Archaebacterial Citrate Synthases: The Enzymes from the Thermoacidophiles Sulfolobus acidocaldarius and Thermoplasma acidophilum Show pro-S Stereospecificity

Citrate synthase from the thermoacidophilic archaebacterium Sulfolobus acidocaldarius was purified 365-fold to electrophoretic homogeneity. At 40 °C and pH 8.1 the homogeneous enzyme shows a specific activity of 73 units per mg, which corresponds to a turnover number of 44 sec-1. Citrate synthase from S. acidocaldarius shows pro-S stereospecificity, as is found with a partially purified preparation of the enzyme from Thermoplasma acidophilum, another thermoacidophilic archaebacterium.

A complex regulatory network controls aerobic ethanol oxidation in Pseudomonas aeruginosa: indication of four levels of sensor kinases and response regulators

In addition to the known response regulator ErbR (former AgmR) and the two-component regulatory system EraSR (former ExaDE), three additional regulatory proteins have been identified as being involved in controlling transcription of the aerobic ethanol oxidation system in Pseudomonas aeruginosa. Two putative sensor kinases, ErcS and ErcS', and a response regulator, ErdR, were found, all of which show significant similarity to the two-component flhSR system that controls methanol and formaldehyde metabolism in Paracoccus denitrificans. All three identified response regulators, EraR (formerly ExaE), ErbR (formerly AgmR) and ErdR, are members of the luxR family. The three sensor kinases EraS (formerly ExaD), ErcS and ErcS' do not contain a membrane domain. Apparently, they are localized in the cytoplasm and recognize cytoplasmic signals. Inactivation of gene ercS caused an extended lag phase on ethanol. Inactivation of both genes, ercS and ercS', resulted in no growth at all on ethanol, as did inactivation of erdR. Of the three sensor kinases and three response regulators identified thus far, only the EraSR (formerly ExaDE) system forms a corresponding kinase/regulator pair. Using reporter gene constructs of all identified regulatory genes in different mutants allowed the hierarchy of a hypothetical complex regulatory network to be established. Probably, two additional sensor kinases and two additional response regulators, which are hidden among the numerous regulatory genes annotated in the genome of P. aeruginosa, remain to be identified.

The PQQ biosynthetic operons and their transcriptional regulation in Pseudomonas aeruginosa

Gene PA1990 of Pseudomonas aeruginosa, located downstream of pqqE and encoding a putative peptidase, was shown to be involved in excretion of PQQ into the culture supernatant. This gene is cotranscribed with the pqqABCDE cluster and was named pqqH. A PA1990::Km(r) mutant (VK3) did not show any effect in growth behaviour; however, in contrast to the wild-type, no excretion of PQQ into the culture supernatant was observed. The putative pqqF gene of P. aeruginosa was shown to be essential for PQQ biosynthesis. A pqqF::Km(r) mutant did not grow aerobically on ethanol, because of its inability to produce PQQ. Transcription of the pqqABCDEH operon was induced upon aerobic growth on ethanol, 1-propanol, 1,2-propanediol and 1-butanol, while on glycerol, succinate and acetate, transcription was low. Transcription of the pqqABCDEH operon was also found upon anoxic growth on ethanol with nitrate as electron acceptor, but no PQQ was produced. Expression of the pqqABCDEH operon is regulated at the transcriptional level. In contrast, the pqqF operon appeared to be transcribed constitutively at a very low level under all growth conditions studied.

Function and transcriptional regulation of the isocitrate lyase in Pseudomonas aeruginosa

Pseudomonas aeruginosa ATCC 17933 is capable of growing aerobically on ethanol as sole source of carbon and energy. This requires the glyoxylate cycle for replenishing C4-compounds to the TCA cycle. The enzyme isocitrate lyase (ICL) catalyzes the first step of this glyoxylate shunt. Its activity was induced more than 10-fold in response to the carbon sources ethanol or acetate instead of glucose or succinate. We could prove that in P. aeruginosa ICL is essential for aerobic as well as anaerobic utilization of C2-sources. Transcriptional regulation of icl gene (aceA) expression was monitored on different carbon sources by using an aceA-lacZ gene fusion. A strong correlation between promoter and ICL activity indicated regulation at the transcriptional level. But ICL was not simply induced by the mere presence of ethanol in the growth medium as was demonstrated by cultivation on mixed substrates. P. aeruginosa showed diauxic growth on media containing ethanol-succinate or ethanol-glucose mixtures and did not transcribe the aceA gene to metabolize ethanol until succinate or glucose, respectively, were exhausted. Inactivation of the chromosomal aceA gene in P. aeruginosa led to an inability to grow on ethanol and acetate. Promoter activity studies showed that all genes necessary to oxidize ethanol were downregulated in the ICL-negative mutant. But on mixed substrates like ethanol-succinate or ethanol-glucose the mutant exhibited growth and utilized ethanol as well, probably as energy source only.

Growth of Dehalococcoides strains with chlorophenols as electron acceptors

Dehalococcoides strains reductively dechlorinate a wide variety of halogenated compounds including chlorinated benzenes, biphenyls, naphthalenes, dioxins, and ethenes. Recent genome sequencing of the two Dehalococcoides strains CBDB1 and 195 revealed the presence of 32 and 18 reductive dehalogenase homologous genes, respectively, and therefore suggested an even higher dechlorinating potential than previously anticipated. Here, we demonstrate reductive dehalogenation of chlorophenol congeners by Dehalococcoides strains CBDB1 and 195. Strain CBDB1 completely converted 2,3-dichlorophenol, all six trichlorophenols, all three tetrachlorophenols, and pentachlorophenol to lower chlorinated phenols. Observed dechlorination rates in batch cultures with cell numbers of 10(7) mL(-1) amounted up to 35 microM day(-1). Chlorophenols were preferentially dechlorinated in the ortho position, but also doubly flanked and singly flanked meta- or para-chlorine substituents were removed. We used a newly designed computer-assisted direct cell counting protocol and quantitative PCR to demonstrate that strain CBDB1 uses chlorophenols as electron acceptors for respiratory growth. The growth yield of strain CBDB1 with 2,3-dichlorophenol was 7.6 x 10(13) cells per mol of Cl- released, and the growth rate was 0.41 day(-1). For strain 195, fast ortho dechlorination of 2,3-dichlorophenol, 2,3,4-trichlorophenol, and 2,3,6-trichlorophenol was detected, with only the ortho chlorine removed. Because chlorinated phenolic compounds are widely distributed as natural components in anaerobic environments, our results reveal one mode in which the Dehalococcoides species could have survived through earth history.

Identification of membrane-bound quinoprotein inositol dehydrogenase in Gluconobacter oxydans ATCC 621H

The GOX1857 gene, which encodes a putative membrane-bound pyrroloquinoline quinone (PQQ)-dependent dehydrogenase in Gluconobacter oxydans ATCC 621H, was characterized. GOX1857 was disrupted and the oxidizing potential of the resulting mutant strain was compared to that of the wild-type. In contrast to the wild-type, the mutant was unable to grow with myo-inositol as the sole energy source and did not show any myo-inositol dehydrogenase activity in vitro, indicating that GOX1857 encodes an inositol dehydrogenase. The association of inositol dehydrogenase with the membrane and the requirement for the cofactor PQQ were confirmed. Inositol dehydrogenase exhibited optimal activity at pH 8.75. As indicated by cultivation on different substrates, inositol dehydrogenase was repressed by d-glucose.

Modification of the membrane-bound glucose oxidation system in Gluconobacter oxydans significantly increases gluconate and 5-keto-D-gluconic acid accumulation

Gluconobacter oxydans DSM 2343 (ATCC 621H)catalyzes the oxidation of glucose to gluconic acid and subsequently to 5-keto-D-gluconic acid (5-KGA), a precursor of the industrially important L-(+)-tartaric acid. To further increase 5-KGA production in G. oxydans, the mutant strain MF1 was used. In this strain the membrane-bound gluconate-2-dehydrogenase activity, responsible for formation of the undesired by-product 2-keto-D-gluconic acid, is disrupted. Therefore, high amounts of 5-KGA accumulate in the culture medium. G. oxydans MF1 was equipped with plasmids allowing the overexpression of the membrane-bound enzymes involved in 5-KGA formation. Overexpression was confirmed on the transcript and enzymatic level. Furthermore, the resulting strains overproducing the membrane-bound glucose dehydrogenase showed an increased gluconic acid formation, whereas the overproduction of gluconate-5-dehydrogenase resulted in an increase in 5-KGA of up to 230 mM. Therefore, these newly developed recombinant strains provide a basis for further improving the biotransformation process for 5-KGA production.

Knockout and overexpression of pyrroloquinoline quinone biosynthetic genes in Gluconobacter oxydans 621H

In Gluconobacter oxydans, pyrroloquinoline quinone (PQQ) serves as the cofactor for various membrane-bound dehydrogenases that oxidize sugars and alcohols in the periplasm. Proteins for the biosynthesis of PQQ are encoded by the pqqABCDE gene cluster. Our reverse transcription-PCR and promoter analysis data indicated that the pqqA promoter represents the only promoter within the pqqABCDE cluster of G. oxydans 621H. PQQ overproduction in G. oxydans was achieved by transformation with the plasmid-carried pqqA gene or the complete pqqABCDE cluster. A G. oxydans mutant unable to produce PQQ was obtained by site-directed disruption of the pqqA gene. In contrast to the wild-type strain, the pqqA mutant did not grow with d-mannitol, d-glucose, or glycerol as the sole energy source, showing that in G. oxydans 621H, PQQ is essential for growth with these substrates. Growth of the pqqA mutant, however, was found with d-gluconate as the energy source. The growth behavior of the pqqA mutant correlated with the presence or absence of the respective PQQ-dependent membrane-bound dehydrogenase activities, demonstrating the vital role of these enzymes in G. oxydans metabolism. A different PQQ-deficient mutant was generated by Tn5 transposon mutagenesis. This mutant showed a defect in a gene with high homology to the Escherichia coli tldD gene, which encodes a peptidase. Our results indicate that the tldD gene in G. oxydans 621H is involved in PQQ biosynthesis, possibly with a similar function to that of the pqqF genes found in other PQQ-synthesizing bacteria.

Substrate binding in quinoprotein ethanol dehydrogenase from Pseudomonas aeruginosa studied by electron-nuclear double resonance

Binding of methanol to the quinoprotein ethanol dehydrogenase from Pseudomonas aeruginosa has been studied by pulsed electron-nuclear double resonance at 9 GHz. Shifts in the hyperfine couplings of the pyrroloquinoline quinone radical provide direct evidence for a change in the environment of the cofactor when substrate is present. By performing experiments with deuteriated methanol, we confirmed that methanol was the cause of the effect. Density functional theory calculations show that these shifts can be understood if a water molecule, which is often found in x-ray structures of the active site of quinoprotein alcohol dehydrogenases, is displaced by the substrate. The difference between the binding of water and methanol is that the water molecule forms a hydrogen bond to O5 of pyrroloquinoline quinone, which the methanol, by virtue of its methyl group, does not. The results support the proposal that aspartate rather than glutamate is the catalytically active base for a hydride transfer mechanism in quinoprotein alcohol dehydrogenases.

Structure of the Pyrroloquinoline Quinone Radical in Quinoprotein Ethanol Dehydrogenase

Quinoprotein alcohol dehydrogenases use the pyrroloquinoline quinone (PQQ) cofactor to catalyze the oxidation of alcohols. The catalytic cycle is thought to involve a hydride transfer from the alcohol to the oxidized PQQ, resulting in the generation of aldehyde and reduced PQQ. Reoxidation of the cofactor by cytochrome proceeds in two sequential steps via the PQQ radical. We have used a combination of electron nuclear double resonance and density functional theory to show that the PQQ radical is not protonated at either O-4 or O-5, a result that is at variance with the general presumption of a singly protonated radical. The quantum mechanical calculations also show that reduced PQQ is unlikely to be protonated at O-5; rather, it is either singly protonated at O-4 or not protonated at either O-4 or O-5, a result that also challenges the common assumption of a reduced PQQ protonated at both O-4 and O-5. The reaction cycle of PQQ-dependent alcohol dehydrogenases is revised in light of these findings.

Preferential attack of the (S)-configured ether-linked carbons in bis-(1-chloro-2-propyl) ether by Rhodococcus sp. strain DTB

Rhodococcus sp. strain DTB (DSM 44534) was grown on a mixture of (R,R)-, (S,S)- and meso-bis-(1-chloro-2-propyl) ether (BCPE) as the sole source of carbon and energy. During BCPE degradation 1'-chloro-2'-propyl-3-chloro-2-prop-1-enyl-ether (DVE), 1-chloro-2-propanol and chloroacetone intermediates were formed. The BCPE or DVE stereoisomers were metabolized in consecutive order via scission of the ether bond, with discrimination against the (R) configuration. Resting cell suspensions of Rhodococcus pregrown on BCPE showed a preferential attack of the (S)-configured ether-linked carbons, resulting in an enantioselective enrichment of (R,R)-BCPE. Microbial discrimination of BCPE or DVE isomers and chemical conversion of the intermediates to 1-chloro-2-propanol allowed the identification of the configuration of all BCPE isomers and the DVE enantiomers. Elucidation of the absolute configuration of the 1-chloro-2-propanol isomers was achieved by enantioselective chemical synthesis.

Transient accumulation of γ-butyrolactone during degradation of bis(4-chloro-n-butyl) ether by diethylether-grown Rhodococcus sp. strain DTB

Rhodococcus sp. strain DTB (DSM 44534) grows aerobically on diethylether as sole source of carbon and energy. Dense cell suspension experiments showed that the induced ether-cleaving enzyme system attacks a broad range of ethers like tetrahydrofuran, phenetole and chlorinated alkylethers including Calpha-substituted alkylethers. Identification of metabolites revealed that degradation of the ethers started by an initial attack of the ether bond. Diethylether-grown cells degraded bis(4-chloro-n-butyl) ether via an initial ether scission followed by the transient accumulation of gamma-butyrolactone as intermediate at nearly stoichiometric concentrations.

Substrate-Binding in Quinoprotein Ethanol Dehydrogenase from Pseudomonas aeruginosa Studied by Electron Paramagnetic Resonance at 94 GHz

Pyrroloquinoline quinone (2,7,9-tricarboxypyrroloquinoline quinone, PQQ) is one of several quinone cofactors that is utilized in a class of dehydrogenases known as quinoproteins. In this contribution, we have used continuous-wave high-field/high-frequency electron paramagnetic resonance (EPR) at 94 GHz (W-band) to study substrate binding in ethanol dehydrogenase (QEDH) from Pseudomonas aeruginosa, taking advantage of the fact that the enzyme is isolated with a substantial proportion of the PQQ cofactor in the paramagnetic semiquinone form. In the substrate-free enzyme, the principal values of the g-tensor, obtained by spectral simulation are: gx = 2.00585(2), gy = 2.00518(2), and gz = 2.00212(2), giving giso = 2.00438(2). All three principal values of the g-tensor decrease when ethanol is bound to the protein: gx = 2.00574(2), gy = 2.00511(2), and gz = 2.00207(2), giving giso = 2.00431(2). The results represent the first direct evidence for the tight binding of an alcohol to a PQQ-dependent alcohol dehydrogenase and show that ethanol also binds to the enzyme even when the PQQ cofactor is in the semiquinone form. The decrease in g is consistent with an increase in polarity in the immediate vicinity of the PQQ cofactor and probably reflects a changed geometry of the PQQ-Ca2+ complex when ethanol binds.

Multiple nonidentical reductive-dehalogenase-homologous genes are common in Dehalococcoides

Degenerate primers were used to amplify large fragments of reductive-dehalogenase-homologous (RDH) genes from genomic DNA of two Dehalococcoides populations, the chlorobenzene- and dioxin-dechlorinating strain CBDB1 and the trichloroethene-dechlorinating strain FL2. The amplicons (1,350 to 1,495 bp) corresponded to nearly complete open reading frames of known reductive dehalogenase genes and short fragments (approximately 90 bp) of genes encoding putative membrane-anchoring proteins. Cloning and restriction analysis revealed the presence of at least 14 different RDH genes in each strain. All amplified RDH genes showed sequence similarity with known reductive dehalogenase genes over the whole length of the sequence and shared all characteristics described for reductive dehalogenases. Deduced amino acid sequences of seven RDH genes from strain CBDB1 were 98.5 to 100% identical to seven different RDH genes from strain FL2, suggesting that both strains have an overlapping substrate range. All RDH genes identified in strains CBDB1 and FL2 were related to the RDH genes present in the genomes of Dehalococcoides ethenogenes strain 195 and Dehalococcoides sp. strain BAV1; however, sequence identity did not exceed 94.4 and 93.1%, respectively. The presence of RDH genes in strains CBDB1, FL2, and BAV1 that have no orthologs in strain 195 suggests that these strains possess dechlorination activities not present in strain 195. Comparative sequence analysis identified consensus sequences for cobalamin binding in deduced amino acid sequences of seven RDH genes. In conclusion, this study demonstrates that the presence of multiple nonidentical RDH genes is characteristic of Dehalococcoides strains.

Studies on hydrogenase activity and chlorobenzene respiration in Dehalococcoides sp. strain CBDB1

Hydrogen oxidation and electron transport were studied in the chlorobenzene-utilizing anaerobe Dehalococcoides sp. strain CBDB1. While Cu(2+) and Hg(2+) ions irreversibly inhibited hydrogenase activity in intact cells, Ni(2+) ions inhibited reversibly. About 80% of the initial hydrogenase activity was inactivated within 30 s when the cells were exposed to air. In contrast, hydrogenase was active at a redox potential of +10 mV when this redox potential was established anoxically with a redox indicator. Viologen dyes served both as electron acceptor for hydrogenase and electron donor for the dehalogenase. A menaquinone analogue, 2,3-dimethyl 1,4-naphthoquinone, served neither as electron acceptor for the hydrogenase nor as electron donor for the dehalogenase. In addition, the menaquinone antagonist 2-n-heptyl-4-hydroxyquinoline-N-oxide had no effect on dechlorination catalyzed by cell suspensions or isolated membranes with hydrogen as electron donor, lending further support to the notion that menaquinone is not involved in electron transport. The ionophores tetrachlorosalicylanilide and carbonylcyanide m-chlorophenylhydrazone did not inhibit dechlorination by cell suspensions, indicating that strain CBDB1 does not require reverse electron transport. The ATP-synthase inhibitor N,N'-dicyclohexylcarbodiimide inhibited the dechlorination reaction with cell suspensions; however, the latter effect was partially relieved by the addition of tetrachlorosalicylanilide. 1,2,3,4-tetrachlorobenzene strongly inhibited dechlorination of other chlorobenzenes by cell suspensions with hydrogen as electron donor, but it did not interfere with either hydrogenase or dehalogenase activity.

A Gluconobacter oxydans mutant converting glucose almost quantitatively to 5-keto-D-gluconic acid

Gluconobacter oxydans converts glucose to gluconic acid and subsequently to 2-keto-D-gluconic acid (2-KGA) and 5-keto-D-gluconic acid (5-KGA) by membrane-bound periplasmic pyrroloquinoline quinone-dependent and flavin-dependent dehydrogenases. The product pattern obtained with several strains differed significantly. To increase the production of 5-KGA, which can be converted to industrially important L-(+)-tartaric acid, growth parameters were optimized. Whereas resting cells of G. oxydans ATCC 621H converted about 11% of the available glucose to 2-KGA and 6% to 5-KGA, with growing cells and improved growth under defined conditions (pH 5, 10% pO2, 0.05% pCO2) a conversion yield of about 45% 5-KGA from the available glucose was achieved. As the accumulation of the by-product 2-KGA is highly disadvantageous for an industrial application of G. oxydans, a mutant was generated in which the membrane-bound gluconate-2-dehydrogenase complex was inactivated. This mutant, MF1, grew in a similar way to the wild type, but formation of the undesired 2-KGA was not observed. Under improved growth conditions, mutant MF1 converted the available glucose almost completely (84%) into 5-KGA. Therefore, this newly developed recombinant strain is suitable for the industrial production of 5-KGA.

AgmR controls transcription of a regulon with several operons essential for ethanol oxidation in Pseudomonas aeruginosa ATCC 17933

The response regulator AgmR was identified to be involved in the regulation of the quinoprotein ethanol oxidation system of Pseudomonas aeruginosa ATCC 17933. Interruption of the agmR gene by insertion of a kanamycin-resistance cassette resulted in mutant NG3, unable to grow on ethanol. After complementation with the intact agmR gene, growth on ethanol was restored. Transcriptional lacZ fusions were used to identify four operons which are regulated by the AgmR protein: the exaA operon encodes the pyrroloquinoline quinone (PQQ)-dependent ethanol dehydrogenase, the exaBC operon encodes a soluble cytochrome c(550) and an aldehyde dehydrogenase, the pqqABCDE operon carries the PQQ biosynthetic genes, and operon exaDE encodes a two-component regulatory system which controls transcription of the exaA operon. Transcription of exaA was restored by transformation of NG3 with a pUCP20T derivative carrying the exaDE genes under lac-promoter control. These data indicate that the AgmR response regulator and the exaDE two-component regulatory system are organized in a hierarchical manner. Gene PA1977, which appears to form an operon with the agmR gene, was found to be non-essential for growth on ethanol.

Characterisation of the PQQ cofactor radical in quinoprotein ethanol dehydrogenase of Pseudomonas aeruginosa by electron paramagnetic resonance spectroscopy

The binding pocket of the pyrroloquinoline quinone (PQQ) cofactor in quinoprotein alcohol dehydrogenases contains a characteristic disulphide ring formed by two adjacent cysteine residues. To analyse the function of this unusual structural motif we have investigated the wild-type and a double cysteine:alanine mutant of the quinoprotein ethanol dehydrogenase from Pseudomonas aeruginosa by electron paramagnetic resonance (EPR) spectroscopy. Thus, we have obtained the principal values for the full rhombic g-tensor of the PQQ semiquinone radical by high-field (94 GHz) EPR necessary for a discrimination of radical species in dehydrogenases containing PQQ together with other redox-active cofactors. Our results show that the characteristic disulphide ring is no prerequisite for the formation of the functionally important semiquinone form of PQQ.

Dehalorespiration with hexachlorobenzene and pentachlorobenzene by Dehalococcoides sp. strain CBDB1

The chlororespiring anaerobe Dehalococcoides sp. strain CBDB1 used hexachlorobenzene and pentachlorobenzene as electron acceptors in an energy-conserving process with hydrogen as electron donor. Previous attempts to grow Dehalococcoides sp. strain CBDB1 with hexachlorobenzene or pentachlorobenzene as electron acceptors failed if these compounds were provided as solutions in hexadecane. However, Dehalococcoides sp. strain CBDB1 was able to grow with hexachlorobenzene or pentachlorobenzene when added in crystalline form directly to cultures. Growth of Dehalococcoides sp. strain CBDB1 by dehalorespiration resulted in a growth yield ( Y) of 2.1+/-0.24 g protein/mol Cl(-) released with hexachlorobenzene as electron acceptor; with pentachlorobenzene, the growth yield was 2.9+/-0.15 g/mol Cl(-). Hexachlorobenzene was reductively dechlorinated to pentachlorobenzene, which was converted to a mixture of 1,2,3,5- and 1,2,4,5-tetrachlorobenzene. Formation of 1,2,3,4-tetrachlorobenzene was not detected. The final end-products of hexachlorobenzene and pentachlorobenzene dechlorination were 1,3,5-trichlorobenzene, 1,3- and 1,4-dichlorobenzene, which were formed in a ratio of about 3:2:5. As reported previously, Dehalococcoides sp. strain CBDB1 converted 1,2,3,5-tetrachlorobenzene exclusively to 1,3,5-trichlorobenzene, and 1,2,4,5-tetrachlorobenzene exclusively to 1,2,4-trichlorobenzene. The organism therefore catalyzes two different pathways to dechlorinate highly chlorinated benzenes. In the route leading to 1,3,5-trichlorobenzene, only doubly flanked chlorine substituents were removed, while in the route leading to 1,3-and 1,4-dichlorobenzene via 1,2,4-trichlorobenzene singly flanked chlorine substituents were also removed. Reductive dehalogenase activity measurements using whole cells pregrown with different chlorobenzene congeners as electron acceptors indicated that different reductive dehalogenases might be induced by the different electron acceptors. To our knowledge, this is the first report describing reductive dechlorination of hexachlorobenzene and pentachlorobenzene via dehalorespiration by a pure bacterial culture.

Reductive dehalogenation of chlorobenzene congeners in cell extracts of Dehalococcoides sp. strain CBDB1

Enzymatic reductive dehalogenation of tri-, tetra-, penta-, and hexachlorobenzenes was demonstrated in cell extracts with low protein concentration (0.5 to 1 micro g of protein/ml) derived from the chlorobenzene-respiring anaerobe Dehalococcoides sp. strain CBDB1. 1,2,3-trichlorobenzene dehalogenase activity was associated with the membrane fraction. Light-reversible inhibition by alkyl iodides indicated the presence of a corrinoid cofactor.

The ethanol oxidation system and its regulation in Pseudomonas aeruginosa

Pseudomonas aeruginosa ATCC 17933, when growing on ethanol, uses a pyrroloquinoline quinone (PQQ)-dependent ethanol oxidation system. The genes coding for the ethanol oxidizing enzyme, a quinoprotein ethanol dehydrogenase (QEDH), cytochrome c(550), which is an essential component of the electron transport chain and accepts the electrons from QEDH, and an NAD-dependent acetaldehyde dehydrogenase form the exaABC gene cluster. Downstream of the exaBC genes the pqqABCDE gene cluster is found, which codes for proteins essential for biosynthesis of the cofactor PQQ. Also essential for growth on ethanol are an acetyl-CoA synthetase encoded by the acsA gene and a malate:quinone oxidoreductase encoded by the mqo gene. The X-ray structure of the soluble QEDH from P. aeruginosa was solved. It is a homodimeric enzyme and, aside from differences in some loops, the folding of QEDH is very similar to the large subunit of the soluble methanol dehydrogenase of methylotrophs, and the PQQ domain of the quinohemoprotein alcohol dehydrogenase from Comamonas testosteroni and P. fluorescens. Transcription from the QEDH (exaA) promoter is regulated by a two component system: a histidine sensor kinase (ExaD), which is presumably located in the cytoplasm, and a response regulator (ExaE). The phenotypic characterization and transcription studies with six regulatory mutants indicate that seven different genes in an hierarchical organization may be involved in regulating the transcription of the ethanol oxidation system and components of acetate metabolism in P. aeruginosa.

Biodegradation of bis(1-chloro-2-propyl) ether via initial ether scission and subsequent dehalogenation by Rhodococcus sp. strain DTB

Rhodococcus sp. strain DTB (DSM 44534) grows on bis(1-chloro-2-propyl) ether (DDE) as sole source of carbon and energy. The non-chlorinated diisopropyl ether and bis(1-hydroxy-2-propyl) ether, however, did not serve as substrates. In ether degradation experiments with dense cell suspensions, 1-chloro-2-propanol and chloroacetone were formed, which indicated that scission of the ether bond is the first step while dehalogenation of the chlorinated C(3)-compounds occurs at a later stage of the degradation pathway. Inhibition of ether scission by methimazole suggested that the first step in degradation is catalyzed by a flavin-dependent enzyme activity. The non-chlorinated compounds 1,2-propanediol, hydroxyacetone, lactate, pyruvate, 1-propanol, propanal, and propionate also supported growth, which suggested that the intermediates 1,2-propanediol and hydroxyacetone are converted to pyruvate or to propionate, which can be channeled into the citric acid cycle by a number of routes. Total release of chloride and growth-yield experiments with DDE and non-chlorinated C(3)-compounds suggested complete biodegradation of the chlorinated ether.

Reductive dehalogenation of chlorinated dioxins by an anaerobic bacterium

Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs and PCDFs) are among the most notorious environmental pollutants. Some congeners, particularly those with lateral chlorine substitutions at positions 2, 3, 7 and 8, are extremely toxic and carcinogenic to humans. One particularly promising mechanism for the detoxification of PCDDs and PCDFs is microbial reductive dechlorination. So far only a limited number of phylogenetically diverse anaerobic bacteria have been found that couple the reductive dehalogenation of chlorinated compounds--the substitution of a chlorine for a hydrogen atom--to energy conservation and growth in a process called dehalorespiration. Microbial dechlorination of PCDDs occurs in sediments and anaerobic mixed cultures from sediments, but the responsible organisms have not yet been identified or isolated. Here we show the presence of a Dehalococcoides species in four dioxin-dechlorinating enrichment cultures from a freshwater sediment highly contaminated with PCDDs and PCDFs. We also show that the previously described chlorobenzene-dehalorespiring bacterium Dehalococcoides sp. strain CBDB1 (ref. 3) is able to reductively dechlorinate selected dioxin congeners. Reductive dechlorination of 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD) demonstrates that environmentally significant dioxins are attacked by this bacterium.

Malate:quinone oxidoreductase is essential for growth on ethanol or acetate in Pseudomonas aeruginosa

Pseudomonas aeruginosa ATCC 17933 growing aerobically on ethanol uses a pyrroloquinoline quinone-dependent ethanol oxidation system. A mutant with an interrupted putative mqo gene, in which malate:quinone oxidoreductase (MQO), an enzyme involved in the citric acid cycle/glyoxylate cycle, was defective, showed a severe growth defect on ethanol and was unable to grow on acetate. Glucose, lactate, succinate or malate supported growth of the mutant. However, an NAD-dependent malate dehydrogenase activity could not be detected. Complementation of the mutant by the wild-type allele of the mqo gene restored wild-type behaviour. The wild-type expressed the dye-dependent MQO and NAD(P)-dependent malic enzymes (MEs). Pyruvate carboxylase (PC) was found upon growth of the wild-type and the mutant on all substrates studied. PC activity in the wild-type was induced on glucose and lactate and was always higher on all substrates in the mqo mutant. In P. aeruginosa ATCC 17933, an active MQO is required for growth on ethanol or acetate, while with glucose, lactate, succinate or malate an apparent bypass route operates, with MEs using malate for generating pyruvate, which is carboxylated to oxaloacetate by PC. To the authors' knowledge, this is the first time that a specific mutant MQO phenotype has been observed, caused by the inactivation of a gene encoding MQO activity. mqo of P. aeruginosa ATCC 17933 corresponds to mqoB (PA4640) of the P. aeruginosa PAO1 genome project.

Microbial transformation of chlorinated benzenes under anaerobic conditions

Chlorobenzenes are reductively dechlorinated by anaerobic bacterial cultures obtained from sediments and sludge. Recently a strain was isolated that couples reductive dechlorination of chlorobenzenes with energy conservation. The results reviewed in this article suggest that additional anaerobic bacteria, thriving by dehalogenation of chlorobenzenes or chlorobiphenylic compounds, can be isolated.

The Pseudomonas aeruginosa acsA gene, encoding an acetyl-CoA synthetase, is essential for growth on ethanol

Pseudomonas aeruginosa ATCC 17933 uses a pyrroloquinoline quinone-dependent ethanol oxidation system. Two mutants of P. aeruginosa, unable to grow on ethanol and showing no acetyl-CoA synthetase (ACS) activity under standard test conditions, were complemented by cosmid pTB3018. Subcloning led to the isolation of a gene which encodes a protein with high similarity to acetyl-CoA synthetases. Interruption of the putative acsA gene by a kanamycin-resistance cassette resulted in a mutant also unable to grow on ethanol and with very low residual acetyl-CoA-forming activity. Complementation by the wild-type allele of the acsA gene restored growth and led to the expression of ACS activity in excess of that of wild-type cells. In wild-type P. aeruginosa, ACS activity was induced upon growth on ethanol, 2,3-butanediol, malonate and acetate. The wild-type and mutants defective in ACS activity showed an active acetate kinase (ACK) under the growth conditions used; however, phosphotransacetylase (PTA) could not be detected. The data indicate that P. aeruginosa requires active acsA gene product for growth on ethanol.

Transformation of 2,2'-dichlorodiisopropyl ether in mixed and pure culture

An aerobic enrichment culture derived from a groundwater contaminated with organic and chloroorganic compounds was adapted to the transformation of 2,2'-dichlorodiisopropyl ether (DDE) in a continuous fixed-bed bioreactor. Continuous DDE removal efficiencies over 90% were achieved with a model water containing 3.3 mM methanol as co-substrate at DDE loading rates of up to 150 micromol l(-1) day(-1) with a hydraulic retention time of 24 h. In batch cultures, a stoichiometric release of 2 micromol chloride per micromol DDE transformed was observed. From the mixed culture, a strain was isolated that is able to grow on DDE as the sole energy and carbon source, tolerating DDE concentrations of up to 1 mM. Based on 16S rRNA sequencing, the strain is affiliated with the genus Rhodococcus.

Bacterial growth based on reductive dechlorination of trichlorobenzenes

An anaerobic mixed bacterial culture was enriched for bacteria dechlorinating 1,2,3- and 1,2,4-trichlorobenzene (TCB) to dichlorobenzenes by exclusive use of non-fermentable substrates and the application of vancomycin. Growth and dechlorination occurred in a purely synthetic medium with formate or hydrogen, acetate, and TCB. Neither acetogenesis nor methanogenesis was detected in the culture. Repeated subculturing maintaining high dechlorinating activities was also achieved when only hydrogen and TCB were supplied. This indicated that reductive dechlorination of TCB was the primary energy conservating process. The number of dechlorinating bacteria was strictly limited by the amount of TCB supplied in the medium. In addition, the dechlorinating activity could be maintained only in the presence of TCB. A most probable number analysis showed that the dechlorinating species amounted to at least 6 x 10(5) cells per ml at a total cell number of about 2 x 10(6) cells per ml. Vitamin B12 significantly stimulated the dechlorinating activity.

A soluble two-component regulatory system controls expression of quinoprotein ethanol dehydrogenase (QEDH) but not expression of cytochrome c(550) of the ethanol-oxidation system in Pseudomonas aeruginosa

The regulation of the divergent promoters of the exaAB genes in Pseudomonas aeruginosa ATCC 17933, in which exaA encodes a quinoprotein ethanol dehydrogenase and exaB codes for a cytochrome c(550), was studied. Using transcriptional lacZ fusions, promoter activity during growth on several substrates was measured. These promoter-probe vectors were also used to identify regulatory mutants defective in exaAB induction. Transcription from both exaA and exaB was reduced significantly in four mutants. Two other mutants showed transcription from exaA that was reduced, but higher than wild-type transcription from exaB. The genes that are needed for exaA promoter induction were sequenced and found to encode a two-component regulatory system: a histidine sensor kinase, which lacks a transmembrane helical N-terminus and is presumably located in the cytoplasm, and a response regulator. The phenotypic characterization and restoration of the wild-type behaviour of the different regulatory mutants produced by different cosmids and subclones indicate that six different genes may be involved in regulating ethanol oxidation in P. aeruginosa.

Bacterial dehalorespiration with chlorinated benzenes

Chlorobenzenes are toxic, highly persistent and ubiquitously distributed environmental contaminants that accumulate in the food chain. The only known microbial transformation of 1,2,3,5-tetrachlorobenzene (TeCB) and higher chlorinated benzenes is the reductive dechlorination to lower chlorinated benzenes under anaerobic conditions observed with mixed bacterial cultures. The lower chlorinated benzenes can subsequently be mineralized by aerobic bacteria. Here we describe the isolation of the oxygen-sensitive strain CBDB1, a pure culture capable of reductive dechlorination of chlorobenzenes. Strain CBDB1 is a highly specialized bacterium that stoichiometrically dechlorinates 1,2,3-trichlorobenzene (TCB), 1,2,4-TCB, 1,2,3,4-TeCB, 1,2,3,5-TeCB and 1,2,4,5-TeCB to dichlorobenzenes or 1,3,5-TCB. The presence of chlorobenzene as an electron acceptor and hydrogen as an electron donor is essential for growth, and indicates that strain CBDB1 meets its energy needs by a dehalorespiratory process. According to their 16S rRNA gene sequences, strain CBDB1, Dehalococcoides ethenogenes and several uncultivated bacteria form a new bacterial cluster, of which strain CBDB1 is the first, so far, to thrive on a purely synthetic medium.

X-ray structure of the quinoprotein ethanol dehydrogenase from Pseudomonas aeruginosa: basis of substrate specificity

The homodimeric enzyme form of quinoprotein ethanol dehydrogenase from Pseudomonas aeruginosa ATCC 17933 crystallizes readily with the space group R3. The X-ray structure was solved at 2.6 A resolution by molecular replacement. Aside from differences in some loops, the folding of the enzyme is very similar to the large subunit of the quinoprotein methanol dehydrogenases from Methylobacterium extorquens or Methylophilus W3A1. Eight W-shaped beta-sheet motifs are arranged circularly in a propeller-like fashion forming a disk-shaped superbarrel. No electron density for a small subunit like that in methanol dehydrogenase could be found. The prosthetic group is located in the centre of the superbarrel and is coordinated to a calcium ion. Most amino acid residues found in close contact with the prosthetic group pyrroloquinoline quinone and the Ca(2+) are conserved between the quinoprotein ethanol dehydrogenase structure and that of the methanol dehydrogenases. The main differences in the active-site region are a bulky tryptophan residue in the active-site cavity of methanol dehydrogenase, which is replaced by a phenylalanine and a leucine side-chain in the ethanol dehydrogenase structure and a leucine residue right above the pyrrolquinoline quinone group in methanol dehydrogenase which is replaced by a tryptophan side-chain. Both amino acid exchanges appear to have an important influence, causing different substrate specificities of these otherwise very similar enzymes. In addition to the Ca(2+) in the active-site cavity found also in methanol dehydrogenase, ethanol dehydrogenase contains a second Ca(2+)-binding site at the N terminus, which contributes to the stability of the native enzyme.

Cytochrome c550 is an essential component of the quinoprotein ethanol oxidation system in Pseudomonas aeruginosa: cloning and sequencing of the genes encoding cytochrome c550 and an adjacent acetaldehyde dehydrogenase

Pseudomonas aeruginosa ATCC 17933 grown aerobically on ethanol produces a soluble cytochrome c550 together with a quinoprotein ethanol dehydrogenase. A 3.2 kb genomic DNA fragment containing the gene encoding cytochrome c550 was cloned and sequenced. Two other complete and two truncated ORFs were also identified. A truncated ORF encoding the quinoprotein ethanol dehydrogenase (exaA) was found upstream of the cytochrome c550 gene (exaB) and in reverse orientation. An ORF encoding a NAD(+)-dependent acetaldehyde dehydrogenase (exaC) was located downstream of the cytochrome c550 gene and in the same orientation. Another ORF showed similarity to the pqqA gene and a truncated ORF similarity to the pqqB gene, both involved in the biosynthesis of the prosthetic group PQQ. The organization of these genes was found to be different from the well-studied methanol oxidation system in methylotrophic bacteria. The deduced amino acid sequence of cytochrome c550 from P. aeruginosa showed some similarity to cytochrome c6 of the alga Chlamydomonas reinhardtii and the haem domain of quinohaemoprotein alcohol dehydrogenases of acetic acid bacteria, but no similarity to the soluble cytochrome cL of the quinoprotein methanol oxidation system of methylotrophs could be detected. A mutant of P. aeruginosa with an interrupted cytochrome c550 gene was unable to grow on ethanol, which proves that cytochrome c550 is an essential component of the ethanol oxidation system in this organism.

Quinoprotein ethanol dehydrogenase of Pseudomonas aeruginosa is a homodimer--sequence of the gene and deduced structural properties of the enzyme

The gene coding for the periplasmic quinoprotein ethanol dehydrogenase of Pseudomonas aeruginosa ATCC 17933 was cloned and sequenced. The deduced amino acid sequence contained a signal peptide of 34 residues and the major protein of 589 amino acids showed high similarities to pyrroloquinoline-quinone-dependent periplasmic and membrane-bound dehydrogenases acting on alcohols, glucose and quinate or shikimate. It was demonstrated by alignment with the amino acid sequence of the large subunit of the quinoprotein methanol dehydrogenase from Methylobacterium extorquens, whose X-ray structure is known, that the amino acid residues involved in the binding of pyrroloquinoline quinone and Ca2+ at the active site are conserved in the quinoprotein ethanol dehydrogenase of P. aeruginosa. Also, the glycine/tryptophan docking motifs involved in stabilizing the superbarrel structure of the quinoprotein methanol dehydrogenase of M. extorquens were conserved. The known sequences of pyrroloquinoline-quinone-dependent dehydrogenases were used to derive new, more specific sequence motifs for detecting members of this family of enzymes. Despite the sequence similarity between the large a subunit of quinoprotein methanol dehydrogenase from M. extorquens and the quinoprotein ethanol dehydrogenase from P. aeruginosa, the two enzyme systems were quite different. In the presence of the prosthetic group, pyrroloquinoline quinone expression of the Pseudomonas gene encoding the 60-kDa subunit of quinoprotein ethanol dehydrogenase in Escherichia coli resulted in formation of active enzyme. The formation of active quinoprotein methanol dehydrogenase, however, is known to require, in addition to the large alpha subunit, the expression of a small beta subunit, and helper proteins [Lidstrom, M. E. (1995) Genetics of bacterial quinoproteins, Methods Enzymol. 258, 217-227].

Physiological characterization of a bacterial consortium reductively dechlorinating 1,2,3- and 1,2,4-trichlorobenzene

A bacterial mixed culture reductively dechlorinating trichlorobenzenes was established in a defined, synthetic mineral medium without any complex additions and with pyruvate as the carbon and energy source. The culture was maintained over 39 consecutive transfers of small inocula into fresh media, enriching the dechlorinating activity. In situ probing with fluorescence-labeled rRNA-targeted oligonucleotide probes revealed that two major subpopulations within the microbial consortium were phylogenetically affiliated with a sublineage within the Desulfovibrionaceae and the gamma subclass of Proteobacteria. The bacterial consortium grew by fermentation of pyruvate, forming acetate, propionate, CO2, formate, and hydrogen. Acetate and propionate supported neither the reduction of trichlorobenzenes nor the reduction of sulfate when sulfate was present. Hydrogen and formate were used for sulfate reduction to sulfide. Sulfate strongly inhibited the reductive dechlorination of trichlorobenzenes. However, when sulfate was depleted in the medium due to sulfate reduction, dechlorination of trichlorobenzenes started. Similar results were obtained when sulfite was present in the cultures. Molybdate at a concentration of 1 mM strongly inhibited the dechlorination of trichlorobenzenes. Cultures supplied with molybdate plus sulfate did not reduce sulfate, but dechlorination of trichlorobenzenes occurred. Supplementation of electron-depleted cultures with various electron sources demonstrated that formate was used as a direct electron donor for reductive dechlorination, whereas hydrogen was not.

Enzymology of the degradation of (di)chlorobenzenes by Xanthobacter flavus 14p1

Xanthobacter flavus 14p1 used 1,4-dichlorobenzene as the sole source of carbon and energy but did not grow on other (chloro)aromatic compounds. 1,4-Dichlorobenzene was attacked by a chlorobenzene dioxygenase, and the intermediate chlorocatechol was metabolized by the modified ortho pathway. All enzymes necessary to convert 1, 4-dichlorobenzene to 3-oxoadipate showed a low substrate specificity and also accepted the respective intermediates of chlorobenzene or 1, 3-dichlorobenzene degradation. Of the three compounds chlorobenzene, 1,4-dichlorobenzene, and 1,3-dichlorobenzene, the latter was the most toxic for X. flavus 14p1. Furthermore, 1,3-dichlorobenzene did not induce chlorocatechol 1,2-dioxygenase activity of the organism. Chlorobenzene, however, induced chlorocatechol 1,2-dioxygenase, dienelactone hydrolase, and maleylacetate reductase activities. As demonstrated by chloride release, also chlorobenzene dioxygenase, chlorobenzene cis-dihydrodiol dehydrogenase, and chloromuconate cycloisomerase activities were present in chlorobenzene-induced cells, but chlorobenzene failed to support growth. Presumably a toxic compound was formed from one of the intermediates.

Purification and characterization of chlorobenzene cis-dihydrodiol dehydrogenase from Xanthobacter flavus 14p1

Chlorobenzene cis-dihydrodiol dehydrogenase was purified to homogeneity from Xanthobacter flavus 14p1, which used 1,4-dichlorobenzene as the sole source of carbon and energy. The enzyme converted a number of halogenated substrates with high specific activity. The pI of the native chlorobenzene cis-dihydrodiol dehydrogenase was 5.4, and the molecular mass was approximately 100 kDa, as determined by gel filtration. The enzyme was composed of four apparently identical subunits with a molecular mass of 26.5 kDa. The Michaelis constant for 3,6-dichlorobenzene cis-dihydrodiol (210 microM) was lower than for benzene cis-dihydrodiol (780 microM), while the specific activity with benzene cis-dihydrodiol (63 units/ mg) was higher than with 3,6-dichlorobenzene cis-dihydrodiol (32 units/mg). Chlorobenzene cis-dihydrodiol dehydrogenase accepted also NADP+ as cosubstrate; however, the activity was reduced to 14% of that with NAD+. The enzymic activity was inhibited by mercuric chloride and to a lesser extent by the metal-ion chelators 8-hydroxyquinoline and KCN.

Degradation of 1,4-dichlorobenzene by Xanthobacter flavus 14p1

Xanthobacter flavus 14p1 was isolated from sludge of the river Mulde by selective enrichment with 1,4-dichlorobenzene as the sole source of carbon and energy. The bacterium did not use other aromatic or chloroaromatic compounds as growth substrates. During growth on 1,4-dichlorobenzene, stoichiometric amounts of chloride ions were released. Degradation products of 1,4-dichlorobenzene were identified by gas chromatography-mass spectrometry analysis. 3,6-Dichloro-cis-1,2-dihydroxycyclohexa-3,5-diene and 3,6-dichlorocatechol were isolated from culture fluid. 2,5-Dichloromuconic acid and 2-chloromaleylacetic acid as well as the decarboxylation product 2-chloroacetoacrylic acid were identified after enzymatic conversion of 3,6-dichlorocatechol by cell extract. 1,4-Dichlorobenzene dioxygenase, dihydrodiol dehydrogenase, and catechol 1,2-dioxygenase activity were induced in cells grown on 1,4-dichlorobenzene. The results demonstrate that 1,4-dichlorobenzene degradation is initiated by dioxygenation and that ring opening proceeds via ortho cleavage.

Cytochrome c550 from Pseudomonas aeruginosa

In cells of Pseudomonas aeruginosa A.T.C.C. 17933 grown on ethanol the synthesis of a soluble c-type cytochrome, together with quinoprotein ethanol dehydrogenase, is induced. The cytochrome, with an alpha-absorption band at 550 nm, was purified to homogeneity. The molecular mass of the monomeric protein is 15 kDa, the pI is 4.8, and it contains one haem prosthetic group. The midpoint potential of the autoxidizable, but not autoreducible, cytochrome is 280 mV. Cytochrome c550 mediates electron transfer between quinoprotein ethanol dehydrogenase and ferricyanide. In a system composed of membrane particles with NN'NN'-tetramethyl-p-phenylenediamine oxidase activity and quinoprotein ethanol dehydrogenase, oxygen consumption is only observed in the presence of cytochrome c550. This indicates the participation of the cytochrome in the electron-transport chain linked to quinoprotein ethanol dehydrogenase in P. aeruginosa. The electron transport from ethanol dehydrogenase to oxygen is inhibited by myxothiazol and antimycin, indicating that a cytochrome bc1-like complex is involved.

A highly specific D-hydroxyisovalerate dehydrogenase from the enniatin producer Fusarium sambucinum

A highly specific D-hydroxyisovalerate (D-HIV) dehydrogenase, which is a key enzyme in depsipeptide synthesis, was purified to near homogeneity from the enniatin-producing fungus Fusarium sambucinum. The enzyme catalyzes the reversible reaction of 2-ketoisovalerate (2-KIV) to D-HIV. It is strictly dependent on NADPH and exhibits a high substrate specificity with respect to 2-KIV. NADH was not accepted by the enzyme. Km values for 2-KIV and NADPH were found to be 200 and 333 microM, respectively. D-HIV dehydrogenase consists of a single polypeptide chain with a molecular mass of about 53 kDa. Optimum temperature for the reduction of 2-KIV was 35 degrees C and for the oxidation reaction was 45 degrees C. The optimum pH was found to be 7 for the reduction and 8-9 for the oxidation reaction.

Preliminary X-ray crystallographic study of malate dehydrogenases from the thermoacidophilic Archaebacteria Thermoplasma acidophilum and Sulfolobus acidocaldarius

Malate dehydrogenases from the thermoacidophilic Archaebacteria Thermoplasma acidophilum and Sulfolobus acidocaldarius have been crystallized and characterized by X-ray diffraction measurements. Crystals of the enzyme from T. acidophilum display space-group symmetry P2(1), a = 63 A, b = 135 A, c = 83 A and beta = 105 degrees; they scattered to approximately 4 A resolution. Two crystal modifications of malate dehydrogenase from S. acidocaldarius were characterized; one displayed trigonal symmetry corresponding to space groups P321, P3(1)21 or P3(2)21 with lattice parameters a = 151 A and c = 248 A and with resolution approximately to 5 A, whereas the other modification displayed space group symmetry I23 or I2(1)3 with lattice parameters a = 129 A and approximately 4.5 A resolution.

Further kinetic and molecular characterization of an extremely heat-stable carboxylesterase from the thermoacidophilic archaebacterium Sulfolobus acidocaldarius

The carboxylesterase (serine esterase, EC 3.1.1.1) from Sulfolobus acidocaldarius was purified 940-fold to homogeneity by an improved purification procedure with a yield of 57%. In the presence of alcohols the enzyme catalyses the transfer of the substrate acyl group to alcohols in parallel to hydrolysis. The results show the existence of an alcohol-binding site and a competitive partitioning of the acyl-enzyme intermediate between water and alcohols. Aniline acts also as a nucleophilic acceptor for the acyl group. On the basis of titration with diethyl p-nitrophenyl phosphate, a number of four active centres is determined for the tetrameric carboxylesterase. The sequence of 20 amino acid residues at the esterase N-terminus and the amino acid composition are reported.

Reversible thermal inactivation of the quinoprotein glucose dehydrogenase from Acinetobacter calcoaceticus. Ca2+ ions are necessary for re-activation

The soluble form of the homogeneous quinoprotein glucose dehydrogenase from Acinetobacter calcoaceticus is reversibly inactivated at temperatures above 35 degrees C. An equilibrium is established between active and denatured enzyme, this depending on the protein concentration and the inactivation temperature used. Upon thermal inactivation the enzyme dissociates into the prosthetic group pyrroloquinoline quinone and the apo form of glucose dehydrogenase. After inactivation at 50 degrees C active enzyme is re-formed again at 25 degrees C. Ca2+ ions are necessary for the re-activation process. The velocity of re-activation depends on the protein concentration, the concentration of the prosthetic group pyrroloquinoline quinone and the Ca2+ concentration. The apo form of glucose dehydrogenase can be isolated, and in the presence of pyrroloquinoline quinone and Ca2+ active holoenzyme is formed. Even though native glucose dehydrogenase is not inactivated in the presence of EDTA or trans-1,2-diaminocyclohexane-NNN'NH-tetra-acetic acid, Ca2+ stabilizes the enzyme against thermal inactivation. Two Ca2+ ions are found per subunit of glucose dehydrogenase. The data suggest that pyrroloquinoline quinone is bound at the active site via a Ca2+ bridge. Mn2+ and Cd2+ can replace Ca2+ in the re-activation mixture.