Catabolism of 4-hydroxybenzoate in Candida parapsilosis proceeds through initial oxidative decarboxylation by a FAD-dependent 4-hydroxybenzoate 1-hydroxylase

Engineering of a chromogenic enzyme screening system based on an auxiliary indole-3-carboxylic acid monooxygenase

Here, we present a proof‐of‐principle for a new high‐throughput functional screening of metagenomic libraries for the selection of enzymes with different activities, predetermined by the substrate being used. By this approach, a total of 21 enzyme‐coding genes were selected, including members of xanthine dehydrogenase, aldehyde dehydrogenase (ALDH), and amidohydrolase families. The screening system is based on a pro‐chromogenic substrate, which is transformed by the target enzyme to indole‐3‐carboxylic acid. The later compound is converted to indoxyl by a newly identified indole‐3‐carboxylate monooxygenase (Icm). Due to the spontaneous oxidation of indoxyl to indigo, the target enzyme‐producing colonies turn blue. Two types of pro‐chromogenic substrates have been tested. Indole‐3‐carboxaldehydes and the amides of indole‐3‐carboxylic acid have been applied as substrates for screening of the ALDHs and amidohydrolases, respectively. Both plate assays described here are rapid, convenient, easy to perform, and adaptable for the screening of a large number of samples both in Escherichia coli and Rhodococcus sp. In addition, the fine‐tuning of the pro‐chromogenic substrate allows screening enzymes with the desired substrate specificity.

Hydroquinone: environmental pollution, toxicity, and microbial answers

Hydroquinone is a major benzene metabolite, which is a well-known haematotoxic and carcinogenic agent associated with malignancy in occupational environments. Human exposure to hydroquinone can occur by dietary, occupational, and environmental sources. In the environment, hydroquinone showed increased toxicity for aquatic organisms, being less harmful for bacteria and fungi. Recent pieces of evidence showed that hydroquinone is able to enhance carcinogenic risk by generating DNA damage and also to compromise the general immune responses which may contribute to the impaired triggering of the host immune reaction. Hydroquinone bioremediation from natural and contaminated sources can be achieved by the use of a diverse group of microorganisms, ranging from bacteria to fungi, which harbor very complex enzymatic systems able to metabolize hydroquinone either under aerobic or anaerobic conditions. Due to the recent research development on hydroquinone, this review underscores not only the mechanisms of hydroquinone biotransformation and the role of microorganisms and their enzymes in this process, but also its toxicity.

Elucidation of the 4-hydroxyacetophenone catabolic pathway in Pseudomonas fluorescens ACB

The catabolism of 4-hydroxyacetophenone in Pseudomonas fluorescens ACB is known to proceed through the intermediate formation of hydroquinone. Here, we provide evidence that hydroquinone is further degraded through 4-hydroxymuconic semialdehyde and maleylacetate to beta-ketoadipate. The P. fluorescens ACB genes involved in 4-hydroxyacetophenone utilization were cloned and characterized. Sequence analysis of a 15-kb DNA fragment showed the presence of 14 open reading frames containing a gene cluster (hapCDEFGHIBA) of which at least four encoded enzymes are involved in 4-hydroxyacetophenone degradation: 4-hydroxyacetophenone monooxygenase (hapA), 4-hydroxyphenyl acetate hydrolase (hapB), 4-hydroxymuconic semialdehyde dehydrogenase (hapE), and maleylacetate reductase (hapF). In between hapF and hapB, three genes encoding a putative intradiol dioxygenase (hapG), a protein of the Yci1 family (hapH), and a [2Fe-2S] ferredoxin (hapI) were found. Downstream of the hap genes, five open reading frames are situated encoding three putative regulatory proteins (orf10, orf12, and orf13) and two proteins possibly involved in a membrane efflux pump (orf11 and orf14). Upstream of hapE, two genes (hapC and hapD) were present that showed weak similarity with several iron(II)-dependent extradiol dioxygenases. Based on these findings and additional biochemical evidence, it is proposed that the hapC and hapD gene products are involved in the ring cleavage of hydroquinone.

Aerobic metabolism of 4-hydroxybenzoic acid in Archaea via an unusual pathway involving an intramolecular migration (NIH shift)

A novel haloarchaeal strain, Haloarcula sp. strain D1, grew aerobically on 4-hydroxybenzoic acid (4HBA) as a sole carbon and energy source and is the first member of the domain Archaea reported to do so. Unusually, D1 metabolized 4HBA via gentisic acid rather than via protocatechuic acid, hydroquinone, or catechol. Gentisate was detected in 4HBA-grown cultures, and gentisate 1,2-dioxygenase activity was induced in 4HBA-grown cells. Stoichiometric accumulation of gentisate from 4HBA was demonstrated in 4HBA-grown cell suspensions containing 2,2'-dipyridyl (which strongly inhibits gentisate 1,2-dioxygenase). To establish whether initial 1-hydroxylation of 4HBA with concomitant 1,2-carboxyl group migration to yield gentisate occurred, 2,6-dideutero-4HBA was synthesized and used as a substrate. Deuterated gentisate was recovered from cell suspensions and identified as 3-deutero-gentisate, using gas chromatography-mass spectrometry and proton nuclear magnetic resonance spectroscopy. This structural isomer would be expected only if a 1,2-carboxyl group migration had taken place, and it provides compelling evidence that the 4HBA pathway in Haloarcula sp. strain D1 involves a hydroxylation-induced intramolecular migration. To our knowledge, this is the first report of a pathway which involves such a transformation (called an NIH shift) in the domain Archaea.

Fungal metabolism of toluene: monitoring of fluorinated analogs by (19)F nuclear magnetic resonance spectroscopy

We used isomeric fluorotoluenes as model substrates to study the catabolism of toluene by five deuteromycete fungi and one ascomycete fungus capable of growth on toluene as the sole carbon and energy source, as well as by two fungi (Cunninghamella echinulata and Aspergillus niger) that cometabolize toluene. Whole cells were incubated with 2-, 3-, and 4-fluorotoluene, and metabolites were characterized by (19)F nuclear magnetic resonance. Oxidation of fluorotoluene by C. echinulata was initiated either at the aromatic ring, resulting in fluorinated o-cresol, or at the methyl group to form fluorobenzoate. The initial conversion of the fluorotoluenes by toluene-grown fungi occurred only at the side chain and resulted in fluorinated benzoates. The latter compounds were the substrate for the ring hydroxylation and, depending on the fluorine position, were further metabolized up to catecholic intermediates. From the (19)F nuclear magnetic resonance metabolic profiles, we propose that diverse fungi that grow on toluene assimilate toluene by an initial oxidation of the methyl group.

Identification of fluoropyrogallols as new intermediates in biotransformation of monofluorophenols in Rhodococcus opacus 1cp

The transformation of monofluorophenols by whole cells of Rhodococcus opacus 1cp was investigated, with special emphasis on the nature of hydroxylated intermediates formed. Thin-layer chromatography, mass spectrum analysis, and (19)F nuclear magnetic resonance demonstrated the formation of fluorocatechol and trihydroxyfluorobenzene derivatives from each of three monofluorophenols. The (19)F chemical shifts and proton-coupled splitting patterns of the fluorine resonances of the trihydroxyfluorobenzene products established that the trihydroxylated aromatic metabolites contained hydroxyl substituents on three adjacent carbon atoms. Thus, formation of 1,2, 3-trihydroxy-4-fluorobenzene (4-fluoropyrogallol) from 2-fluorophenol and formation of 1,2,3-trihydroxy-5-fluorobenzene (5-fluoropyrogallol) from 3-fluorophenol and 4-fluorophenol were observed. These results indicate the involvement of fluoropyrogallols as previously unidentified metabolites in the biotransformation of monofluorophenols in R. opacus 1cp.

Purification and properties of 4-hydroxybenzoate 1-hydroxylase (decarboxylating), a novel flavin adenine dinucleotide-dependent monooxygenase from Candida parapsilosis CBS604

A novel flavoprotein monooxygenase, 4-hydroxybenzoate 1-hydroxylase (decarboxylating), from Candida parapsilosis CBS604 was purified to apparent homogeneity. The enzyme is induced when the yeast is grown on either 4-hydroxybenzoate, 2,4-dihydroxybenzoate, or 3,4-dihydroxybenzoate as the sole carbon source. The purified monooxygenase is a monomer of about 50 kDa containing flavin adenine dinucleotide as weakly bound cofactor. 4-Hydroxybenzoate 1-hydroxylase from C. parapsilosis catalyzes the oxidative decarboxylation of a wide range of 4-hydroxybenzoate derivatives with the stoichiometric consumption of NAD(P)H and oxygen. Optimal catalysis is reached at pH 8, with NADH being the preferred electron donor. By using (18)O2, it was confirmed that the oxygen atom inserted into the product 1,4-dihydroxybenzene is derived from molecular oxygen. 19F nuclear magnetic resonance spectroscopy revealed that the enzyme catalyzes the conversion of fluorinated 4-hydroxybenzoates to the corresponding hydroquinones. The activity of the enzyme is strongly inhibited by 3,5-dichloro-4-hydroxybenzoate, 4-hydroxy-3,5-dinitrobenzoate, and 4-hydroxyisophthalate, which are competitors with the aromatic substrate. The same type of inhibition is exhibited by chloride ions. Molecular orbital calculations show that upon deprotonation of the 4-hydroxy group, nucleophilic reactivity is located in all substrates at the C-1 position. This, and the fact that the enzyme is highly active with tetrafluoro-4-hydroxybenzoate and 4-hydroxy-3-nitrobenzoate, suggests that the phenolate forms of the substrates play an important role in catalysis. Based on the substrate specificity, a mechanism is proposed for the flavin-mediated oxidative decarboxylation of 4-hydroxybenzoate.