Interactions of substrates with a purified 4-methoxybenzoate monooxygenase system (O-demethylating) from Pseudomonas putida

Crystal structure of dicamba monooxygenase: a Rieske nonheme oxygenase that catalyzes oxidative demethylation

Dicamba (3,6-dichloro-2-methoxybenzoic acid) is a widely used herbicide that is efficiently degraded by soil microbes. These microbes use a novel Rieske nonheme oxygenase, dicamba monooxygenase (DMO), to catalyze the oxidative demethylation of dicamba to 3,6-dichlorosalicylic acid (DCSA) and formaldehyde. We have determined the crystal structures of DMO in the free state, bound to its substrate dicamba, and bound to the product DCSA at 2.10-1.75 A resolution. The structures show that the DMO active site uses a combination of extensive hydrogen bonding and steric interactions to correctly orient chlorinated, ortho-substituted benzoic-acid-like substrates for catalysis. Unlike other Rieske aromatic oxygenases, DMO oxygenates the exocyclic methyl group, rather than the aromatic ring, of its substrate. This first crystal structure of a Rieske demethylase shows that the Rieske oxygenase structural scaffold can be co-opted to perform varied types of reactions on xenobiotic substrates.

Single turnover chemistry and regulation of O2 activation by the oxygenase component of naphthalene 1,2-dioxygenase

Naphthalene 1,2-dioxygenase (NDOS) is a three-component enzyme that catalyzes cis-(1R,2S)-dihydroxy-1,2-dihydronaphthalene formation from naphthalene, O2, and NADH. We have determined the conditions for a single turnover of NDOS for the first time and studied the regulation of catalysis. As isolated, the alpha3beta3 oxygenase component (NDO) has up to three catalytic pairs of metal centers (one mononuclear Fe2+ and one diferric Rieske iron-sulfur cluster). This form of NDO is unreactive with O2. However, upon reduction of the Rieske cluster and exposure to naphthalene and O2, approximately 0.85 cis-diol product per occupied mononuclear iron site rapidly forms. Substrate binding is required for oxygen reactivity. Stopped-flow and chemical quench analyses indicate that the rate constant of the single turnover product-forming reaction significantly exceeds the NDOS turnover number. UV-visible and electron paramagnetic resonance spectroscopies show that during catalysis, one mononuclear iron and one Rieske cluster are oxidized per product formed, satisfying the two-electron reaction stoichiometry. The addition of oxidized or reduced NDOS ferredoxin component (NDF) increases both the product yield and rate of oxidation of formerly unreactive Rieske clusters. The results show that NDO alone catalyzes dioxygenase chemistry, whereas NDF appears to serve only an electron transport role, in this case redistributing electrons to competent active sites.

Substrate and solvent isotope effects on the fate of the active oxygen species in substrate-modulated reactions of putidamonooxin

Using 4-methoxybenzoate monooxygenase from Pseudomonas putida, the substrate deuterium isotope effect on product formation and the solvent isotope effect on the stoichiometry of oxygen uptake, NADH oxidation, product and/or H2O2 (D2O2) formation for tight couplers, partial uncouplers, and uncouplers as substrates were measured. These studies revealed for the true, intrinsic substrate deuterium isotope effect on the oxygenation reaction a k1H/k2H ratio of < 2.0, derived from the inter- and intramolecular substrate isotope effects. This value favours a concerted oxygenation mechanism of the substrate. Deuterium substitution in a tightly coupling substrate initiated a partial uncoupling of oxygen reduction and substrate oxygenation, with release of H2O2 corresponding to 20% of the overall oxygen uptake. This H2O2 (D2O2) formation (oxidase reaction) almost completely disappeared when the oxygenase function was increased by deuterium substitution in the solvent. The electron transfer from NADH to oxygen, however, was not affected by deuterium substitution in the substrate and/or the solvent. With 4-trifluoromethylbenzoate as uncoupling substrate and D2O as solvent, a reduction (peroxidase reaction) of the active oxygen complex was initiated in consequence of its extended lifetime. These additional two electron-transfer reactions to the active oxygen complex were accompanied by a decrease of both NADH oxidation and oxygen uptake rates. These findings lead to the following conclusions: (a) under tightly coupling conditions the rate-limiting step must be the formation time and lifetime of an active transient intermediate within the ternary complex iron/peroxo/substrate, rather than an oxygenative attack on a suitable C-H bond or electron transfer from NADH to oxygen. Water is released after the monooxygenation reaction; (b) under uncoupling conditions there is competition in the detoxification of the active oxygen complex between its protonation (deuteronation), with formation of H2O2 (D2O2) and its further reduction to water. The additional two electron-transfer reactions onto the active oxygen complex then become rate limiting for the oxygen uptake rate.

The oxygenase component of the 2-aminobenzenesulfonate dioxygenase system from Alcaligenes sp. strain O-1

Growth of Alcaligenes sp. strain O-1 with 2-aminobenzenesulfonate (ABS; orthanilate) as sole source of carbon and energy requires expression of the soluble, multicomponent 2-aminobenzenesulfonate 2,3-dioxygenase system (deaminating) (ABSDOS) which is plasmid-encoded. ABSDOS was separated by anion-exchange chromatography to yield a flavin-dependent reductase component and an iron-dependent oxygenase component. The oxygenase component was purified to about 98% homogeneity and an alpha2beta2 subunit structure was deduced from the molecular masses of 134,45 and 16 kDa for the native complex, and the alpha and beta subunits, respectively. Analysis of the amount of acid labile sulfur and total iron, and the UV spectrum of the purified oxygenase component indicated one [2Fe-2S] Rieske centre per alpha subunit. The inhibition of activity by the iron-specific chelator o-phenanthroline indicated the presence of an additional iron-binding site. Recovery of active protein required strictly anoxic conditions during all purification steps. The FAD-containing reductase could not be purified. ABSDOS oxygenated nine sulfonated compounds; no oxygen uptake was detected with carboxylated aromatic compounds or with aliphatic sulfonated compounds. Km values of 29, 18 and 108 microM and Vmax values of 140, 110 and 72 pkat for ABS, benzenesulfonate and 4-toluenesulfonate, respectively, were observed. The N-terminal amino acid sequences of the alpha- and beta-subunits of the oxygenase component allowed PCR primers to be deduced and the DNA sequence of the alpha-subunit was thereafter determined. Both redox centres were detected in the deduced amino acid sequence. Sequence data and biochemical properties of the enzyme system indicate a novel member of the class IB ring-hydroxylating dioxygenases.

2-Oxo-1,2-dihydroquinoline 8-monooxygenase, a two-component enzyme system from Pseudomonas putida 86

2-Oxo-1,2-dihydroquinoline 8-monooxygenase, which catalyzes the NADH-dependent oxygenation of 2-oxo-1,2-dihydroquinoline to 8-hydroxy-2-oxo-1,2-dihydroquinoline, is the second enzyme in the quinoline degradation pathway of Pseudomonas putida 86. This enzyme system consists of two inducible protein components, which were purified, characterized, and identified as reductase and oxygenase. The yellow reductase is a monomeric iron-sulfur flavoprotein (M(r), 38,000), containing flavin adenine dinucleotide and plant-type ferredoxin [2Fe-2S]. It transferred electrons from NADH to the oxygenase or to some artificial electron acceptors. The red-brown oxygenase (M(r), 330,000) consists of six identical subunits (M(r), 55,000) and was identified as an iron-sulfur protein, possessing about six Rieske-type [2Fe-2S] clusters and additional iron. It was reduced by NADH plus catalytic amounts of reductase. For monooxygenase activity, reductase, oxygenase, NADH, molecular oxygen, and substrate were required. The activity was considerably enhanced by the addition of polyethylene glycol and Fe2+. 2-Oxo-1,2-dihydroquinoline 8-monooxygenase revealed a high substrate specificity toward 2-oxo-1,2-dihydroquinoline, since none of 25 other tested compounds was converted. Based on its physical, chemical, and catalytic properties, we presume 2-oxo-1,2-dihydroquinoline 8-monooxygenase to belong to the class IB multicomponent non-heme iron oxygenases.

Terephthalate 1,2-dioxygenase system from Comamonas testosteroni T-2: purification and some properties of the oxygenase component

Comamonas testosteroni T-2, grown in terephthalate (TER)-salts medium, synthesizes inducible enzymes that convert TER to (1R,2S)-dihydroxy-3,5-cyclohexadiene-1,4-dicarboxylic acid (DCD) and protocatechuate (PC). Anion-exchange chromatography of cell extracts yielded two sets of fractions, R and Z, that were necessary for oxygenation of TER to DCD; we termed this activity the TER dioxygenase system (TERDOS). An NAD(+)-dependent DCD dehydrogenase, which converted DCD to PC, overlapped all fractions R. No significant purification from fraction R, which contained an NADH-dependent reductase function(s) of TERDOS, was attained. Fraction Z, at the end of the gradient, contained essentially one protein, which was further purified by hydrophobic interaction chromatography. This component, Z, had the UV-visible spectrum and electron paramagnetic resonance characteristics of a Rieske [2Fe-2S] protein and was considered to be the oxygenase. M(r)s of about 126,000 for oxygenase Z under native conditions were observed. Oxygenase Z consisted of two subunits, alpha and beta, with M(r)s of 49,000 and 18,000, respectively, under denaturing conditions. We presume that this oxygenase has an alpha 2 beta 2 structure. The sequences of the N-terminal amino acids of each subunit were determined. The activity of the purified enzyme was enhanced about fivefold by addition of Fe2+. In the presence of O2, NADH, and fraction R, component Z catalyzed the stoichiometric transformation of TER to PC, with the intermediate formation of DCD. The reaction was confirmed as a dioxygenation when we observed incorporation of two oxygen atoms from 18O2 into PC. The substrate range of TERDOS appeared to be narrow; apart from TER, only 2,5-dicarboxypyridine and 1,4-dicarboxynaphthalene (of 11 compounds tested) were converted to a product.

Oxygenation and spontaneous deamination of 2-aminobenzenesulphonic acid in Alcaligenes sp. strain O-1 with subsequent meta ring cleavage and spontaneous desulphonation to 2-hydroxymuconic acid

2-Aminobenzenesulphonic acid (2AS) is degraded by Alcaligenes sp. strain O-1 via a previously detected but unidentified intermediate. A mutant of strain O-1 was found to excrete this intermediate, which was isolated and identified by m.s., 1H- and 13C-n.m.r. as 3-sulphocatechol (3SC). Proteins from cell extracts of strain O-1 were separated by anion-exchange chromatography. A multicomponent oxygenase was observed to convert 1 mol each of NADH, O2 and 2AS into 1 mol each of 3SC, NH3 and NAD+. The enzyme presumably catalysed formation of the ring of a 2-amino-2,3-diol moiety, and elimination in the amino group led to a rearomatization. 3SC was further degraded via meta ring cleavage, which could be prevented by inactivation of the 3-sulphocatechol-2,3-dioxygenase (3SC23O) with 3-chlorocatechol. In Tris buffer, the separated 3SC23O catalysed the reaction of 1 mol each of 3SC and O2 involving a transient yellow intermediate, and release of 1 mol of sulphite and two organic products. The major product was identified by n.m.r. and by g.c./m.s. as 5-carboxypenta-2,4-dien-5-olide (CPDO), an indicator of formation of 2-hydroxymuconic acid (2HM). The second product was identified as the Z,E isomer of 2HM by comparison with authentic material. When the CPDO in the product mixture was chemically hydrolysed to (Z,E)-2HM, 1 mol of (Z,E)-2HM/mol of 3SC was observed. If oxygenation of 3SC by 3SC23O was carried out in phosphate buffer, only a single product was detected, a keto form of 2HM. This dioate was also formed from authentic (Z,E)-2HM in phosphate buffer. Formation of the natural product (Z,E)-2HM from the xenobiotic, 3SC, seems to involve oxygenation to the unstable 2-hydroxy-6-sulphonomuconic acid semialdehyde, which hydrolyses spontaneously to 2HM. There would appear to be at least one spontaneous reaction per enzyme reaction in this pathway.

Effects of oxygen concentration on the metabolism of anisole homologues by rat liver microsomes

1. The effects of oxygen concentration were studied on the metabolic pathways of anisole homologues (anisole, phenetole and isopropoxybenzene) catalysed by liver microsomes from phenobarbital-treated rats. 2. With increase of oxygen concentration, the rate of anisole o-hydroxylation reached a plateau at about 35 microM O2, while the rates of O-demethylation and aromatic p-hydroxylation were still increasing at 223 microM O2 (air). 3. The rates of all three metabolic reactions of phenetole reached plateau levels at about 80 microM O2. 4. The rates of all three metabolic reactions of iso-propoxybenzene were still increasing as 223 microM O2 (air). 5. The ratio of aromatic p-hydroxylation or O-dealkylation to aromatic o-hydroxylation decreased in anisole metabolism, and showed no uniform change in phenetole and isopropoxybenzene metabolism with decreasing oxygen concentration. 6. The ratio of aromatic p-hydroxylation to O-dealkylation was essentially constant over the range of oxygen concentration studied in anisole and phenetole metabolism, while in iso-propoxybenzene metabolism the ratio was different between higher and lower oxygen concentrations than 60 microM. 7. This series of compounds with increasing chain length did not show homologous changes in rates of product formation or O2 dependent of product formation.

Purification and some properties of 2-halobenzoate 1,2-dioxygenase, a two-component enzyme system from Pseudomonas cepacia 2CBS

The two components of the inducible 2-halobenzoate 1,2-dioxygenase from Pseudomonas cepacia 2CBS were purified to homogeneity. Yellow component B is a monomer (Mr, 37,500) with NADH-acceptor reductase activity. Ferricyanide, 2,6-dichlorophenol indophenol, and cytochrome c acted as electron acceptors. Component B was identified as an iron-sulfur flavoprotein containing 0.8 mol of flavin adenine dinucleotide, 1.7 mol of iron, and 1.7 mol of acid-labile sulfide per mol of enzyme. The isoelectric point was estimated to be pH 4.2. Component B was reduced by the addition of NADH. Red-brown component A (Mr, 200,000 to 220,000) is an iron-sulfur protein containing 5.8 mol of iron and 6.0 mol of acid-labile sulfide. The isoelectric point was within the range of pH 4.5 to 5.3. Component A could be reduced by dithionite or by NADH plus catalytic amounts of component B. Component A consisted of nonidentical subunits alpha (Mr, 52,000) and beta (Mr, 20,000). It contained approximately equimolar amounts of alpha and beta, and cross-linking studies suggested an alpha 3 beta 3 subunit structure of component A. The NADH- and Fe(2+)-dependent enzyme system was named 2-halobenzoate 1,2-dioxygenase, because it catalyzes the conversion of 2-fluoro-, 2-bromo-, 2-chloro-, and 2-iodobenzoate to catechol. 2-Halobenzoate 1,2-dioxygenase exhibited a very broad substrate specificity, but benzoate analogs with electron-withdrawing substituents at the ortho position were transformed preferentially.

4-Toluene sulfonate methyl-monooxygenase from Comamonas testosteroni T-2: purification and some properties of the oxygenase component

Comamonas testosteroni T-2 synthesizes an inducible enzyme system that oxygenates 4-toluene sulfontate (TS) to 4-sulfobenzyl alcohol when grown in TS-salts medium. We purified this TS methyl-monooxygenase system (TSMOS) and found it to consist of two components. A monomeric, iron-sulfur flavoprotein (component B), which has been shown to act as a reductase in the 4-sulfobenzoate dioxygenase system of this organism (H. H. Locher, T. Leisinger, and A. M. Cook, Biochem. J. 274:833-842, 1991), carried electrons from NADH to component M, an oxygenase. This oxygenase had the UV-visible spectral characteristics of an iron-sulfur protein. Mrs of about 152,000 for the native oxygenase and of 43,000 under denaturing conditions indicated a homotri- or homotetrameric enzyme, whose N-terminal amino acids and amino acid composition were determined. The activity of the purified enzyme was enhanced about fivefold by the addition of Fe2+. In the presence of O2 and NADH, components B and M together catalyzed the stoichiometric transformation of TS or p-toluate to the corresponding alcohol. The reaction was confirmed as oxygenation of the methyl group by observation of an oxygen atom from 18O2 in carboxybenzyl alcohol. The substrate range of TSMOS included carboxylated analogs of TS (p- and m-toluates and 4-ethylbenzoate), whereas p-xylene, toluene, and p-cresol were not substrates. TSMOS also catalyzed demethylation; 4-methoxybenzoate was transformed to 4-hydroxybenzoate and formaldehyde.

Oxidative metabolism of 14C-pyridine by human and rat tissue subcellular fractions

1. The oxidative metabolism of 14C-pyridine by human and rat microsomal fractions has been studied. Metabolites were separated by h.p.l.c. employing continuous radioactivity monitoring of the column effluent. 2. Human liver microsomal fractions incubated with an NADPH-generating system and oxygen formed pyridine N-oxide (PNO) at an average rate of 275 pmol/min per mg protein, 2-pyridone (2PO) at 207 pmol/min per mg and 4-pyridone (4PO) at 154 pmol/min per mg. One human subject formed 3-hydroxypyridine N-oxide in addition to the other metabolites. 3. Human kidney microsomal fractions formed PNO, 2PO and 4PO at rates similar to those with human liver microsomal fractions, whereas human lung microsomal fractions formed the metabolites at less than half the rate. 4. Metabolism of pyridine by human liver microsomal fractions was inhibited 54% by nitrogen, 34% by 80% carbon monoxide-20% oxygen, and 20% by metyrapone. 2-Diethylaminoethyl-2,2-diphenylvalerate HCl (SKF-525A) did not inhibit pyridine metabolism. 5. Liver microsomal fractions from non-induced rats metabolized pyridine to PNO at a rate of 19 pmol/min per mg protein, 2PO at 17 pmol/min per mg and 4PO at 61 pmol/min per mg. 6. There was no pyridine metabolism by human or rat tissue cytosolic fractions incubated under the same conditions.

Substrate-modulated reactions of putidamonooxin. The nature of the active oxygen species formed and its reaction mechanism

1. 4-Methoxybenzoate monooxygenase is fairly nonspecific. The enzyme system with putidamonooxin as its oxygen-activating component catalyses: (a) O-, S- and N-demethylation; (b) the oxygenation of 4-methylbenzoate and 4-methylmercaptobenzoate, with formation of 4-carboxybenzyl alcohol and 4-carboxyphenylmethyl sulfoxide, respectively, and (c) attack of the aromatic ring of 4- and 3-hydroxybenzoate and 4-aminobenzoate, yielding 3,4-dihydroxybenzoate and 4-amino-3-hydroxybenzoate, respectively. 2. Compounds which are bound by the active sites of putidamonooxin have two essential features in common: a planar aromatic ring system, and a free carboxyl group attached to it. 3. By a substrate-modulated reaction putidamonooxin can be induced to function not only as a monooxygenase but also as a peroxotransferase, i.e. it incorporates both atoms of the activated oxygen molecule into a substrate molecule. This finding supports the hypothesis that a mesomeric state of the iron.peroxo complex, [FeO2]+, is indeed the active oxygenating species of putidamonooxin. 4. The lifetime of the ternary complex consisting of enzyme.iron-peroxo-complex.substrate is significantly prolonged by uncoupling and partially uncoupling substrates, except when it is inactivated by protonation of the iron.peroxo complex by a proton transported into the active sites by a special kind of substrate (i.e. isomers of monoaminobenzoate), with the direct formation of H2O2. 5. The lifetime of the active oxygen species is determined by (a) the rate of the oxygenation reaction in the presence of tight-coupling substrates and (b) the rate of the oxygenation reaction as well as detoxification by the availability of a dissociable proton in the presence of partial uncoupling (and uncoupling) substrate analogues. 6. The rate of the oxygenation reaction depends on the lifetime of the active oxygen species, [FeO2]+, in the presence of partial uncoupling substrates. 7. The iron.peroxo complex attacks an aromatic ring system according to the empiric rules of electrophilic substitution, whereas the attack of aliphatic substituents at the aromatic ring is controlled by steric criteria.

Cloning and sequencing of Pseudomonas genes encoding vanillate demethylase

A 2,598-base-pair (bp) SalI-HincII DNA fragment has been cloned which codes for vanillate demethylase, the enzyme responsible for the demethylation of vanillate (3-methoxy-4-hydroxybenzoate) to protocatechuate (3,4-dihydroxybenzoate). Complementation and insertional inactivation experiments have shown that this fragment carries two genes (vanA and vanB) which are predominantly cotranscribed from a promoter upstream of vanA. Nucleotide sequencing of the SalI-HincII fragment confirmed the genetic data: two open reading frames of 987 and 942 bp were present in the transcribed orientation. These had a very high G + C content in the third base of each codon, which is characteristic of Pseudomonas chromosomal genes. Expression of the genes in Escherichia coli with the T7 RNA polymerase-promoter system gave rise to two polypeptides of 36 and 33 kilodaltons which could be identified by deletion analysis as the products of vanA and vanB, respectively. A search of the protein sequence data bank indicated that the vanB gene product was related to the ferredoxin family.

Protein-protein interactions and antigenic relationships between the components of 4-methoxybenzoate monooxygenase and of benzene 1,2-dioxygenase from Pseudomonas putida

The investigations presented in this paper were performed on two enzyme systems from Pseudomonas putida: (a) 4-methoxybenzoate monooxygenase, consisting of a NADH: putidamonooxin oxidoreductase and putidamonooxin, the oxygen-activating component, and (b) benzene 1,2-dioxygenase, a three-component enzyme system with an NADH: ferredoxin oxidoreductase, functioning together with a plant-type ferredoxin as electron-transport chain, and an oxygen-activating component similar to putidamonooxin in its active sites. The influence of temperature, ionic strength, and pH on the activities of 4-methoxybenzoate monooxygenase and of NADH: putidamonooxin oxidoreductase were investigated. The studies revealed that the activity of 4-methoxybenzoate monooxygenase is determined by the behaviour of the reductase. Spectroscopic measurements showed that the interaction between the two components of 4-methoxybenzoate monooxygenase influences the optical-absorption behaviour of one or both components. As a criterion for the affinity between the two components of 4-methoxybenzoate monooxygenase, the Km value of the reductase for putidamonooxin was determined and found to be 31 +/- 11 microM. Antibodies against both components of 4-methoxybenzoate monooxygenase were obtained from rabbits. The antibodies against putidamonooxin inhibited the O-demethylation reaction (up to 80%) and also the reduction of putidamonooxin by the reductase (up to 40%). The antibodies against putidamonooxin did not interact with the oxygen-activating component of benzene 1,2-dioxygenase. The electron-transport chains of 4-methoxybenzoate monooxygenase and benzene 1,2-dioxygenase could not be replaced by one another without a complete loss of enzyme activity.

Enzymatic release of halogens or methanol from some substituted protocatechuic acids

Four strains of gram-negative bacteria capable of growing at the expense of 5-chlorovanillate were isolated from soil, and the metabolism of one strain was studied in particular detail. In the presence of alpha, alpha'-bipyridyl, a suspension of 5-chlorovanillate-grown cells accumulated 5-chloroprotocatechuate from 5-chlorovanillate; in the absence of inhibitor these compounds, and various other 5-substituted protocatechuates and vanillates, were oxidized to completion. Cell suspensions of this strain grown on 5-chlorovanillate or vanillate released chloride quantitatively from 5-chlorovanillate and released methanol from syringate. Extracts of cells grown with 4-hydroxybenzoate, vanillate, or syringate possessed high levels of both protocatechuate 4,5-dioxygenase and 2-pyrone-4,6-dicarboxylate hydrolase; extracts from acetate-grown cells did not. Protocatechuate 4,5-dioxygenase, purified from strains that could grow with 5-chlorovanillate, oxidized 5-halogeno-protocatechuates and 3-O-methylgallate with the formation of 2-pyrone-4,6-dicarboxylate. A crude extract converted 5-chloroprotocatechuate into pyruvate plus oxaloacetate. On the basis of these observations, a meta-fission reaction sequence is proposed for the bacterial degradation of vanillate and protocatechuate substituted at C-5 of the benzene ring with halogen or methoxyl.

Dioxygen-activating iron center in putidamonooxin. Electron spin resonance investigation of the nitrosylated putidamonooxin

The mononuclear non-haem iron center is the dioxygen-binding site of putidamonooxin which is the dioxygen-activating component of the 4-methoxybenzoate monooxygenase. Replacement of dioxygen by nitrosyl leads to the formation of a rather stable Fe3+ X NO- complex which is characterized by electron spin resonance (ESR) at g approximately equal to 4 and g approximately equal to 2. The ESR features can be composed by two spectral components which are characterized by different tetragonal distortions of the axial symmetry. Binding of 4-hydroxybenzoate, which is the product of the enzymatic reaction, leads to the formation of an ESR spectrum with pure axial symmetry. After binding of 4-methoxybenzoate, i.e. the physiological substrate of the monooxygenase, only one spectral component, i.e. that with a small tetragonal distortion, is observed. Binding of substrate analogues, like 4-aminobenzoate and 4-trifluoromethylbenzoate, leads to a spectral heterogeneity with variable amounts of the ESR component with a large tetragonal distortion. Benzoate induces an ESR spectrum with only that spectral component with large tetragonal distortion. The iron-depleted substrate-free form of the enzyme, ligated with NO, also shows ESR heterogeneity, i.e. both spectral components overlap, with 60% of the component with large tetragonal distortion. Binding of 4-methoxybenzoate leads to the occurrence of a pure spectrum, i.e. with small tetragonal distortion, whereas binding of benzoate leads to a pure spectrum with large tetragonal distortion. Thus, the structural heterogeneity is removed by binding of both the ligand (NO) and substrate. The Fe3+ X NO- complex is discussed as an analogue of the native oxy complex Fe3+ X O2-.

Bacterial O-Methylation of Chloroguaiacols: Effect of Substrate Concentration, Cell Density, and Growth Conditions

O-methylation of chloroguaiacols has been examined in a number of gram-positive and gram-negative bacteria to elucidate the effects of substrate concentration, growth conditions, and cell density. Substrate concentrations between 0.1 and 20.0 mg liter −1 were used, and it was found that (i) yields of the O-methylated products were significantly higher at the lowest concentrations and (ii) rates of O-methylation were not linear functions of concentration. With 3,4,5-trichloroguaiacol, the nature of the metabolites also changed with concentration. During growth with a range of substrates, O-methylation of chloroguaiacols also took place. With vanillate, however, de-O-methylation occurred: the chlorocatechol formed from 4,5,6-trichloroguaiacol was successively O-methylated to 3,4,5-trichloroguaiacol and 3,4,5-trichloroveratrole, whereas that produced from 4,5-dichloroguaiacol was degraded without O-methylation. Effective O-methylation in nonproliferating suspensions occurred at cell densities as low as 10 5 cells ml −1 , although both the yields and the rates were lower than in more dense cultures. By using disk assays, it was shown that, compared with their precursors, all of the O-methylated metabolites were virtually nontoxic to the strains examined. It is therefore proposed that O-methylation functions as a detoxification mechanism for cells exposed to chloroguaiacols and chlorophenols. In detail, significant differences were observed in the response of gram-positive and gram-negative cell strains to chloroguaiacols. It is concluded that bacterial O-methylation is to be expected in the natural environment subjected to discharge of chloroguaiacols.

The steroid-9 alpha-hydroxylation system from Nocardia species

The steroid 9 alpha-hydroxylase from Nocardia species M117 was found to be an electron-transport chain consisting of an NADH-dependent flavoprotein reductase and two iron-sulfur proteins named protein II and protein III. The components were partially purified. The flavoprotein reductase from Nocardia species M117 was enriched 20-fold to 100 units/mg and protein III 200-fold to 2400 units/mg protein. Protein II has a molecular weight of approximately 214 000. The purification factor of protein II was not determined. The absorption spectrum of protein II shows a maximum at 425 nm in the oxidized form and maxima at 510 nm, 415 nm and 370 nm in the reduced state; whereas protein III has a prominent maximum at 452 nm in the oxidized form and no absorption maximum in the reduced state. Carbon monoxide treatment of the reduced forms of protein II and protein III showed no maximum at 450 nm. Both proteins II and III are sensitive to oxygen. The hydroxylase activity can be reconstituted from the isolated components. Activity of the combined proteins was demonstrated by product analysis and NADH consumption produced by the addition of progesterone. Protein III catalyzes the reduction of cytochrome c in the presence of NADH and Nocardia flavoprotein reductase. The reconstitution of hydroxylase activity from cytochrome-free enzyme preparations excludes the participation of cytochrome P-450 in this steroid hydroxylase system.

Mössbauer studies on the active Fe ... [2Fe-2S] site of putidamonooxin, its electron transport and dioxygen activation mechanism

Putidamonooxin, the oxygenase of a 4-methoxybenzoate monooxygenase enzyme system, catalyzes the oxidative O-demethylation of the substrate 4-methoxybenzoate in conjunction with the NADH:putidamonooxin oxidoreductase. Putidamonooxin is a conjugated iron-sulfur protein which needs iron ions as cofactors for its enzymatic activity. Putiamonooxin was isolated from Pseudomonas putida, which was grown on a 57Fe-enriched culture medium. Thus putidamonooxin was enriched in vivo with 57Fe up to about 80%. During our Mössbauer study of putidamonooxin a number of parameters have been varied: (a) the oxidation state of putidamonooxin (oxidized, reduced and aerobically reoxidized); (b) the substrate bound to putidamonooxin (4-methoxybenzoate, benzoate, 4-tert-butylbenzoate); (c) the temperature between 2.7 K and 245 K; (d) the applied magnetic field between 0 and 0.1 T and (e) the amount of iron cofactor. From our Mössbauer results it is obvious that the iron-sulfur centers of putidamonooxin are [2 Fe-2S] clusters similar to those of the plant-type ferredoxins. Further, we have evidence for the existence of iron ions (one per [2 Fe-2S] cluster), which serve as cofactors for the dioxygen activation, functioning as the dioxygen binding site and mediating the electron flow from the [2 Fe-2S] cluster to dioxygen.

Dioxygen activation by putidamonooxin. The oxygen species formed and released under uncoupling conditions

In the presence of substrates not favourable for hydroxylation, more than 80% of the dioxygen consumed by purified, reconstituted 4-methoxybenzoate monooxygenase appears in the reaction mixture as hydrogen peroxide. We have investigated whether under these conditions (a) reduced putidamonooxin, the oxygenase of this enzyme system, either autoxidizes in the presence of dioxygen, with liberation of superoxide anion radicals which then disproportionate to H2O2 and O2, or (b) dioxygen is reduced by two sequential single-electron steps leading to the active oxygen species that forms hydrogen peroxide directly when inactivated by protonation. Quantitative estimation of O-2 radicals, with either succinylated ferricytochrome c or epinephrine used as O-2 scavengers, revealed that only about 6% of the total electron flux channelled via putidamonooxin to dioxygen led to the monovalent reduction on dioxygen. This means that not more than 3% of the hydrogen peroxide found under uncoupling conditions arises from the rapid bimolecular disproportionation of initially formed O-2 radicals. Inconsistent results were obtained when lactoperoxidase was used as an O-2 trap. Our measurements indicate that the conversion of lactoperoxidase into compound III is an inappropriate method of detecting any O-2 radicals that may be found by the uncoupled 4-methoxybenzoate monooxygenase. The stoichiometry of about 1:1 for O2 uptake: H2O2 formation indicates that under uncoupling conditions H2O is virtually not formed. The role of [FeO2]+ as the active oxygenating species of putidamonooxin is discussed.

Production of methanol from aromatic acids by Pseudomonas putida

When grown at the expense of 3,4,5-trimethoxybenzoic acid, a strain of Pseudomonas putida oxidized this compound and also 3,5-dimethoxy-4-hydroxybenzoic (syringic) and 3,4-dihydroxy-5-methoxybenzoic (3-O-methylgallic) acids; but other hydroxy- or methoxy-benzoic acids were oxidized slowly or not at all. Radioactivity appeared exclusively in carbon dioxide when cells were incubated with [4-methoxyl-14C]trimethoxybenzoic acid, but was found mainly in methanol when[methoxyl-14C]3-O-methylgallic acid was metabolized. The identity of methanol was proved by analyzing the product from [methoxyl-13C]3-O-methylgallic acid by nuclear magnetic resonance spectroscopy and by isolating the labeled 3,5-dinitrobenzoic acid methyl ester, which was examined by mass spectrometry. These results, together with measurements of oxygen consumed in demethylations catalyzed by cell extracts, showed that two methoxyl groups of 3,4,5-trimethoxybenzoate and one of syringate were oxidized to give carbon dioxide and 3-O-methylgallate. This was then metabolized to pyruvate; the other product was presumed to be the 4-methyl ester of oxalacetic acid, for which cell extracts contained an inducible, specific esterase. P. putida did not metabolize the methanol released from this compound by hydrolysis. Support for the proposed reaction sequence was obtained by isolating mutants which, although able to convert 3,4,5-trimethoxybenzoic acid into 3-O-methylgallic acid, were unable to use either compound for growth.

Chemical and spectral properties of putidamonooxin, the iron-containing and acid-labile-sulfur-containing monooxygenase of a 4-methoxybenzoate O-demethylase from Pseudomonas putida

Gel chromatography indicates that putidamonooxin has a molecular weight of about 126,000. On the other hand, the amino acid composition and the iron-to-protein ratio point to a minimal molecular weight of 33,000 and 31,000 respectively. On sodium dodecylsulfate/polyacrylamide gel electrophoresis the enzyme migrated as a homogeneous band corresponding to a molecular weight of about 40,000. The number of spots found in the tryptic peptide map of the carboxymethylated and digested enzyme indicates that putidamonooxin is composed of three or four identical subunits. After covalent cross-linking of the subunits with dimethyl suberimidate and subsequent dodecylsulfate electrophoresis the main bands were in the molecular weight range of 40,000, 87,000 and 124,000. These findings lead us to propose that putidamonooxin is either a trimer or tetramer. The amino acid composition of putidamonooxin and related data calculated from this are given. The isoelectric point was shown by isoelectric focusing to be a pH 4.7. Low-temperature optical spectra of the reduced and oxidized enzyme as well as of three different putidamonooxin.substrate complexes are given together with those recorded at 10 degrees C. Enzyme.substrate binding spectra are observed with the oxidized putidamonooxin but not with the reduced enzyme. For the oxidized putidamonooxin a molar absorption coefficient at 455nm of 14.7mM-1 cm-1 was determined. Ks values of putidamonooxin towards different substrates and substrate analogues (i.e. tight couplers, partial uncouplers and uncouplers) are presented and possible reasons for the difference between the Ks values here obtained and the previously reported Km values are discussed.

Purification and properties of pyrazon dioxygenase from pyrazon-degrading bacteria

Chromatography on DEAE-cellulose and gel filtration on Sephadex revealed that pyrazon dioxygenase from pyrazon-degrading bacteria consists of three different enzyme components. No component alone oxidizes the phenyl moiety of pyrazon, only when the three components are combined can oxidation be detected. Following electron paramagnetic resonance and ultraviolet measurements the protein nature of the three components was determined: component A1 (molecular weight about 180000,red-brown in colour) is an iron-sulphur protein. The existence of approximately two moles of iron and two moles of inorganic sulphur per mole of protein was demonstrated. This enzyme component was purified to homogeneity in disc electrophoresis. Component A2 is a yellow protein of a molecular weight of about 67000. FAD was shown to be the prosthetic group of this protein. Component B (molecular weight about 12000, brown in colour) is a protein of the ferredoxin type, which was purified to homogeneity, as demonstrated by disc electrophoresis. A hypothetical scheme for the cooperation of the three components is proposed: component A2 accepts as cosubstrate NADH and functions as a ferredoxin reductase. The ferredoxin, component B, has the function of an electron carrier. The conversion of the substrates is effected by component A1, the terminal dioxygenase.

Kinetic studies on a 4-methoxybenzoate O-demethylase from Pseudomonas putida

A direct, sensitive and reliable photometric assay procedure for monitoring the activity of non-specific 4-methoxybenzoate O-demethylases of microorganisms is described. The assay is based on the O-demethylation of 3-nitro-4-methoxybenzoate to the yellow-coloured product 3-nitro-4-hydroxybenzoate. Using this assay and by monitoring the oxidation rate of reduced pyridine nucleotides, the kinetic properties of a purified, reconstituted enzyme system composed of 4-methoxybenzoate monooxygenase (O-demethylating) and a reductase from Pseudomonas putida have been investigated. It has been found that the KM value of the monoxygenase of this enzyme system towards different substrates (i.e. tight couplers, uncouplers and partial uncouplers) rises from the extremely low value of 0.07 muM for the tight couplers to about 55 muM for the uncouplers. The effect of possible inhibitors and metal ions on the reconstituted enzyme system was investigated. The inhibition pattern was almost identical to that found for the purified reductase, only batho-phenanthrolinedisulfonate showing a greater inhibition of the reconstituted enzyme system. The affinity of the reductase towards NADH was found to be approximately 200-fold greater than that towards NADPH. Futhermore, the affinity of this reductase to NADH depended on the nature of the electron acceptor. The affinity to NADH was more than 10 times higher when the monooxygenase-substrate complex was used as the electron acceptor, than when cytochrome c or 2,6-dichloroindophenol was used. These differences are discussed on the basis of enzyme-enzyme interactions between the reductase and the monooxygenase.

A 4-methoxybenzoate O-demethylase from Pseudomonas putida. A new type of monooxygenase system

A strain of Pseudomonas putida grown on 4-methoxybenzoate as sole carbon source contains an enzyme system for the O-demethylation of this substrate. The enzyme system is purifiable and can be separated into two components: an NADH-dependent reductase and an iron-containing and acid-labile-sulfur-containing monooxygenase. The reductase, of molecular weight 42000 and containing two chromophores, an FMN and an iron-sulfur complex (EPR at g = 1.95), reduces both one-electron and two-electron acceptors (i.e., ferricyanide, 2,6-dichloroindophenol, cytochrome c, and cytochrome b5) at an optimum pH of 8.0. Increasing ionic strength affects these activities differently. The absolute spectrum of the oxidized displays distinct absorption peaks at 409 and 463 nm and a small shoulder between 538 and 554 nm. Treatment with dithionite or NADH reduces the absorbance throughout the visible range, yielding a spectrum with small maxima at 402 and 538 nm. Spectroscopic characteristics of the reductase indicate a tight coupling between its two chromophores. The iron-containing and acid-labile-sulfur-containing monooxygenase, which has a molecular weight of about 120000, contains an iron-sulfur chromophore with an EPR signal at g = 1.90. This protein is a dimer whose subunits each have a molecular weight of about 50000 and are perhaps identical. The optical absorption properties are somewhat unusual. In contrast to other iron-sulfur proteins, there is no significant peak near 415 nm in the absorption spectrum of the oxidized protein, but rather one at 455 nm. The presence of the substrate 4-methoxybenzoate increases both the NADH-dependent reductase. Hydroxylation can be achieved by the monooxygenase also in absence of the reductase with artifical reductants. This enzyme opens a new group of oxygenases within the classification scheme, i.e., iron-containing and labile-sulfur-containing monooxygenases. From the reported data, a scheme for the interaction of the isolated pigments and their relationship to various acceptors is proposed.