Purification and some properties of component B of the 4-chlorophenylacetate 3,4-dioxygenase from Pseudomonas species strain CBS 3

Role of Dehalogenases in Aerobic Bacterial Degradation of Chlorinated Aromatic Compounds

This review was conducted to provide an overview of dehalogenases involved in aerobic biodegradation of chlorinated aromatic compounds. Additionally, biochemical and molecular characterization of hydrolytic, reductive, and oxygenolytic dehalogenases was reviewed. This review will increase our understanding of the process of dehalogenation of chlorinated aromatic compounds.

A novel hydrolytic dehalogenase for the chlorinated aromatic compound chlorothalonil

Dehalogenases play key roles in the detoxification of halogenated aromatics. Interestingly, only one hydrolytic dehalogenase for halogenated aromatics, 4-chlorobenzoyl-coenzyme A (CoA) dehalogenase, has been reported. Here, we characterize another novel hydrolytic dehalogenase for a halogenated aromatic compound from the 2,4,5,6-tetrachloroisophthalonitrile (chlorothalonil)-degrading strain of Pseudomonas sp. CTN-3, which we have named Chd. Chd catalyzes a hydroxyl substitution at the 4-chlorine atom of chlorothalonil. The metabolite of the Chd dehalogenation, 4-hydroxy-trichloroisophthalonitrile, was identified by reverse-phase high-performance liquid chromatography (HPLC), tandem mass spectrometry (MS/MS), and nuclear magnetic resonance (NMR). Chd dehalogenates chlorothalonil under anaerobic and aerobic conditions and does not require the presence of cofactors such as CoA and ATP. Chd contains a putative conserved domain of the metallo-beta-lactamase superfamily and shows the highest identity with several metallohydrolases (24 to 29%). Chd is a monomer (36 kDa), and the isoelectric point (pI) of Chd is estimated to be 4.13. Chd has a dissociation constant (K(m)) of 0.112 mM and an overall catalytic rate (k(cat)) of 207 s(-1) for chlorothalonil. Chd is completely inhibited by 1,10-phenanthroline, diethyl pyrocarbonate, and N-bromosuccinic acid. Site-directed mutagenesis of Chd revealed that histidines 128 and 157, serine 126, aspartates 45, 130 and 184, and tryptophan 241 were essential for the dehalogenase activity. Chd differs from other reported hydrolytic dehalogenases based on the analysis of amino acid sequences and catalytic mechanisms. This study provides an excellent dehalogenase candidate for mechanistic study of hydrolytic dehalogenation of halogenated aromatic compound.

Biological dehalogenation and halogenation reactions

A large number of halogenated compounds is produced by chemical synthesis. Some of these compounds are very toxic and cause enormous problems to human health and to the environment. Investigations on the degradation of halocompounds by microorganisms have led to the detection of various dehalogenating enzymes catalyzing the removal of halogen atoms under aerobic and anaerobic conditions involving different mechanisms. On the other hand, more than 3500 halocompounds are known to be produced biologically, some of them in great amounts. Until 1997, only haloperoxidases were thought to be responsible for incorporation of halogen atoms into organic compounds. However, recent investigations into the biosynthesis of halogenated metabolites by bacteria have shown that a novel type of halogenating enzymes, FADH(2)-dependent halogenases, are involved in biosyntheses of halogenated metabolites. In every gene cluster coding for the biosynthesis of a halogenated metabolite, isolated so far, one or several genes for FADH(2)-dependent halogenases have been identified.

Purification and characterization of gallic acid decarboxylase from pantoea agglomerans T71

Oxygen-sensitive gallic acid decarboxylase from Pantoea (formerly Enterobacter) agglomerans T71 was purified from a cell extract after stabilization by reducing agents. This enzyme has a molecular mass of approximately 320 kDa and consists of six identical subunits. It is highly specific for gallic acid. Gallic acid decarboxylase is unique among similar decarboxylases in that it requires iron as a cofactor, as shown by plasma emission spectroscopy (which revealed an iron content of 0.8 mol per mol of enzyme subunit), spectrophotometric analysis (absorption shoulders at 398 and 472 nm), and inhibition of the enzyme activity by 2,2'-bipyridyl, o-phenanthroline, and EDTA. Another interesting feature of this strain is the fact that it contains a tannase, which is used together with the gallic acid decarboxylase in a two-enzyme resting cell bioconversion to synthesize valuable pyrogallol from readily available tannic acid.

Studies of the redox properties of CDP-6-deoxy-L-threo-D-glycero-4-hexulose-3-dehydrase (E1) and CDP-6-deoxy-L-threo-D-glycero-4-hexulose-3-dehydrase reductase (E3): two important enzymes involved in the biosynthesis of ascarylose

Studies of the biosynthesis of ascarylose, a 3,6-dideoxyhexose found in the lipopolysaccharide of Yersinia pseudotuberculosis V, have shown that the C-3 deoxygenation is a process consisting of two enzymatic steps. The first enzyme involved in this transformation is CDP-6-deoxy-L-threo-D-glycero-4-hexulose-3-dehydrase (E1), which is a pyridoxamine 5'-phosphate dependent iron-sulfur protein. The second catalyst, CDP-6-deoxy-L-threo-D-glycero-4-hexulose-3-dehydrase reductase, formally called CDP-6-deoxy-delta(3,4)-glucoseen reductase (E3), is an NADH dependent plant type [2Fe-2S] containing flavoenzyme. To better understand the electron transfer carried out by these two enzymes, the potentials of the E1 and E3 redox cofactors were determined spectroelectrochemically. At pH 7.5, the midpoint potential of the E3 FAD was found to be -212 mV, with the FADox/FADsq couple (E1o') and the FADsq/FADhq couple (E2o') calculated to be -231 and -192 mV, respectively. However, the E1o' and E2o' of the FAD in E3(apoFeS) at pH 7.5 were estimated to be -215 and -240 mV, respectively, which are quite different from those of the holo-E3, suggesting a significant effect of the iron-sulfur center on the redox properties of the flavin coenzyme. Our data also showed that the midpoint potential of the E3 iron-sulfur is -257 mV and that of the E1 [2Fe-2S] center is -209 mV. These values indicated a thermodynamic barrier to the proposed electron transfer of NADH->FAD=>E3[2Fe-2S]->E1[2Fe-2S] at pH 7.5. Regulation of electron transfer by several mechanisms is possible and experiments were performed to examine ways of overcoming the unfavorable electron transfer energetics in the E1/E3 system. It was found that both binding of E3 with NAD+ and complex formation between E3 and E1 showed no effect on the midpoint potentials of the E3 FAD and iron-sulfur center. Interestingly, the midpoint potential of the E3 FAD shifts dramatically to -273 mV (E1o' approximately -345 mV and E2o' approximately -200 mV) at pH 8.4, with very little semiquinone stabilization (< 5%). The potential of the E3 [2Fe-2S] center at pH 8.4 was also found to undergo a negative shift to -279 mV, and that of the E1 iron sulfur center remained essentially the same at -206 mV. These data indicated that the redox properties of this system may be regulated by pH and the electron transfer between the E3 redox centers may be prototropically controlled. These results also demonstrated that E3 is unique among this class of enzymes.

The 2Fe2S centres of the 2-oxo-1,2-dihydroquinoline 8-monooxygenase from Pseudomonas putida 86 studied by EPR spectroscopy

The 2-oxo-1,2-dihydroquinoline 8-monooxygenase from Pseudomonas putida 86 comprises two components with four redox active sites necessary for activity. We present an EPR characterization of the iron-sulfur centres in the purified reductase and oxygenase component of this novel enzyme system. The oxygenase component was identified as a Rieske [2Fe2S] protein on the basis of its characteristic EPR spectrum with gz,y,x = 2.01, 1.91, 1.76 and gav = 1.893. The reductase component, an iron-sulfur flavoprotein, contained a [2Fe2S] cluster with gz,y,x = 2.03, 1.94, 1.89 and the average g-value (gav) of 1.953, typical of a ferredoxin-type centre. In redox titrations at pH 7, the midpoint potentials were determined to be -180 mV +/- 30 mV and -100 mV +/- 10 mV for the reductase and oxygenase component, respectively. A detailed comparison to other multicomponent enzyme systems is presented pointing out the EPR and redox properties of the FeS centres involved.

Quinoline 2-oxidoreductase and 2-oxo-1,2-dihydroquinoline 5,6-dioxygenase from Comamonas testosteroni 63. The first two enzymes in quinoline and 3-methylquinoline degradation

The enzymes catalysing the first two steps of quinoline and 3-methylquinoline degradation by Comamonas testosteroni 63 were investigated. Quinoline 2-oxidoreductase, which catalyses the hydroxylation of (3-methyl-)quinoline to (3-methyl-)2-oxo-1,2-dihydroquinoline, was purified to apparent homogeneity. The native enzyme, with a molecular mass of 360 kDa, is composed of three non-identical subunits (87, 32, and 22 kDa), occurring in a ratio of 1.16:1:0.83. Containing FAD, molybdenum, iron, and acid-labile sulfur in the stoichiometric ratio of 2:2:8:8, the enzyme belongs to the molybdo-iron/sulfur flavoproteins. Molybdopterin cytosine dinucleotide is the organic part of the pterin molybdenum cofactor. Comparison of N-terminal amino acid sequences revealed similarities to a number of procaryotic molybdenum-containing hydroxylases. Especially the N-termini of the beta-subunits of the quinoline 2-oxidoreductases from Comamonas testosteroni 63, Pseudomonas putida 86, and Rhodococcus spec. B1, and of quinoline-4-carboxylic acid 2-oxidoreductase from Agrobacterium spec. 1B showed striking similarities. Further degradation of (3-methyl-)2-oxo-1,2-dihydroquinoline proceeds via dioxygenation at the benzene ring, i.e. at 5,6-position [Schach, S., Schwarz, G., Fetzner, S. & Lingens, F. (1993) Biol. Chem. Hoppe-Seyler 374, 175-181]. 2-Oxo-1,2-dihydroquinoline 5,6-dioxygenase was partially purified; NADH and oxygen are required for the reaction, and the enzymic activity is enhanced 1.5-fold by addition of Fe2+ ions. Unexpectedly, this aromatic ring dioxygenase did not separate into distinct protein components, but is apparently a single-component enzyme. The molecular mass was estimated to be about 260 kDa. 2-Oxo-1,2-dihydroquinoline 5,6-dioxygenase is very thermolabile. However, dithioerythritol and low concentrations of substrate had a moderately stabilizing effect. 2-Oxo-1,2-dihydroquinoline 5,6-dioxygenase is inhibited by sulfhydryl-blocking agents, by metal-chelating agents, and by the flavin analogues quinacrine and acriflavin.

Dehalogenation of 4-chlorobenzoate. Characterisation of 4-chlorobenzoyl-coenzyme A dehalogenase from Pseudomonas sp. CBS3

Pseudomonas sp. CBS3 is capable of growing with 4-chlorobenzoate as sole source of carbon and energy. The removal of the chlorine of 4-chlorobenzoate is performed in the first degradation step by an enzyme system consisting of three proteins. A 4-halobenzoate-coenzyme A ligase activates 4-chlorobenzoate in a coenzyme A, ATP and Mg2+ dependent reaction to 4-chlorobenzoyl-coenzyme A. This thioester intermediate is dehalogenated by the 4-chlorobenzoyl-coenzyme A dehalogenase. Finally coenzyme A is split off by a 4-hydroxybenzoyl-CoA thioesterase to form 4-hydroxybenzoate. The involved 4-chlorobenzoyl-coenzyme A dehalogenase was purified to apparent homogeneity by a five-step purification procedure. The native enzyme had an apparent molecular mass of 120,000 and was composed of four identical polypeptide subunits of 31 kDa. The enzyme displayed an isoelectric point of 6.7. The maximal initial rate of catalysis was achieved at pH 10 at 60 degrees C. The apparent Km value for 4-chlorobenzoyl-coenzyme A was 2.4-2.7 microM. Vmax was 1.1 x 10(-7) M sec-1 (2.2 mumol min-1 mg-1 of protein). The NH2-terminal amino acid sequence was determined. All 4-halobenzoyl-coenzyme A thioesters, except 4-fluorobenzoyl-coenzyme A, were dehalogenated by the 4-chlorobenzoyl-CoA dehalogenase.

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.

Cloning, nucleotide sequence, and expression of the plasmid-encoded genes for the two-component 2-halobenzoate 1,2-dioxygenase from Pseudomonas cepacia 2CBS

The two-component nonheme iron dioxygenase system 2-halobenzoate 1,2-dioxygenase from Pseudomonas cepacia 2CBS catalyzes the double hydroxylation of 2-halobenzoates with concomitant release of halogenide and carbon dioxide, yielding catechol. The gene cluster encoding this enzyme, cbdABC, was localized on a 70-kbp conjugative plasmid designated pBAH1. The nucleotide sequences of cbdABC and flanking regions were determined. In the deduced amino acid sequence of the large subunit of the terminal oxygenase component (CbdA), a conserved motif proposed to bind the Rieske-type [2Fe-2S] cluster was identified. In the NADH:acceptor reductase component (CbdC), a putative binding site for a chloroplast-type [2Fe-2S] center and possible flavin adenine dinucleotide- and NAD-binding domains were identified. The cbdABC sequences show significant homology to benABC, which encode benzoate 1,2-dioxygenase from Acinetobacter calcoaceticus (52% identity at the deduced amino acid level), and to xylXYZ, which encode toluate 1,2-dioxygenase from Pseudomonas putida mt-2 (51% amino acid identity). Recombinant pkT231 harboring cbdABC and flanking regions complemented a plasmid-free mutant of wild-type P. cepacia 2CBS for growth on 2-chlorobenzoate, and it also allowed recombinant P. putida KT2440 to metabolize 2-chlorobenzoate. Functional NADH:acceptor reductase and oxygenase components of 2-halobenzoate 1,2-dioxygenase were enriched from recombinant Pseudomonas clones.

Stereospecificity of hydride removal from NADH by reductases of multicomponent nonheme iron oxygenase systems

The stereospecificity of hydride removal from the 4 position of the pyridine ring of NADH by reductases from all three classes of multicomponent nonheme iron oxygenases was examined. The class I and II reductases, modules of which show significant sequence similarity with and which belong to the ferredoxin-NADP+ reductase family of flavin-dependent oxidoreductases, transferred the pro-R hydrogen. By contrast, the class II enzymes, which do not show significant sequence similarity to the class I and III enzymes but modules of which belong to the glutathione reductase family of flavoenzymes, transferred the pro-S hydrogen.

Site-directed mutagenesis of the 2-haloalkanoic acid dehalogenase I gene from Pseudomonas sp. strain CBS3 and its effect on catalytic activity

Two 2-haloalkanoic acid dehalogenases from Pseudomonas sp. strain CBS3 catalyse hydrolytic dehalogenation of chloroacetate and 2-chloropropionate. We used site-directed mutagenesis to introduce specific changes in the dehalogenase I encoding gene (dehCI). Substitution of Asp-10 by Ala-10 resulted in complete loss of dehalogenating activity although expression of the 2-haloacid dehalogenase I was not affected in the mutant as shown by western blot analysis, and although comparison of the mutated enzyme with the wild type enzyme indicated that extensive rearrangements in the three-dimensional structure of the enzyme had not occurred. From these data we suggest that Asp-10 of 2-haloacid dehalogenases I from Pseudomonas sp. strain CBS3 may be the nucleophilic residue in the active-site of this enzyme essential for halide release.

Molecular mechanisms of genetic adaptation to xenobiotic compounds

Microorganisms in the environment can often adapt to use xenobiotic chemicals as novel growth and energy substrates. Specialized enzyme systems and metabolic pathways for the degradation of man-made compounds such as chlorobiphenyls and chlorobenzenes have been found in microorganisms isolated from geographically separated areas of the world. The genetic characterization of an increasing number of aerobic pathways for degradation of (substituted) aromatic compounds in different bacteria has made it possible to compare the similarities in genetic organization and in sequence which exist between genes and proteins of these specialized catabolic routes and more common pathways. These data suggest that discrete modules containing clusters of genes have been combined in different ways in the various catabolic pathways. Sequence information further suggests divergence of catabolic genes coding for specialized enzymes in the degradation of xenobiotic chemicals. An important question will be to find whether these specialized enzymes evolved from more common isozymes only after the introduction of xenobiotic chemicals into the environment. Evidence is presented that a range of genetic mechanisms, such as gene transfer, mutational drift, and genetic recombination and transposition, can accelerate the evolution of catabolic pathways in bacteria. However, there is virtually no information concerning the rates at which these mechanisms are operating in bacteria living in nature and the response of such rates to the presence of potential (xenobiotic) substrates. Quantitative data on the genetic processes in the natural environment and on the effect of environmental parameters on the rate of evolution are needed.

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.

Purification and characterization of 2-halocarboxylic acid dehalogenase II from Pseudomonas spec. CBS 3

2-Halocarboxylic acid dehalogenase II from Pseudomonas spec. CBS 3 (EC 3.8.1.2), which had been cloned in E. coli Hb 101 was purified to electrophoretic homogeneity from crude extracts of E. coli Hb 101 clone 1164. Ammonium sulfate fractionation and three subsequent chromatographic purification steps yielded a pure enzyme in a 230-fold enrichment. The relative molecular masses as determined by gelfiltration on Superose 12 and SDS-polyacrylamide gel electrophoresis were 64,000 Da for the holoenzyme and 29,000 Da for the subunit. The isoelectric point, determined by isoelectric focusing, was at pH 6.2. Substrate specificity towards chlorinated and brominated substrates was limited to short chain monosubstituted 2-halocarboxylic acids. Fluorocompounds were not converted. The reaction proceeded best at a pH above 9.5 and at a reaction temperature of 40-45 degrees C.

Degradation of 1,2,4-Trichloro- and 1,2,4,5-Tetrachlorobenzene by Pseudomonas Strains

Two Pseudomonas sp. strains, capable of growth on chlorinated benzenes as the sole source of carbon and energy, were isolated by selective enrichment from soil samples of an industrial waste deposit. Strain PS12 grew on monochlorobenzene, all three isomeric dichlorobenzenes, and 1,2,4-trichlorobenzene (1,2,4-TCB). Strain PS14 additionally used 1,2,4,5-tetrachlorobenzene (1,2,4,5-TeCB). During growth on these compounds both strains released stoichiometric amounts of chloride ions. The first steps of the catabolism of 1,2,4-TCB and 1,2,4,5-TeCB proceeded via dioxygenation of the aromatic nuclei and furnished 3,4,6-trichlorocatechol. The intermediary cis -3,4,6-trichloro-1,2-dihydroxycyclohexa-3,5-diene (TCB dihydrodiol) formed from 1,2,4-TCB was rearomatized by an NAD + -dependent dihydrodiol dehydrogenase activity, while in the case of 1,2,4,5-TeCB oxidation the catechol was obviously produced by spontaneous elimination of hydrogen chloride from the initially formed 1,3,4,6-tetrachloro-1,2-dihydroxycyclohexa-3,5-diene. Subsequent ortho cleavage was catalyzed by a type II catechol 1,2-dioxygenase producing the corresponding 2,3,5-trichloromuconate which was channeled into the tricarboxylic acid pathway via an ordinary degradation sequence, which in the present case included 2-chloro-3-oxoadipate. From the structure-related compound 2,4,5-trichloronitrobenzene the nitro group was released as nitrite, leaving the above metabolite as 3,4,6-trichlorocatechol. Enzyme activities for the oxidation of chlorobenzenes and halogenated metabolites were induced by both strains during growth on these haloaromatics and, to a considerable extent, during growth of strain PS12 on acetate.

Cloning and characterization of plasmid-encoded genes for the degradation of 1,2-dichloro-, 1,4-dichloro-, and 1,2,4-trichlorobenzene of Pseudomonas sp. strain P51

Pseudomonas sp. strain P51 is able to use 1,2-dichlorobenzene, 1,4-dichlorobenzene, and 1,2,4-trichlorobenzene as sole carbon and energy sources. Two gene clusters involved in the degradation of these compounds were identified on a catabolic plasmid, pP51, with a size of 110 kb by using hybridization. They were further characterized by cloning in Escherichia coli, Pseudomonas putida KT2442, and Alcaligenes eutrophus JMP222. Expression studies in these organisms showed that the upper-pathway genes (tcbA and tcbB) code for the conversion of 1,2-dichlorobenzene and 1,2,4-trichlorobenzene to 3,4-dichlorocatechol and 3,4,6-trichlorocatechol, respectively, by means of a dioxygenase system and a dehydrogenase. The lower-pathway genes have the order tcbC-tcbD-tcbE and encode a catechol 1,2-dioxygenase II, a cycloisomerase II, and a hydrolase II, respectively. The combined action of these enzymes degrades 3,4-dichlorocatechol and 3,4,6-trichlorocatechol to a chloromaleylacetic acid. The release of one chlorine atom from 3,4-dichlorocatechol takes place during lactonization of 2,3-dichloromuconic acid.

Degradation of 2-chlorobenzoate by Pseudomonas cepacia 2CBS

A bacterium was isolated from water by enrichment on 2-chlorobenzoate as sole source of carbon and energy. Based on morphological and physiological properties, this microorganism was assigned to the species Pseudomonas cepacia. The organism was designated Pseudomonas cepacia 2CBS. During growth on 2-chlorobenzoate, the chlorine substituent was released quantitatively, and a small amount of 2,3-dihydroxybenzoate accumulated in the culture medium. Mutants of Pseudomonas cepacia 2CBS were induced by treatment with N-methyl-N'-nitro-N-nitrosoguanidine. Some of these mutants produced catechol from 2-chlorobenzoate. Other mutants accumulated the meta-cleavage product of catechol, 2-hydroxy-cis,cis-muconic acid semialdehyde. In crude cell-free extracts of Pseudomonas cepacia 2CBS, an enzyme was detected which catalysed the conversion of 2-chlorobenzoate to catechol. Molecular oxygen, NADH and exogenous Fe2+ were required for activity. Stoichiometric amounts of chloride were released. Experiments with 18O2 revealed that both oxygen atoms in the hydroxyl groups of the product were derived from molecular oxygen. Thus, the enzyme catalysing the conversion of 2-chlorobenzoate was identified as 2-chlorobenzoate 1,2-dioxygenase (1,2-hydroxylating, dehalogenating, decarboxylating). 2-Chlorobenzoate 1,2-dioxygenase from Pseudomonas cepacia 2CBS was shown to be a multicomponent enzyme system. The activities of catechol 2,3-dioxygenase and catechol 1,2-dioxygenase were detected in crude cell-free extracts. The activity of catechol 2,3-dioxygenase was 60 times higher than the activity of catechol 1,2-dioxygenase, indicating that catechol is mainly degraded via meta-cleavage in Pseudomonas cepacia 2CBS. No enzyme was found which converted 2,3-dihydroxybenzoate, suggesting that this compound is a dead-end metabolite of 2-chlorobenzoate catabolism. A pathway for the degradation of 2-chlorobenzoate by Pseudomonas cepacia 2CBS is proposed.

Enzymatic dehalogenation of pentachlorophenol by extracts from Arthrobacter sp. strain ATCC 33790

Arthrobacter sp. strain ATCC 33790 was grown with pentachlorophenol (PCP) as the sole source of carbon and energy. Crude extracts, which were prepared by disruption of the bacteria with a French pressure cell, showed no dehalogenating activity with PCP as the substrate. After sucrose density ultracentrifugation of the crude extract at 145,000 x g, various layers were found in the gradient. One yellow layer showed enzymatic conversion of PCP. One chloride ion was released per molecule of PCP. The product of the enzymatic conversion was tetrachlorohydroquinone. NADPH and oxygen were essential for this reaction. EDTA stimulated the enzymatic activity by 67%. The optimum pH for the enzyme activity was 7.5, and the temperature optimum was 25 degrees C. Enzymatic activity was also detected with 2,4,5-trichlorophenol, 2,3,4-trichlorophenol, 2,4,6-trichlorophenol, and 2,3,4,5-tetrachlorophenol as substrates, whereas 3,4,5-trichlorophenol, 2,4-dichlorophenol, 3,4-dichlorophenol, and 4-chlorophenol did not serve as substrates.

A novel metabolite in the microbial degradation of 2-chlorobenzoate

A bacterium which utilizes 2-chlorobenzoate as sole source of carbon and energy was isolated from water and tentatively identified as a Pseudomonas sp. During growth on 2-chlorobenzoate, the chlorine substituent was released quantitatively and a metabolite accumulated in the culture medium. Based on thin-layer chromatography, high-pressure liquid chromatography and gas chromatography linked to mass spectrometry this compound was unequivocally identified as 2,3-dihydroxybenzoate, a compound not described before in the degradation of 2-chlorobenzoate.

Microbial enzymes for oxidation of organic molecules

Enzymatic systems employed by microorganisms for oxidative transformation of various organic molecules include laccases, ligninases, tyrosinases, monooxygenases, and dioxygenases. Reactions performed by these enzymes play a significant role in maintaining the global carbon cycle through either transformation or complete mineralization of organic molecules. Additionally, oxidative enzymes are instrumental in modification or degradation of the ever-increasing man-made chemicals constantly released into our environment. Due to their inherent stereo- and regioselectivity and high efficiency, oxidative enzymes have attracted attention as potential biocatalysts for various biotechnological processes. Successful commercial application of these enzymes will be possible through employing new methodologies, such as use of organic solvents in the reaction mixtures, immobilization of either the intact microorganisms or isolated enzyme preparations on various supports, and genetic engineering technology.