4-Hydroxybenzoate-coenzyme A ligase from Rhodopseudomonas palustris: purification, gene sequence, and role in anaerobic degradation.

Anaerobic metabolism of most aromatic acids is initiated by coenzyme A thioester formation. Rhodopseudomonas palustris grows well under anaerobic, phototrophic conditions with many aromatic acids, including benzoate and 4-hydroxybenzoate, as a carbon source. A coenzyme A ligase that reacts with 4-hydroxybenzoate was purified from 4-hydroxybenzoate-grown cells of R. palustris. This enzyme required MgATP, reduced coenzyme A, and 4-hydroxybenzoate, benzoate, or cyclohex-1,4-dienecarboxylate for optimal activity but also used phosphopantetheine, cyclohex-2,5-dienecarboxylate, and 4-fluorobenzoate at lower rates. The 4-hydroxybenzoate-coenzyme A ligase differed in molecular characteristics from a previously described benzoate-coenzyme A ligase from R. palustris, and the two ligases did not cross-react immunologically. The gene encoding the 4-hydroxybenzoate enzyme was cloned and sequenced. The deduced gene product showed about 20% amino acid identity with bacterial coenzyme A ligases involved in aerobic degradation of aromatic acids. An R. palustris mutant carrying a disrupted 4-hydroxybenzoate-coenzyme A ligase gene was unable to grow with 4-hydroxybenzoate under anaerobic conditions, indicating that the enzyme is essential for anaerobic degradation of this compound.     

New aerobic benzoate oxidation pathway via benzoyl-coenzyme A and 3-hydroxybenzoyl-coenzyme A in a denitrifying Pseudomonas sp.

A denitrifying Pseudomonas sp. is able to oxidize aromatic compounds compounds completely to CO2, both aerobically and anaerobically. It is shown that benzoate is aerobically oxidized by a new degradation pathway via benzoyl-coenzyme A (CoA) and 3-hydroxybenzoyl-CoA. The organism grew aerobically with benzoate, 3-hydroxybenzoate, and gentisate; catechol, 2-hydroxybenzoate, and protocatechuate were not used, and 4-hydroxybenzoate was a poor substrate. Mutants were obtained which were not able to utilize benzoate as the sole carbon source aerobically but still used 3-hydroxybenzoate or gentisate. Simultaneous adaptation experiments with whole cells seemingly suggested a sequential induction of enzymes of a benzoate oxidation pathway via 3-hydroxybenzoate and gentisate. Cells grown aerobically with benzoate contained a benzoate-CoA ligase (AMP forming) (0.1 mumol min-1 mg-1) which converted benzoate but not 3-hydroxybenzoate into its CoA thioester. The enzyme of 130 kDa composed of two identical subunits of 56 kDa was purified and characterized. Cells grown aerobically with 3-hydroxybenzoate contained a similarly active CoA ligase for 3-hydroxybenzoate, 3-hydroxybenzoate-CoA ligase (AMP forming). Extracts from cells grown aerobically with benzoate catalyzed a benzoyl-CoA- and flavin adenine dinucleotide-dependent oxidation of NADPH with a specific activity of at least 25 nmol NADPH oxidized min-1 mg of protein-1; NADH and benzoate were not used. This new enzyme, benzoyl-CoA 3-monooxygenase, was specifically induced during aerobic growth with benzoate and converted [U-14C]benzoyl-CoA stoichiometrically to [14C]3-hydroxybenzoyl-CoA.     

圈卷产色链霉菌尼可霉素生物合成基因：—sanJ的克隆及功能研究

以尼可霉素生物合成相关的基因片段为探针,从圈卷产色链霉菌cosmid基因文库中筛选到１个大约７．５kb的ＤＮＡ片段,交ＤＮＡ片段克隆到载体pBluescripM13-的KpnⅠ位点,得到了重组质粒pNL2200.对pNL2200中外源ＤＮＡ片段进行了一系列的亚克隆及部分核苷酸序列分析。结果表明,２．３kb的SalⅠ－ＢaｍＨⅠＤＮＡ片段中含有１个完整的开放阅读框,起始密码子为２７１位的ＧＴＧ,终止密码子为１９５４位的ＴＧＡ,该基因的大小为１６８６bp,编码１个大小为５６１个氨基酸的蛋白质产物。利用blastx程序的蛋白质数据库中进行同源比较,结果揭示此基因产物与腺苷酸形成酶超家族的连接酶有４４％的一致性,此外,该基因的破坏导致圈卷产色链霉菌尼可霉素生物合成能力的丧失,证明它是尼可霉素生物合成所必需的,命名为其为sanＪ。     

Anaerobic metabolism of 3-hydroxybenzoate by the denitrifying bacterium Thauera aromatica

The anaerobic metabolism of 3-hydroxybenzoate was studied in the denitrifying bacterium Thauera aromatica. Cells grown with this substrate were adapted to grow with benzoate but not with 4-hydroxybenzoate. Vice versa, 4-hydroxybenzoate-grown cells did not utilize 3-hydroxybenzoate. The first step in 3-hydroxybenzoate metabolism is a coenzyme A (CoA) thioester formation, which is catalyzed by an inducible 3-hydroxybenzoate-CoA ligase. The enzyme was purified and characterized. Further metabolism of 3-hydroxybenzoyl-CoA by cell extract required MgATP and was coupled to the oxidation of 2 mol of reduced viologen dyes per mol of substrate added. Purification of the 3-hydroxybenzoyl-CoA reducing enzyme revealed that this activity was due to benzoyl-CoA reductase, which reduced the 3-hydroxy analogue almost as efficiently as benzoyl-CoA. The further metabolism of the alicyclic dienoyl-CoA product containing the hydroxyl substitution obviously required additional specific enzymes. Comparison of the protein pattern of 3-hydroxybenzoate-grown cells with benzoate-grown cells revealed several 3-hydroxybenzoate-induced proteins; the N-terminal amino acid sequences of four induced proteins were determined and the corresponding genes were identified and sequenced. A cluster of six adjacent genes contained the genes for substrate-induced proteins 1 to 3; this cluster may not yet be complete. Protein 1 is a short-chain alcohol dehydrogenase. Protein 2 is a member of enoyl-CoA hydratase enzymes. Protein 3 was identified as 3-hydroxybenzoate-CoA ligase. Protein 4 is another member of the enoyl-CoA hydratases. In addition, three genes coding for enzymes of beta-oxidation were present. The anaerobic 3-hydroxybenzoate metabolism here obviously combines an enzyme (benzoyl-CoA reductase) and electron carrier (ferredoxin) of the general benzoyl-CoA pathway with enzymes specific for the 3-hydroxybenzoate pathway. This raises some questions concerning the regulation of both pathways.     

Benzoate-coenzyme A ligase from Thauera aromatica: an enzyme acting in anaerobic and aerobic pathways

In the denitrifying member of the beta-Proteobacteria Thauera aromatica, the anaerobic metabolism of aromatic acids such as benzoate or 2-aminobenzoate is initiated by the formation of the coenzyme A (CoA) thioester, benzoyl-CoA and 2-aminobenzoyl-CoA, respectively. Both aromatic substrates were transformed to the acyl-CoA intermediate by a single CoA ligase (AMP forming) that preferentially acted on benzoate. This benzoate-CoA ligase was purified and characterized as a 57-kDa monomeric protein. Based on V(max)/K(m), the specificity constant for 2-aminobenzoate was 15 times lower than that for benzoate; this may be the reason for the slower growth on 2-aminobenzoate. The benzoate-CoA ligase gene was cloned and sequenced and was found not to be part of the gene cluster encoding the general benzoyl-CoA pathway of anaerobic aromatic metabolism. Rather, it was located in a cluster of genes coding for a novel aerobic benzoate oxidation pathway. In line with this finding, the same CoA ligase was induced during aerobic growth with benzoate. A deletion mutant not only was unable to grow anaerobically on benzoate or 2-aminobenzoate, but also aerobic growth on benzoate was affected. This suggests that benzoate induces a single benzoate-CoA ligase. The product of benzoate activation, benzoyl-CoA, then acts as inducer of separate anaerobic or aerobic pathways of benzoyl-CoA, depending on whether oxygen is lacking or present.     

Involvement of coenzyme A thioesters in anaerobic metabolism of 4-hydroxybenzoate by Rhodopseudomonas palustris.

The initial steps of anaerobic 4-hydroxybenzoate degradation were studied in whole cells and cell extracts of the photosynthetic bacterium Rhodopseudomonas palustris. Illuminated suspensions of cells that had been grown anaerobically on 4-hydroxybenzoate and were assayed under anaerobic conditions took up [U-14C]4-hydroxybenzoate at a rate of 0.6 nmol min-1 mg of protein-1. Uptake occurred with high affinity (apparent Km = 0.3 microM), was energy dependent, and was insensitive to external pH in the range of 6.5 to 8.2 Very little free 4-hydroxybenzoate was found associated with cells, but a range of intracellular products was formed after 20-s incubations of whole cells with labeled substrate. When anaerobic pulse-chase experiments were carried out with cells incubated on ice or in darkness, 4-hydroxybenzoyl coenzyme A (4-hydroxybenzoyl-CoA) was formed early and disappeared immediately after addition of excess unlabeled substrate, as would be expected of an early intermediate in 4-hydroxybenzoate metabolism. A 4-hydroxybenzoate-CoA ligase activity with an average specific activity of 0.7 nmol min-1 mg of protein-1 was measured in the soluble protein fraction of cells grown anaerobically on 4-hydroxybenzoate. 4-Hydroxybenzoyl-CoA was the sole product formed from labeled 4-hydroxybenzoate in the ligase reaction mixture. 4-Hydroxybenzoate uptake and ligase activities were present in cells grown anaerobically with benzoate, 4-hydroxybenzoate, and 4-aminobenzoate and were not detected in succinate-grown cells. These results indicate that the high-affinity uptake of 4-hydroxybenzoate by R. palustris is due to rapid conversion of the free acid to its CoA derivative by a CoA ligase and that this is also the initial step of anaerobic 4-hydroxybenzoate degradation.     

Enzymes of the mandelate pathway in bacterium N.C.I.B. 8250

1. Bacterium N.C.I.B. 8250 was grown on dl-mandelate, benzyl alcohol, benzoyl-formate, benzaldehyde and benzoate and also on 2-hydroxy, 4-hydroxy, 3,4-dihydroxy and 4-hydroxy-3-methoxy analogues of these compounds. The enzymic complements of the cells were determined and the specificities of some of the enzymes examined. 2. Growth on mandelate or benzoylformate induces l-mandelate dehydrogenase, benzoylformate decarboxylase, benzyl alcohol dehydrogenase and a heat-stable as well as a heat-labile benzaldehyde dehydrogenase. Growth on benzyl alcohol or benzaldehyde induces benzyl alcohol dehydrogenase and the heat-labile benzaldehyde dehydrogenase. 3. The enzymes of the mandelate-to-benzoate pathway are non-specifically active on, and induced by, all the substituted analogues that support growth. 4. Benzoate oxidase is induced by growth on benzoate or on 2-hydroxybenzoate. 2-Hydroxybenzoate hydroxylase, 4-hydroxybenzoate hydroxylase and 4-hydroxy-3-methoxybenzoate O-demethylase are induced only by growth on homologous substrates. 5. The results of the investigation are discussed with regard to the possible regulation of the enzyme systems.     

SanT, a bidomain protein, is essential for nikkomycin biosynthesis of Streptomyces ansochromogenes

Nikkomycins act as a competitive inhibitor of chitin synthetase and display potent activities against phytopathogenic and human pathogenic fungi. sanT is located in the gene cluster of nikkomycin biosynthesis in Streptomyces ansochromogenes. Sequence analysis revealed that the deduced product of sanT has an unusual domain structure, which consists of an N-terminal acyl carrier protein (ACP) domain and a C-terminal aminotransferase (AMT) domain. Gene disruption and complementation indicated that sanT is essential for nikkomycin biosynthesis. Each domain of SanT was overexpressed in Escherichia coli and then purified. ACP domain is posttranslationally modified with phosphopantetheine (Ppant) prosthetic group at Ser-33. AMT domain catalyzes the transamination of 4-pyridyl-2-oxo-4-hydroxyisovalerate (POHIV), a precursor of peptidyl moiety of nikkomycins, to pyridylhomothreonine (PHT) in vitro. The two domains function independently but both are essential for nikkomycin biosynthesis. The biochemical and genetic evidences suggested that SanT is possibly a bifunctional protein, participating in the biosynthesis of peptidyl moiety and the assembly of nikkomycins.     

Identification of the pcaRKF gene cluster from Pseudomonas putida: involvement in chemotaxis, biodegradation, and transport of 4-hydroxybenzoate.

Pseudomonas putida PRS2000 is chemotactic to 4-hydroxybenzoate and other aromatic acids. This behavioral response is induced when cells are grown on 4-hydroxybenzoate or benzoate, compounds that are degraded via the beta-ketoadipate pathway. Isolation of a transposon mutant defective in 4-hydroxybenzoate chemotaxis allowed identification of a new gene cluster designated pcaRKF. DNA sequencing, mutational analysis, and complementation studies revealed that pcaR encodes a regulatory protein required for induction of at least four of the enzymes of the beta-ketoadipate pathway and that pcaF encodes beta-ketoadipyl-coenzyme A thiolase, the last enzyme in the pathway. The third gene, pcaK, encodes a transporter for 4-hydroxybenzoate, and this protein is also required for chemotaxis to aromatic acids. The predicted PcaK protein is 47 kDa in size, with a deduced amino acid sequence indicative of membership in the major facilitator superfamily of transport proteins. The protein, expressed in Escherichia coli, catalyzed 4-hydroxybenzoate transport. In addition, whole cells of P. putida pcaK mutants accumulated 4-hydroxybenzoate at reduced rates compared with that in wild-type cells. The pcaK mutation did not impair growth at the expense of 4-hydroxybenzoate under most conditions; however, mutant cells grew somewhat more slowly than the wild type on 4-hydroxybenzoate at a high pH. The finding that 4-hydroxybenzoate chemotaxis can be disrupted without an accompanying effect on metabolism indicates that this chemotactic response is receptor mediated. It remains to be determined, however, whether PcaK itself is a chemoreceptor for 4-hydroxybenzoate or whether it plays an indirect role in chemotaxis. These findings indicate that aromatic acid detection and transport are integral features of aromatic degradation pathways.     

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.     

Benzoate-coenzyme A ligase, encoded by badA, is one of three ligases able to catalyze benzoyl-coenzyme A formation during anaerobic growth of Rhodopseudomonas palustris on benzoate.

The first step of anaerobic benzoate degradation is the formation of benzoyl-coenzyme A by benzoate-coenzyme A ligase. This enzyme, purified from Rhodopseudomonas palustris, is maximally active with 5 microM benzoate. To study the molecular basis for this reaction, the benzoate-coenzyme A ligase gene (badA) was cloned and sequenced. The deduced amino acid sequence of badA showed substantial similarity to other coenzyme A ligases, with the highest degree of similarity being that to 4-hydroxybenzoate-coenzyme A ligase (50% amino acid identity) from R. palustris. A badA mutant that was constructed had barely detectable levels of ligase activity when cell extracts were assayed at 10 microM benzoate. Despite this, the mutant grew at wild-type rates on benzoate under laboratory culture conditions (3 mM benzoate), and mutant cell extracts had high levels of ligase activity when assayed at a high concentration of benzoate (1 mM). This suggested that R. palustris expresses, in addition to BadA, a benzoate-activating enzyme(s) with a relatively low affinity for benzoate. A possible role of 4-hydroxybenzoate-coenzyme A ligase (encoded by hbaA) in this capacity was investigated by constructing a badA hbaA double mutant. Although the double mutant grew more slowly on benzoate than badA cells, growth rates were still significant, suggesting the involvement of a third enzyme in benzoate activation. Competition experiments involving the addition of a small amount of cyclohexanecarboxylate to ligase assay mixtures implicated cyclohexanecarboxylate-coenzyme A ligase as being this third enzyme. These results show that wild-type R. palustris cells synthesize at least three enzymes that can catalyze the initial step in anaerobic benzoate degradation during growth on benzoate. This observation supports previous suggestions that benzoyl-coenzyme A formation plays a central role in anaerobic aromatic compound biodegradation.     

4-hydroxybenzoyl coenzyme A reductase (dehydroxylating) is required for anaerobic degradation of 4-hydroxybenzoate by Rhodopseudomonas palustris and shares features with molybdenum-containing hydroxylases.

The anaerobic degradation of 4-hydroxybenzoate is initiated by the formation of 4-hydroxybenzoyl coenzyme A, with the next step proposed to be a dehydroxylation to benzoyl coenzyme A, the starting compound for a central pathway of aromatic compound ring reduction and cleavage. Three open reading frames, divergently transcribed from the 4-hydroxybenzoate coenzyme A ligase gene, hbaA, were identified and sequenced from the phototrophic bacterium Rhodopseudomonas palustris. These genes, named hbaBCD, specify polypeptides of 17.5, 82.6, and 34.5 kDa, respectively. The deduced amino acid sequences show considerable similarities to a group of hydroxylating enzymes involved in CO, xanthine, and nicotine metabolism that have conserved binding sites for [2Fe-2S] clusters and a molybdenum cofactor. Cassette disruption of the hbaB gene yielded a mutant that was unable to grow anaerobically on 4-hydroxybenzoate but grew normally on benzoate. The hbaB mutant cells did not accumulate [14C]benzoyl coenzyme A during short-term uptake of [14C]4-hydroxybenzoate, but benzoyl coenzyme A was the major radioactive metabolite formed by the wild type. In addition, crude extracts of the mutant failed to convert 4-hydroxybenzoyl coenzyme A to benzoyl coenzyme A. This evidence indicates that the hbaBCD genes encode the subunits of a 4-hydroxybenzoyl coenzyme A reductase (dehydroxylating). The sizes of the specified polypeptides are similar to those reported for 4-hydroxybenzoyl coenzyme A reductase isolated from the denitrifying bacterium Thauera aromatica. The amino acid consensus sequence for a molybdenum cofactor binding site is in HbaC. This cofactor appears to be an essential component because anaerobic growth of R. palustris on 4-hydroxybenzoate, but not on benzoate, was retarded unless 0.1 microM molybdate was added to the medium. Neither tungstate nor vanadate replaced molybdate, and tungstate competitively inhibited growth stimulation by molybdate.     

Purification and properties of benzoate-coenzyme A ligase, a Rhodopseudomonas palustris enzyme involved in the anaerobic degradation of benzoate.

A soluble benzoate-coenzyme A (CoA) ligase was purified from the phototrophic bacterium Rhodopseudomonas palustris. Synthesis of the enzyme was induced when cells were grown anaerobically in light with benzoate as the sole carbon source. Purification by chromatography successively on hydroxylapatite, phenyl-Sepharose, and hydroxylapatite yielded an electrophoretically homogeneous enzyme preparation with a specific activity of 25 mumol/min per mg of protein and a molecular weight of 60,000. The purified enzyme was insensitive to oxygen and catalyzed the Mg2+ ATP-dependent formation of acyl-CoA from carboxylate and free reduced CoA, with high specificity for benzoate and 2-fluorobenzoate. Apparent Km values of 0.6 to 2 microM for benzoate, 2 to 3 microM for ATP, and 90 to 120 microM for reduced CoA were determined. The reaction product, benzoyl-CoA, was an effective inhibitor of the ligase reaction. The kinetic properties of the enzyme match the kinetics of substrate uptake by whole cells and confirm a role for benzoate-CoA ligase in maintaining entry of benzoate into cells as well as in catalyzing the first step in the anaerobic degradation of benzoate by R. palustris.     

Anaerobic metabolism of 2-hydroxybenzoic acid (salicylic acid) by a denitrifying bacterium

The anaerobic metabolism of 2-hydroxybenzoic acid (salicylic acid) was studied in a denitrifying bacterium. Cells grown with 2-hydroxybenzoate were simultaneously adapted to degrade benzoate. Extract of these cells formed benzoate or benzoyl-CoA when incubated under reducing conditions with salicylate, MgATP, and coenzyme A, suggesting a degradation of 2-hydroxybenzoate via benzoate or benzoyl-CoA. This suggestion was supported by enzyme activity measurements. In extracts of 2-hydroxybenzoate-grown cells, the following enzyme activities were detected: two CoA ligases, one specific for 2-hydroxybenzoate, the other for benzoate, and two different enzyme activities catalyzing the reductive transformation of 2-hydroxybenzoyl-CoA. These findings suggest a degradation of salicylic acid by two new enzymes, 2-hydroxybenzoate-CoA ligase (AMP-forming) and 2-hydroxybenzoyl-CoA reductase (dehydroxylating), catalyzing (1) 2-hydroxybenzoate + MgATP + CoASH → 2-hydroxybenzoyl-CoA + MgAMP + PP_i (2) 2-hydroxybenzoyl-CoA + 2[H] → benzoyl-CoA + H₂O Benzoyl-CoA was dearomatized by reduction of the ring. This represents another case in which benzoyl-CoA is a central intermediate in anaerobic aromatic metabolism. Received: 1 February 1996 / Accepted: 24 February 1996     

Genes coding for a new pathway of aerobic benzoate metabolism in Azoarcus evansii

A new pathway for aerobic benzoate oxidation has been postulated for Azoarcus evansii and for a Bacillus stearothermophilus-like strain. Benzoate is first transformed into benzoyl coenzyme A (benzoyl-CoA), which subsequently is oxidized to 3-hydroxyadipyl-CoA and then to 3-ketoadipyl-CoA; all intermediates are CoA thioesters. The genes coding for this benzoate-induced pathway were investigated in the beta-proteobacterium A. evansii. They were identified on the basis of N-terminal amino acid sequences of purified benzoate metabolic enzymes and of benzoate-induced proteins identified on two-dimensional gels. Fifteen genes probably coding for the benzoate pathway were found to be clustered on the chromosome. These genes code for the following functions: a putative ATP-dependent benzoate transport system, benzoate-CoA ligase, a putative benzoyl-CoA oxygenase, a putative isomerizing enzyme, a putative ring-opening enzyme, enzymes for beta-oxidation of CoA-activated intermediates, thioesterase, and lactone hydrolase, as well as completely unknown enzymes belonging to new protein families. An unusual putative regulator protein consists of a regulator protein and a shikimate kinase I-type domain. A deletion mutant with a deletion in one gene (boxA) was unable to grow with benzoate as the sole organic substrate, but it was able to grow with 3-hydroxybenzoate and adipate. The data support the proposed pathway, which postulates operation of a new type of ring-hydroxylating dioxygenase acting on benzoyl-CoA and nonoxygenolytic ring cleavage. A beta-oxidation-like metabolism of the ring cleavage product is thought to lead to 3-ketoadipyl-CoA, which finally is cleaved into succinyl-CoA and acetyl-CoA.     

Identification of genes coding for hydrolytic dehalogenation in the metagenome derived from a denitrifying 4-chlorobenzoate degrading consortium.

A metagenomic approach was taken to investigate the genetic basis for the ability of an anaerobic consortium to grow on either 4-chlorobenzoate or 4-bromobenzoate under denitrifying conditions. Degenerate PCR primers were designed for the family of 4-chlorobenzoyl-CoA dehalogenase genes. The primers were utilized to screen a metagenome library and two overlapping clones were identified which yield a PCR product. The complete sequence of one metagenome clone was determined and genes encoding 4-chlorobenzoyl-CoA ligase (FcbA) and 4-chlorobenzoyl-CoA dehalogenase (FcbB) were identified. Analysis of the ORFs present in the nucleotide sequence suggests that the metagenome clone originated from an uncultured denitrifying microorganism belonging to the Betaproteobacteria. Interestingly, unlike similar gene clusters reported in aerobes, a gene encoding 4-hydroxybenzoyl-CoA thioesterase was not present in the gene cluster. This suggests that 4-hydroxybenzoyl-CoA is further degraded via the anaerobic reduction pathway in the corresponding microorganism instead of through thioester hydrolysis to yield 4-hydroxybenzoate.     

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-.