Uncoupling of the substrate monooxygenation and reduced pyridine nucleotide oxidation activities of salicylate hydroxylase by flavins

NahAa can convert naphthalene and reduce chromate simultaneously and immobilized on functional multiwall carbon nanotubes for wastewater treatment

Pseudomonas brassicacearum LZ-4 is a facultative anaerobic bacterium, can efficiently degrade naphthalene and reduce chromate simultaneously. In this study, we showed that the naphthalene degradation enzyme NahAa from P. brassicacearum LZ-4 can reduce Cr(VI). Heterologous expression in E. coli S17-1 along with RNA interference of NahAa in strain LZ-4 showed the enzyme can reduce chromate in vivo. In vitro, purified NahAa was identified and can catalyze Cr(VI) reduction by 64.2%. Flavin adenine dinucleotide (FAD) was identified as a cofactor of NahAa, which Cr(VI) could obtain electrons from NADH through NahAa-associated FAD for reduction. Immobilized NahAa on functional multi walled carbon nanotubes via physical adsorption method to produce a stable, high efficient composite MWCNT-NahAa. The maximum efficiency of MWCNT-NahAa composite was obtained in enzyme concentrations of 6 mg/mL and 20 min immobilization time. The optical reaction conditions for MWCNT-NahAa were pH 7.0 and 30 °C, still retaining 50% of its initial activities after five consecutive cycles. Application of composites in wastewater can reduce 90.4% Cr(VI), higher than free NahAa that was 63.5%. To our best knowledge, this is the first report immobilized enzyme in polycyclic aromatic hydrocarbons-degradation pathway for Cr(VI) wastewater treatment, providing a new insights on combined pollution remediation.

The naphthalene catabolic protein NahG plays a key role in hexavalent chromium reduction in Pseudomonas brassicacearum LZ-4

Soil contamination by PAH and heavy metals is a growing problem. Here, we showed that a new isolate, Pseudomonas brassicacearum strain LZ-4, can simultaneously degrade 98% of 6 mM naphthalene and reduce 92.4% of 500 μM hexavalent chromium [Cr (VI)] within 68 h. A draft genome sequence of strain LZ-4 (6,219,082 bp) revealed all the genes in the naphthalene catabolic pathway and some known Cr (VI) reductases. Interestingly, genes encoding naphthalene pathway components were upregulated in the presence of Cr (VI), and Cr (VI) reduction was elevated in the presence of naphthalene. We cloned and expressed these naphthalene catabolic genes and tested for Cr (VI) reduction, and found that NahG reduced 79% of 100 μM Cr (VI) in 5 minutes. Additionally, an nahG deletion mutant lost 52% of its Cr (VI) reduction ability compared to that of the wild-type strain. As nahG encodes a salicylate hydroxylase with flavin adenine dinucleotide (FAD) as a cofactor for electron transfer, Cr (VI) could obtain electrons from NADH through NahG-associated FAD. To the best of our knowledge, this is the first report of a protein involved in a PAH-degradation pathway that can reduce heavy metals, which provides new insights into heavy metal-PAH contamination remediation.

Degradation of the plant defence hormone salicylic acid by the biotrophic fungus Ustilago maydis

Salicylic acid (SA) is a key plant defence hormone which plays an important role in local and systemic defence responses against biotrophic pathogens like the smut fungus Ustilago maydis. Here we identified Shy1, a cytoplasmic U. maydis salicylate hydroxylase which has orthologues in the closely related smuts Ustilago hordei and Sporisorium reilianum. shy1 is transcriptionally induced during the biotrophic stages of development but not required for virulence during seedling infection. Shy1 activity is needed for growth on plates with SA as a sole carbon source. The trigger for shy1 transcriptional induction is SA, suggesting the possibility of a SA sensing mechanism in this fungus.

Pyridoxal phosphate: biosynthesis and catabolism

Vitamin B(6) is an essential cofactor that participates in a large number of biochemical reactions. Pyridoxal phosphate is biosynthesized de novo by two different pathways (the DXP dependent pathway and the R5P pathway) and can also be salvaged from the environment. It is one of the few cofactors whose catabolic pathway has been comprehensively characterized. It is also known to function as a singlet oxygen scavenger and has protective effects against oxidative stress in fungi. Enzymes utilizing vitamin B(6) are important targets for therapeutic agents. This review provides a concise overview of the mechanistic enzymology of vitamin B(6) biosynthesis and catabolism. This article is part of a Special Issue entitled: Pyridoxal Phosphate Enzymology.

Biodegradation of phenanthrene by Alcaligenes sp. strain PPH: partial purification and characterization of 1-hydroxy-2-naphthoic acid hydroxylase

Alcaligenes sp. strain PPH degrades phenanthrene via 1-hydroxy-2-naphthoic acid (1-H2NA), 1,2-dihydroxynaphthalene (1,2-DHN), salicylic acid and catechol. Enzyme activity versus growth profile and heat stability studies suggested the presence of two distinct hydroxylases, namely 1-hydroxy-2-naphthoic acid hydroxylase and salicylate hydroxylase. 1-Hydroxy-2-naphthoic acid hydroxylase was partially purified (yield 48%, fold 81) and found to be a homodimer with a subunit molecular weight of ∼34 kDa. The enzyme was yellow in color, showed UV-visible absorption maxima at 274, 375 and 445 nm, and fluorescence emission maxima at 527 nm suggested it to be a flavoprotein. The apoenzyme prepared by the acid-ammonium sulfate (2 M) dialysis method was colorless, inactive and lost the characteristic flavin absorption spectra but regained ∼90% activity when reconstituted with FAD. Extraction of the prosthetic group and its analysis by HPLC suggests that the holoenzyme contained FAD. The enzyme was specific for 1-H2NA and failed to show activity with any other hydroxynaphthoic acid analogs or salicylic acid. The K(m) for 1-H2NA in the presence of either NADPH or NADH remained unaltered (72 and 75 μM, respectively), suggesting dual specificity for the coenzyme. The K(m) for FAD was determined to be 4.7 μM. The enzyme catalyzed the conversion of 1-H2NA to 1,2-DHN only under aerobic conditions. These results suggested that 1-hydroxy-2-naphthoic acid hydroxylase is a flavoprotein monooxygenase specific for 1-H2NA and different from salicylate-1-hydroxylase.

Metabolism of 2-, 3- and 4-hydroxybenzoates by soil isolatesAlcaligenessp. strain PPH andPseudomonassp. strain PPD

Pseudomonas sp. strain PPD and Alcaligenes sp. strain PPH isolated from soil by enrichment culture technique utilize 2-, 3- and 4-hydroxybenzoates as the sole source of carbon and energy. The degradation pathways were elucidated by performing whole-cell O(2) uptake, enzyme activity and induction studies. Depending on the mixture of carbon source and the preculture condition, strain PPH was found to degrade 2-hydroxybenzoate either via the catechol or gentisate route and has both salicylate 1-hydroxylase and salicylate 5-hydroxylase. Strain PPD utilizes 2-hydroxybenzoate via gentisate. Both strains degrade 3- and 4-hydroxybenzoate via gentisate and protocatechuate, respectively. Enzymes were induced by respective hydroxybenzoate. Growth pattern, O(2) uptake and enzyme activity profiles on the mixture of three hydroxybenzoates as a carbon source suggest coutilization by both strains. When 3- or 4-hydroxybenzoate grown culture was used as an inoculum, strain PPH failed to utilize 2-hydroxybenzoate via catechol, indicating the modulation of the metabolic pathways, thus generating metabolic diversity.

Catabolic role of a three-component salicylate oxygenase from Sphingomonas yanoikuyae B1 in polycyclic aromatic hydrocarbon degradation

Sphingomonas yanoikuyae B1 possesses several different multicomponent oxygenases involved in metabolizing aromatic compounds. Six different pairs of genes encoding large and small subunits of oxygenase iron-sulfur protein components have previously been identified in a gene cluster involved in the degradation of both monocyclic and polycyclic aromatic hydrocarbons. Insertional inactivation of one of the oxygenase large subunit genes, bphA1c, results in a mutant strain unable to grow on naphthalene, phenanthrene, or salicylate. The knockout mutant accumulates salicylate from naphthalene and 1-hydroxy-2-naphthoic acid from phenanthrene indicating the loss of salicylate oxygenase activity. Complementation experiments verify that the salicylate oxygenase in S. yanoikuyae B1 is a three-component enzyme consisting of an oxygenase encoded by bphA2cA1c, a ferredoxin encoded by the adjacent bphA3, and a ferredoxin reductase encoded by bphA4 located over 25kb away. Expression of bphA3-bphA2c-bphA1c genes in Escherichia coli demonstrated the ability of salicylate oxygenase to convert salicylate to catechol and 3-, 4-, and 5-methylsalicylate to methylcatechols.

NahW, a novel, inducible salicylate hydroxylase involved in mineralization of naphthalene by Pseudomonas stutzeri AN10

Two genes, nahG and nahW, encoding two independent salicylate 1-hydroxylases have been identified in the naphthalene-degrading strain Pseudomonas stutzeri AN10. While nahG resides in the same transcriptional unit as the meta-cleavage pathway genes, forming the naphthalene degradation lower pathway, nahW is situated outside but in close proximity to this transcriptional unit. The nahG and nahW genes of P. stutzeri AN10 are induced and expressed upon incubation with salicylate, and the enzymes that are encoded, NahG and NahW, are involved in naphthalene and salicylate metabolism. Both genes, nahG and nahW, have been cloned in Escherichia coli JM109. The overexpression of these genes yields peptides with apparent molecular masses of 46 kDa (NahG) and 43 kDa (NahW), respectively. Both enzymes exhibit broad substrate specificities and metabolize salicylate, methylsalicylates, and chlorosalicylates. However, the relative rates by which the substituted analogs are transformed differ considerably.

Structure of chromosomal DNA coding for Pseudomonas putida S-1 salicylate hydroxylase

A gene coding for the salicylate hydroxylase has been isolated from chromosomal DNA of Pseudomonas putida S-1 and sequenced. The DNA fragment contained an open reading frame of 1266 bp encoding a polypeptide of 421 amino acid residues. The predicted amino acid sequence of the protein gave a good agreement with the sequences determined with the peptides isolated from the enzyme but methionine residue in the amino terminal was deleted in the N-terminal sequence of the enzyme protein. The nucleotide and amino acid sequences of the salicylate hydroxylase shared several common characteristics with those of the enzyme encoded on the plasmid DNA of P. putida PpG7; homology of nucleotide sequence is 58% and that of amino acid sequence is 56%. We could find two large conserved regions of the amino acid sequence at or near FAD- and NADH-binding regions. The FAD-binding site locates on the amino terminal and a lysine residue, functioning as an NADH-binding site (K. Suzuki, M. Mizuguchi, T. Gomi, and E. Itagaki, 1995, J. Biochem. 117,579-585), locates as Lys163.

Production, partial characterization, and potential diagnostic use of salicylate hydroxylase from Pseudomonas putida UUC-1

An unusual strain of Pseudomonas putida UUC-1 capable of growth at high salicylate concentration (10 gl-1) was investigated with the aim of developing an assay and a biosensor system for determining salicylate in body fluids by utilizing the salicylate hydroxylase enzyme. Medium and growth condition optimization were carried out under chemostat conditions. The highest biomass yield was at 4.0 gl-1 salicylate, 25 degrees C, pH 6.5, and 0.2 h-1 dilution rate. Growth occurred at up to 0.45 h-1 dilution rate, producing 236 Ul-1 enzyme activity and an output of 424 U h-1. The activity and productivity were higher than any reported in the literature for this enzyme. It had a Km value of 2.07 +/- 0.32 microM and an M(r) of approximately 43,000. In addition, its specific activity in the crude extract (0.8-0.9 U mg-1 protein) was similar to the commercially available enzyme. No plasmid DNA was detected in this strain, and no salicylate-negative isolates were obtained when curing with mitomycin C. It is therefore proposed that our strain has a chromosomally located inducible salicylate hydroxylase gene that enables it to grow at high salicylate. This strain also offers a means of cheaply producing large quantities of the enzyme through standard fermentation techniques.

Determination of sulfated peptides by differential iodination

A sequential approach was developed to label tyrosine sulfate and peptides containing tyrosine sulfate selectively. Amino acids and peptides containing tyrosine and tyrosine sulfate were first iodinated using chloramine-T-method. Reaction products were determined by RP-HPLC. Mono- and biiodination of tyrosine and several model peptides was achieved within 120 s incubation time. Iodination of free tyrosine sulfate and sulfated cholecystokinin26-33 was less than 5%. After desulfation of the reaction products with 1 N HCl successful radioiodination of desulfated tyrosine was carried out whereas tyrosine did incorporate radioactive iodine only 10%. As shown by RP-HPLC specific labeling of tyrosine sulfate containing peptides with 125iodine was achieved.

Electron transfer reactions in the NADPH oxidase system of neutrophils--involvement of an NADPH-cytochrome c reductase in the oxidase system

Membrane-bound NADPH oxidase of pig blood neutrophils was solubilized with heptylthioglucoside in a high yield. The solubilized preparation from myristate-stimulated cells (sample S) showed high O2- generating activity, and the preparation from resting cells (sample R) had no activity, but the two samples had equal amounts of flavins and cytochrome b-558 (cyt b-558). The electron transfer reactions to exogenous cytochrome c (cyt c) or cyt b-558 in samples S and R were examined. Under anaerobic conditions, NADPH-dependent cyt c reductase activity appeared higher in sample S than in sample R, and the addition of FMN and FAD greatly enhanced the reductase activity of sample S, but not that of sample R. No marked difference between the reductase activities of samples S and R was seen with NADH. Photoreduction of the NADPH oxidase system was examined in the absence of NADPH under anaerobic conditions by monitoring the reduction rates of exogenous cyt c using a flashlight with cut-off filters between 400 and 500 nm. Cyt c reduction was much higher in sample S than in sample R on photoexcitation at about 450 nm. Photoreduction was carried out with a band-pass filter for selective irradiation at 450 nm. Marked reduction of exogenous cyt c was observed only in sample S: the small reduction of cyt c by sample R was independent of the light wavelength and was equal to the blank level. In contrast, no difference in the reduction of cyt b-558 by the two samples was found by either NADPH or photoreduction. Under aerobic conditions, no direct reduction of either cyt c or cyt b-558 was observed. These results suggest that an NADPH-cyt c reductase (a membrane-bound flavoprotein) is involved in the NADPH oxidase system of stimulated neutrophils.

Functional modification of an arginine residue on salicylate hydroxylase

Salicylate hydroxylase from Pseudomonas putida (EC 1.14.13.1, salicylate, NADH:oxygen oxidoreductase) is an FAD-containing monooxygenase, which catalyzes decarboxylative hydroxylation of salicylate to produce catechol in the presence of NADH and O2. By chemical treatment of the enzyme with dicarbonyl reagents, such as glyoxal, the original oxygenase activity was converted to the salicylate-dependent NADH-dehydrogenase activity with free FAD as electron acceptor. One of twenty arginine residues of this enzyme is concerned with this alteration of activity, as shown by the result of its modification at pH 6.9. This result is further supported by the isolation of one arginine-modified enzyme by chromatographic methods on DEAE-Sephadex, A-50 columns. It exhibits the dehydrogenase activity predominantly. This modified enzyme is spectrophotometrically and electrophoretically characterized by a minute conformational change around the active site, and kinetically by a 7-fold increase in an apparent Km for NADH and a decrease of more than 5-fold in an apparent Km for FAD as electron acceptor, with an apparent Vmax of 22 s-1 for the dehydrogenase activity. Flow kinetics also showed a marked decrease in the rate for oxygenation of the reduced enzyme-salicylate complex from 21 s-1 (native enzyme) to 3.3 s-1 (modified enzyme). These facts suggest that one arginine residue of the enzyme is responsible for the NADH binding site, and chemical modification of one arginine residue of the enzyme induces some conformational change around the active site to alter the catalytic activity from oxygenation to dehydrogenation.

Purification and characterization of salicylate hydroxylase from Pseudomonas putida PpG7

The salicylate hydroxylase from P. putida PpG7 was purified and characterized. The enzyme appears to be monomeric, and it showed one major band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with an apparent Mr of 45 kDa. The sequence of the first 25 amino acids of salicylate hydroxylase (PpG7) was determined. Also, the total amino acid composition of salicylate hydroxylase (PpG7) was obtained and compared with that of the known salicylate hydroxylase from P. putida.

Molecular cloning of salicylate hydroxylase genes from Pseudomonas cepacia and Pseudomonas putida

The sal gene encoding Pseudomonas cepacia salicylate hydroxylase was cloned and the sal encoding Pseudomonas putida salicylate hydroxylase was subcloned into plasmid vector pRO2317 to generate recombinant plasmids pTK3 and pTK1, respectively. Both cloned genes were expressed in the host Pseudomonas aeruginosa PAO1. The parental strain can utilize catechol, a product of the salicylate hydroxylase-catalyzed reaction, but not salicylate as the sole carbon source for growth due to a natural deficiency of salicylate hydroxylase. The pTK1- or pTK3-transformed P. aeruginosa PAO1, however, can be grown on salicylate as the sole carbon source and exhibited activities for the cloned salicylate hydroxylase in crude cell lysates. In wild-type P. cepacia as well as in pTK1- or pTK3-transformed P. aeruginosa PAO1, the presence of glucose in addition to salicylate in media resulted in lower efficiencies of sal expression P. cepacia apparently can degrade salicylate via the meta cleavage pathway which, unlike the plasmid-encoded pathway in P. putida, appears to be encoded on chromosome. As revealed by DNA cross hybridizations, the P. cepacia hsd and ht genes showed significant homology with the corresponding plasmid-borne genes of P. putida but the P. cepacia sal was not homologous to the P. putida sal. Furthermore, polyclonal antibodies developed against purified P. cepacia salicylate hydroxylase inactivated the cloned P. cepacia salicylate hydroxylase but not the cloned P. putida salicylate hydroxylase in P. aeruginosa PAO1. It appears that P. cepacia and P. putida salicylate hydroxylases, being structurally distinct, were probably derived through convergent evolution.

Studies of electron-transfer properties of salicylate hydroxylase from Pseudomonas cepacia and effects of salicylate and benzoate binding

The pH dependence of the redox behavior of salicylate hydroxylase from Pseudomonas cepacia as well as the effects of salicylate, benzoate, and chloride binding is described. At pH 7.6 in 0.02 M potassium phosphate buffer E1(0')(EFl ox/EFl.-) is -0.150 V and E2(0')(EFl.-/EFl red H-) is -0.040 V versus the standard hydrogen electrode (SHE). A maximum of 5% of FAD anion semiquinone is thermodynamically stabilized under these conditions. However, in coulometric and dithionite titrations more semiquinone is kinetically formed, indicating slow transfer of the second electron. The potential/pH dependence is consistent with a two-electron, one-proton transfer. Upon salicylate binding the midpoint potential is shifted 0.020 V negative from -0.094 to -0.114 V vs SHE at pH 7.6. A maximum of 7% of the neutral semiquinone is stabilized both in potentiometric and coulometric titrations. This small potential shift indicates that the substrate is bound nearly to the same extent to all three oxidation states of the enzyme. It is clear that the substrate binding does not make the reduction of the flavin thermodynamically more favorable. In contrast to salicylate, the potential shift caused by the effector, benzoate, is much more significant. (A maximum potential shift of -0.07 V is calculated.) Benzoate binds most tightly to the oxidized form and is least tightly bound to the two-electron-reduced form of the enzyme. For the reduction of the free enzyme the transfer of the second electron or the transfer of the proton is rate limiting, as is shown by the kinetic formation of the anionic semiquinone.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetic analysis of enzyme inactivation by an autodecaying reagent

A simple method is described for the determination of both the pseudo-first-order rate constant and the second-order rate constant for enzyme inactivation by a chemical reagent which itself undergoes exponential decay. The validity of this method has been demonstrated in two test cases in which the labile diethyl pyrocarbonate was used to inactivate salicylate hydroxylase and bacterial luciferase.