). The reaction of reduced enzyme and oxygen generated an imino acid and H₂O₂, with no detectable C4a-hydroperoxyflavin. Single-turnover reactions in which l-LOX/MOG was first reduced by l-lysine to form imino acid before mixing with various compounds revealed that under anaerobic conditions, only hydrolysis products are present. Similar results were obtained upon H₂O₂ addition after enzyme denaturation. H₂O₂ addition to active l-LOX/MOG resulted in formation of more 5-aminopentanoic acid, but not 5-aminopentamide, suggesting that H₂O₂ generated from l-LOX/MOG in situ can result in decarboxylation of the imino acid, yielding an amide product, and extra H₂O₂ resulted in decarboxylation only of keto acids. Molecular dynamics simulations and detection of charge transfer species suggested that interactions between the substrate and its binding site on l-LOX/MOG are important for imino acid decarboxylation. Structural analysis indicated that the flavoenzyme oxidases catalyzing decarboxylation of an imino acid all share a common plug loop configuration that may facilitate this decarboxylation.

Chaperone-assisted expression and purification of putrescine monooxygenase from Shewanella putrefaciens-95

A putrescine monooxygenase from Shewanella putrefaciens 95 (SpPMO) is the initial enzyme catalyzing the hydroxylation of putrescine to N-hydroxyl putrescine, the precursor for the synthesis of a siderophore putrebactin was identified. This PMO clustered together with known characterized NMOs from Shewanella baltica, Bordetella pertussis, Erwinia amylovora, Streptomyces sp. Gordonia rubripertincta, Pseudomonas aeruginosa and outgrouped from Escherichia coli, Nocardia farcinica, and Rhodococcus erythropolis. The deduced SpPMO protein showed 53% and 36% sequence identity with other characterized bacterial NMOs from Erwinia amylovora and Gordonia rubripertincta respectively. In this investigation, we have cloned the complete 1518bp coding sequence of pubA from Shewanella putrefaciens 95 encoding the corresponding protein SpPMO. It comprises 505 amino acid residues in length and has approximately a molecular weight of 54 kDa. Chaperone-assisted heterologous expression of SpPMO in pET151Topo expression vector under the control of bacteriophage T7 promoter permitted a stringent IPTG dependent expression. It has been successfully cloned, overexpressed and purified as a soluble His₆ -tagged enzyme using E. coli as a cloning and expression host. The expression of recombinant SpPMO was confirmed by Western blotting using anti-His₆ antibody. The purified protein showed FAD and NADPH dependent N-hydroxylation activity. This study has paved a way to understand the hydroxylation step of putrebactin synthesis which can be further investigated by studying its kinetic mechanism and physiological role.

Mutational and crystallographic analysis of l-amino acid oxidase/monooxygenase from Pseudomonas sp. AIU 813: Interconversion between oxidase and monooxygenase activities

In this study, it was shown for the first time that l-amino acid oxidase of Pseudomonas sp. AIU813, renamed as l-amino acid oxidase/monooxygenase (l-AAO/MOG), exhibits l-lysine 2-monooxygenase as well as oxidase activity. l-Lysine oxidase activity of l-AAO/MOG was increased in a p-chloromercuribenzoate (p-CMB) concentration-dependent manner to a final level that was five fold higher than that of the non-treated enzyme. In order to explain the effects of modification by the sulfhydryl reagent, saturation mutagenesis studies were carried out on five cysteine residues, and we succeeded in identifying l-AAO/MOG C254I mutant enzyme, which showed five-times higher specific activity of oxidase activity than that of wild type. The monooxygenase activity shown by the C254I variant was decreased significantly. Moreover, we also determined a high-resolution three-dimensional structure of l-AAO/MOG to provide a structural basis for its biochemical characteristics. The key residue for the activity conversion of l-AAO/MOG, Cys-254, is located near the aromatic cage (Trp-418, Phe-473, and Trp-516). Although the location of Cys-254 indicates that it is not directly involved in the substrate binding, the chemical modification by p-CMB or C254I mutation would have a significant impact on the substrate binding via the side chain of Trp-516. It is suggested that a slight difference of the binding position of a substrate can dictate the activity of this type of enzyme as oxidase or monooxygenase.

Identification of Tyr413 as an active site residue in the flavoprotein tryptophan 2-monooxygenase and analysis of its contribution to catalysis

The flavoenzyme tryptophan 2-monooxygenase catalyzes the oxidation of tryptophan to indoleacetamide, carbon dioxide, and water. The enzyme is a homologue of l-amino acid oxidase. In the structure of l-amino acid oxidase complexed with aminobenzoate, Tyr372 hydrogen bonds with the carboxylate of the inhibitor in the active site. All 10 conserved tyrosine residues in tryptophan 2-monooxygenase were mutated to phenylalanine; steady state kinetic characterization of the purified proteins identified Tyr413 as the residue homologous to Tyr372 of l-amino acid oxidase. Y413F and Y413A tryptophan 2-monooxygenase were characterized more completely with tryptophan as the substrate to probe the contribution of this residue to catalysis. Mutation of Tyr413 to phenylalanine results in a decrease in the value of the first-order rate constant for reduction of 35-fold and a decrease in the rate constant for oxidation of 11-fold. Mutation to alanine decreases the rate constant for reduction by 200-fold and that for oxidation by 33-fold. Both mutations increase the K(d) value for tryptophan and the K(i) values for the competitive inhibitors indoleacetamide and indole pyruvate by 5-10-fold. Both mutations convert the enzyme to an oxidase, in that the products of the catalytic reactions of both are indolepyruvate and hydrogen peroxide. The V/K(trp)-pH profiles for the Tyr413 mutant enzymes no longer show the pK(a) value of 9.9 seen in that for the wild-type enzyme, allowing identification of Tyr413 as the active site residue in the wild-type enzyme which must be protonated for catalysis. Substitution of Tyr413 abolishes the formation of the long wavelength charge transfer species observed in the wild-type enzyme. The data are consistent with the main role of Tyr413 being to maintain the correct orientation of tryptophan for effective hydride transfer and imino acid decarboxylation.

Characterization of 2-oxo-3-pentynoate as an active-site-directed inactivator of flavoprotein oxidases: identification of active-site peptides in tryptophan 2-monooxygenase

2-oxo-3-pentynoate has been characterized as an active-site-directed inhibitor of selected flavoprotein oxidases. Tryptophan 2-monooxygenase is irreversibly inactivated in an active-site-directed fashion. The addition of FAD affords no protection from inactivation, whereas the competitive inhibitor indole-3-acetamide fully protects the enzyme from inactivation. The inactivation follows first-order kinetics for at least five half-lives. The rate of inactivation shows saturation kinetics, consistent with the formation of a reversible complex between the alkylating agent and the enzyme before inactivation occurs. Values of 0.017 +/- 0.0005 min-1 and 44 +/- 7 microM were determined for the limiting rate of inactivation and the apparent dissociation constant for 2-oxo-3-pentynoate, respectively. Tryptic maps of tryptophan 2-monooxygenase treated with 2-oxo-3-pentynoate show that two peptides are alkylated in the absence of indole-3-acetamide but not in its presence. The two peptides were identified by mass spectrometry as residues 333-349 and 503-536. Based upon sequence analysis, cysteine 511 and either cysteine 339 or histidine 338 are the likely sites of modification. In contrast, incubation of D-amino acid oxidase or nitroalkane oxidase with 2-oxo-3-pentynoate results in a loss of 55% or 100%, respectively, of the initial activity. In neither case does a competitive inhibitor affect the rate of inactivation, suggesting that the effect is not due to modification of active-site residues.

Characterization of an FMN-containing cyclohexanone monooxygenase from a cyclohexane-grown Xanthobacter sp

A soluble cyclohexanone monooxygenase was purified 16.1-fold to homogeneity from a Xanthobacter sp. grown upon cyclohexane as sole source of carbon and energy. The native enzyme is a 50-kDa single polypeptide chain associated with FMN rather than FAD as flavin prosthetic group in a 1:1 stoichiometric relationship. The monooxygenase catalyses the transformation of cyclohexanone to the lactone 1-oxa-2-oxocycloheptane in an oxygen ring insertion reaction. Only related cycloalkanone substrates are accepted for oxygenation, no activity is shown towards straight-chain alkanones. Enzyme activity is strongly inhibited by sulphydryl-reactive agents, but is relatively insensitive to metal chelators, electron transport inhibitors and the metal ions Fe3+ and Cu2+. Cyclohexanone monooxygenase has Km values for cyclohexanone and NADPH of less than 0.5 microM and 12.5 microM respectively. Kinetic investigations under steady-state conditions demonstrate that the flavoprotein prosthetic group, FMN, is involved in the monooxygenase catalytic mechanism. The systematic name for the enzyme is cyclohexanone, NADPH:oxygen oxidoreductase (6-hydroxylating, 1,2-lactonizing) (EC 1.14.13.22).

Oxidation of anionic nitroalkanes by flavoenzymes, and participation of superoxide anion in the catalysis

The reactivities of anionic nitroalkanes with 2-nitropropane dioxygenase of Hansenula mrakii, glucose oxidase of Aspergillus niger, and mammalian D-amino acid oxidase have been compared kinetically. 2-Nitropropane dioxygenase is 1200 and 4800 times more active with anionic 2-nitropropane than D-amino acid oxidase and glucose oxidase, respectively. The apparent Km values for anionic 2-nitropropane are as follows: 2-nitropropane dioxygenase, 1.61 mM; glucose oxidase, 16.7 mM; and D-amino acid oxidase, 11.1 mM. Anionic 2-nitropropane undergoes an oxygenase reaction with 2-nitropropane dioxygenase and glucose oxidase, and an oxidase reaction with D-amino acid oxidase. In contrast, anionic nitroethane is oxidized through an oxygenase reaction by 2-nitropropane dioxygenase, and through an oxidase reaction by glucose oxidase. All nitroalkane oxidations by these three flavoenzymes are inhibited by Cu and Zn-superoxide dismutase of bovine blood, Mn-superoxide dismutases of bacilli, Fe-superoxide dismutase of Serratia marcescens, and other O2-. scavengers such as cytochrome c and NADH, but are not affected by hydroxyl radical scavengers such as mannitol. None of the O2-. scavengers tested affected the inherent substrate oxidation by glucose oxidase and D-amino acid oxidase. Furthermore, the generation of O2-. in the oxidation of anionic 2-nitropropane by 2-nitropropane dioxygenase was revealed by ESR spectroscopy. The ESR spectrum of anionic 2-nitropropane plus 2-nitropropane dioxygenase shows signals at g1 = 2.007 and g11 = 2.051, which are characteristic of O2-.. The O2-. generated is a catalytically essential intermediate in the oxidation of anionic nitroalkanes by the enzymes.

The purification and properties of cyclohexanone oxygenase from Nocardia globerula CL1 and Acinetobacter NCIB 9871

1. Cyclohexanone oxygenases from Norcardia globerula CL1 and Acinetobacter NCIB 9871 have been purified 12-fold and 35-fold respectively and each gives a single symmetrical sedimentation peak in the ultracentrifuge and a single protein band on 2.25 nm average pore radius polyacrylamide gels. 2. The enzyme from N. globerula has a molecular weight of 53000 while that from Acinetobacter has a molecular weight of about 59000. Each is a single polypeptide chain with one mole of bound FAD per mole of protein that does not dissociate during purification. Acidification of the Acinetobacter enzyme in the presence of (NH4)2SO4 releases the bound FAD and yields native apoenzyme from which the active holoenzyme can be reconstituted. The apparent dissociation constant for the FAD is 40 nM.

Purification and properties of cyclopentanone oxygenase of Pseudomonas NCIB 9872

1. Cyclopentanone oxygenase from Pseudomonas NCIB 9872 has been purified some 40-fold. It gives a single peak in the ultracentrifuge and a single major protein band on polyacrylamide gels contaminated with about 5% of a slower migrating impurity. Flavin dissociates from the protein during electrophoresis. 2. The enzyme has a molecular weight of about 200000 and is a homopolymeric assemblage of either three of four subunits of molecular weight 54000-58000. 3. The prosthetic group is FAD and values of about 2.5 are typically obtained for the number of moles bound to each mole of holoenzyme. Some FAD probably dissociates during purification and it seems likely that each subunit binds one FAD in the undamaged protein.