Utilization of cyanide as nitrogenous substrate by Pseudomonas fluorescens NCIMB 11764: evidence for multiple pathways of metabolic conversion

The use of synthetic agonists of quorum sensing N- acyl homoserine lactone pathway improves the bioleaching ability in Acidithiobacillus and Pseudomonas bacteria

Metal solubilization from discarded electrical material and electronic devices (e-waste) using the bioleaching capabilities of bacterial cells is highly effective. However, gaps in understanding about the microbiological processes involved in the bioleaching reaction leads to less efficient metal solubilization in large-scale e-waste processing. In this study, bacterial species belonging to the genera Acidithiobacillus and Pseudomonas were used to leach copper and gold from discarded printed circuit boards (PCB). Through modulation of the cell-to-cell communication system in these bacteria, phenotypic traits directly involved in the bioleaching reaction were regulated in order to improve the metal solubilization. Addition of the long chain synthetic autoinducer molecule N-acyl homoserine lactone (AHL) of the quorum sensing pathway to the bioleaching reaction resulted in a significant enhancement of metal extraction from PCB. Factors such as: cell attachment to PCB, biofilm formation and hydrogen cyanide (HCN) production were regulated by the quorum sensing system and could be directly related to the improvement of metal bioleaching. Bioleaching reactions using bacterial quorum sensing modulation could represent a valuable tool in overcoming limitations at the industrial level imposed by microbiological traits that lead to inefficient metal bioleaching from e-waste.

Extracellular Metabolism Sets the Table for Microbial Cross-Feeding

The transfer of nutrients between cells, or cross-feeding, is a ubiquitous feature of microbial communities with emergent properties that influence our health and orchestrate global biogeochemical cycles. Cross-feeding inevitably involves the externalization of molecules. Some of these molecules directly serve as cross-fed nutrients, while others can facilitate cross-feeding. Altogether, externalized molecules that promote cross-feeding are diverse in structure, ranging from small molecules to macromolecules. The functions of these molecules are equally diverse, encompassing waste products, enzymes, toxins, signaling molecules, biofilm components, and nutrients of high value to most microbes, including the producer cell. As diverse as the externalized and transferred molecules are the cross-feeding relationships that can be derived from them. Many cross-feeding relationships can be summarized as cooperative but are also subject to exploitation. Even those relationships that appear to be cooperative exhibit some level of competition between partners. In this review, we summarize the major types of actively secreted, passively excreted, and directly transferred molecules that either form the basis of cross-feeding relationships or facilitate them. Drawing on examples from both natural and synthetic communities, we explore how the interplay between microbial physiology, environmental parameters, and the diverse functional attributes of extracellular molecules can influence cross-feeding dynamics. Though microbial cross-feeding interactions represent a burgeoning field of interest, we may have only begun to scratch the surface.

Isolation of cyanide-degrading bacteria and molecular characterization of its cyanide-degrading nitrilase

Hydrogen cyanide is an industrially important chemical, and its annual production is more than 1.5 million tons. Because of its toxicity, the cyanide-containing effluents from industries have caused many environmental problems. Among various methods to treat the contaminated soils or water, the biological degradation is regarded to be promising. We isolated two cyanide-degrading microorganisms, Pedobacter sp. EBE-1 and Bacillus sp. EBE-2, from soil contaminated with cyanide. Among these bacteria, Bacillus sp. EBE-2 exhibited significantly a high cyanide-degrading ability. Bacillus sp. EBE-2 might be used for the remediation of cyanide contaminated water or soil. A nitrilase gene was cloned from Bacillus sp. EBE-2. Bacillus nitrilase was expressed in Escherichia coli and purified. Bacillus nitrilase exhibited cyanide-degrading activity as a large oligomer. Since formic acid formation from cyanide was observed, Bacillus nitrilase is likely to be a cyanide hydrolase. Although there exist various homologous enzymes annotated as carbon-nitrogen family hydrolases, this is the first report on the cyanide degrading activity. The structure and catalytic site of Bacillus nitrilase were studied by homology modeling and molecular docking simulation.

Degradation of ferrocyanide by natural isolated bacteria

This study aims at investigating the iron cyanide (CN) degradation potential of two natural bacterial isolates with the purpose of their application in iron CN phytoremediation. The strains were isolated from contaminated soil and incubated over 4 months with 50 mg L^-1 CN (as ferrocyanide) as the sole iron and nitrogen source. Unlike previous reports, the study provides control for bacterial growth, biotic and abiotic CN losses. Bacterial growth, CN, ammonium, and nitrate concentrations were monitored regularly. Both strains grew less rapid with iron CN compared with the positive control. However, the growth was diauxic. The CN concentration in the media decreased with 20% and 25% respectively, while that in the sterile controls remained stable. Ammonium was detected in the media of both strains implying that a fraction of the initially applied ferrocyanide has been converted. The nitrogen lost from the system evened out with that in the cells at the end of the experiments. These results showed that the investigated strains were undoubtedly able to grow on iron CN as an alternative nitrogen source, but contrary to some previous findings, the iron CN utilization is much slower and takes place only after complete exhaustion of the cellular nitrogen reserves.

Microbial (Enzymatic) Degradation of Cyanide to Produce Pterins as Cofactors

Cyanide is one of the most poisonous substances in the environment, which may have originated from natural and anthropogenic sources. There are many enzymes produced by microorganisms which can degrade and utilize cyanide. The major byproducts of cyanide degradation are alanine, glutamic acid, alpha-amino-butyric acid, beta-cyanoalanine, pterin etc. These products have many pharmaceutical and medicinal applications. For the degradation of cyanide, microbes produce necessary cofactors which catalyze the degradation pathways. Pterin is one of the cofactors for cyanide degradation. There are many pathways involved for the degradation of cyanide, cyanate, and thiocyanate. Some of the microorganisms possess resistance to cyanide, since they have developed adaptive alternative pathways for the production of ATP by utilization of cyanide as carbon and nitrogen sources. In this review, we summarized different enzymes, their mechanisms, and corresponding pathways for the degradation of cyanide and production of pterins during cyanide degradation. We aim to enlighten different types of pterin, its classification, and biological significance through this literature review.

Detoxification of ferrocyanide in asoil-plant system

The detoxification of iron cyanide in a soil-plant system was investigated to assess the total cyanide extracted from contaminated soil and allocated in the leaf tissue of willow trees (Salix caprea). They were grown in soil containing up to 1000 mg/kg dry weight (dw) of cyanide (CN), added as ¹⁵N-labeled potassium ferrocyanide and prepared with a new method for synthesis of labeled iron cyanides. CN content and ¹⁵N enrichment were monitored weekly over the exposure in leaf tissue of different age. The ¹⁵N enrichment in the young and old leaf tissue reached up to 15.197‰ and 9063‰, respectively; it increased significantly over the exposure and with increasing exposure concentrations (p < 0.05). Although the CN accumulation in the old leaf tissue was higher, compared to the young leaf tissue (p < 0.05), the ¹⁵N enrichment in the two tissue types did not differ statistically. This indicates a non-uniform CN accumulation but a uniform ¹⁵N allocation throughout the leaf mass. Significant differences were detected between the measured CN content and the C¹⁵N content, calculated from the ¹⁵N enrichment (p < 0.05), revealing a significant CN fraction within the leaf tissue, which could not be detected as ionic CN. The application of labeled iron CN clearly shows that CN is detoxified during uptake by the willows. However, these results do not exclude other detoxification pathways, not related to the trees. Still, they are strongly indicative of the central role the trees played in CN removal and detoxification under the experimental conditions.

Alkaline active cyanide dihydratase of Flavobacterium indicum MTCC 6936: Growth optimization, purification, characterization and in silico analysis

The present work explores a rare cyanide dihydratase of Flavobacterium indicum MTCC 6936 for its potential of cyanide degradation. The enzyme is purified to 12 fold with a yield of 76%. SDS and native-PAGE analysis revealed that enzyme was monomer of 40 kDa size. The enzyme works well in mesophilic range at wide array of pH. The thermostability profile of cyanide dihydratase revealed that the enzyme is quite stable at 30 °C and 35 °C with half-life of 6 h 30 min and 5 h respectively. K_m and V_max for cyanide dihydratase of F. indicum was measured to be 4.76 mM and 45 U mg^-1 with k_cat calculated to be 27.3 s^-1 and specificity constant (k_cat/K_m) to be around 5.67 mM^-1 s^-1. MALDI-TOF analysis of purified protein revealed that the amino acid sequence has 50% and 43% sequence identity with putative amino acid sequence of F. indicum and earlier reported cyanide dihydratase of Bacillus pumilus respectively. Homology modeling studies of cyanide dihydratase of F. indicum predicted the catalytic triad of the enzyme indicating Cys at 164, Glu at 46 and Lys at 130th position. The purified enzyme has potential applications in bioremediation and analytical sector.

Evaluation of Pterin, a Promising Drug Candidate from Cyanide Degrading Bacteria

Pterin is a member of the compounds known as pteridines. They have the same nucleus of 2-amino-4-hydroxypteridine (pterin); however, the side-chain is different at the position 6, and the state of oxidation of the ring may exist in different form viz. tetrahydro, dihydro, or a fully oxidized form. In the present study, the microorganisms able to utilize cyanide, and heavy metals have been tested for the efficient production of pterin compound. The soil samples contaminated with cyanide and heavy metals were collected from Salem steel industries, Tamil Nadu, India. Out of 77 isolated strains, 40 isolates were found to utilize sodium cyanate as nitrogen source at different concentrations. However, only 13 isolates were able to tolerate maximum concentration (60 mM) of sodium cyanate and were screened for pterin production. Among the 13 isolates, only 1 organism showed maximum production of pterin, and the same was identified as Bacillus pumilus SVD06. The compound was extracted and purified by preparative high-performance liquid chromatography and analyzed by UV/visible, FTIR, and fluorescent spectrum. The antioxidant property of the purified pterin compound was determined by cyclic voltammetry. In addition, antimicrobial activity of pterin was also studied which was substantiated by antagonistic activity against Escherichia coli, and Pseudomonas aeruginosa. Besides that the pterin compound was proved to inhibit the formation of biofilm. The extracted pterin compounds could be proposed further not only for antioxidant and antimicrobial but also for its potency to aid as anticancer and psychotic drugs in future.

Analeptic agent from microbes upon cyanide degradation

Microbes being the initial form of life and ubiquitous in occurrence, they adapt to the environment quickly. The microbial metabolism undergoes alteration to ensure conducive environment either by degrading the toxic substances or producing toxins to protect themselves. The presence of cyanide waste triggers the cyanide degrading enzymes in the microbes which facilitate the microbes to utilize the cyanide for its growth. To enable the degradation of cyanide, the microbes also produce the necessary cofactors and enhancers catalyzing the degradation pathways. Pterin, a cofactor of the enzyme cyanide monooxygenase catalyzing the oxidation of cyanide, is considered to be a potentially bioactive compound. Besides that, the pterins also act as cofactor for the enzymes involved in neurotransmitter metabolism. The therapeutic values of pterin as neuromodulating agent validate the necessity to pursue the commercial production of pterin. Even though chemical synthesis is possible, the non-toxic methods of pterin production need to be given greater attention in future.

A Cyanide-Induced 3-Cyanoalanine Nitrilase in the Cyanide-Assimilating Bacterium Pseudomonas pseudoalcaligenes Strain CECT 5344

Pseudomonas pseudoalcaligenes CECT 5344 is a bacterium able to assimilate cyanide as a sole nitrogen source. Under this growth condition, a 3-cyanoalanine nitrilase enzymatic activity was induced. This activity was encoded by nit4, one of the four nitrilase genes detected in the genome of this bacterium, and its expression in Escherichia coli enabled the recombinant strain to fully assimilate 3-cyanoalanine. P. pseudoalcaligenes CECT 5344 showed a weak growth level with 3-cyanoalanine as the N source, unless KCN was also added. Moreover, a nit4 knockout mutant of P. pseudoalcaligenes CECT 5344 became severely impaired in its ability to grow with 3-cyanoalanine and cyanide as nitrogen sources. The native enzyme expressed in E. coli was purified up to electrophoretic homogeneity and biochemically characterized. Nit4 seems to be specific for 3-cyanoalanine, and the amount of ammonium derived from the enzymatic activity doubled in the presence of exogenously added asparaginase activity, which demonstrated that the Nit4 enzyme had both 3-cyanoalanine nitrilase and hydratase activities. The nit4 gene is located downstream of the cyanide resistance transcriptional unit containing cio1 genes, whose expression levels are under the positive control of cyanide. Real-time PCR experiments revealed that nit4 expression was also positively regulated by cyanide in both minimal and LB media. These results suggest that this gene cluster including cio1 and nit4 could be involved both in cyanide resistance and in its assimilation by P. pseudoalcaligenes CECT 5344.IMPORTANCE Cyanide is a highly toxic molecule present in some industrial wastes due to its application in several manufacturing processes, such as gold mining and the electroplating industry. The biodegradation of cyanide from contaminated wastes could be an attractive alternative to physicochemical treatment. P. pseudoalcaligenes CECT 5344 is a bacterial strain able to assimilate cyanide under alkaline conditions, thus avoiding its volatilization as HCN. This paper describes and characterizes an enzyme (Nit4) induced by cyanide that is probably involved in cyanide assimilation. The biochemical characterization of Nit4 provides a segment for building a cyanide assimilation pathway in P. pseudoalcaligenes This information could be useful for understanding, and hopefully improving, the mechanisms involved in bacterial cyanide biodegradation and its application in the treatment of cyanide-containing wastes.

Microbial mutualism dynamics governed by dose-dependent toxicity of cross-fed nutrients

Microbial interactions, including mutualistic nutrient exchange (cross-feeding), underpin the flow of energy and materials in all ecosystems. Metabolic exchanges are difficult to assess within natural systems. As such, the impact of exchange levels on ecosystem dynamics and function remains unclear. To assess how cross-feeding levels govern mutualism behavior, we developed a bacterial coculture amenable to both modeling and experimental manipulation. In this coculture, which resembles an anaerobic food web, fermentative Escherichia coli and photoheterotrophic Rhodopseudomonas palustris obligately cross-feed carbon (organic acids) and nitrogen (ammonium). This reciprocal exchange enforced immediate stable coexistence and coupled species growth. Genetic engineering of R. palustris to increase ammonium cross-feeding elicited increased reciprocal organic acid production from E. coli, resulting in culture acidification. Consequently, organic acid function shifted from that of a nutrient to an inhibitor, ultimately biasing species ratios and decreasing carbon transformation efficiency by the community; nonetheless, stable coexistence persisted at a new equilibrium. Thus, disrupting the symmetry of nutrient exchange can amplify alternative roles of an exchanged resource and thereby alter community function. These results have implications for our understanding of mutualistic interactions and the use of microbial consortia as biotechnology.

Two-step bioleaching of copper and gold from discarded printed circuit boards (PCB)

An effective strategy for environmentally sound biological recovery of copper and gold from discarded printed circuit boards (PCB) in a two-step bioleaching process was experimented. In the first step, chemolithotrophic acidophilic Acidithiobacillus ferrivorans and Acidithiobacillus thiooxidans were used. In the second step, cyanide-producing heterotrophic Pseudomonas fluorescens and Pseudomonas putida were used. Results showed that at a 1% pulp density (10g/L PCB concentration), 98.4% of the copper was bioleached by a mixture of A. ferrivorans and A. thiooxidans at pH 1.0-1.6 and ambient temperature (23±2°C) in 7days. A pure culture of P. putida (strain WCS361) produced 21.5 (±1.5)mg/L cyanide with 10g/L glycine as the substrate. This gold complexing agent was used in the subsequent bioleaching step using the Cu-leached (by A. ferrivorans and A. thiooxidans) PCB material, 44.0% of the gold was mobilized in alkaline conditions at pH 7.3-8.6, and 30°C in 2days. This study provided a proof-of-concept of a two-step approach in metal bioleaching from PCB, by bacterially produced lixiviants.

Gold biorecovery from e-waste: An improved strategy through spent medium leaching with pH modification

Rapid technological advancement and relatively short life time of electronic goods have resulted in an alarming growth rate of electronic waste which often contains significant quantities of toxic and precious metals. Compared to conventional recovery methods, bioleaching is an environmentally friendly process for metal extraction. Gold was bioleached from electronic scrap materials (ESM) via gold-cyanide complexation using cyanide produced from pure and mixed cultures of cyanogenic bacteria Chromobacterium violaceum, Pseudomonas aeruginosa and Pseudomonas fluorescens. As ESM was toxic to the bacteria, a two-step bioleaching approach was adopted where the solid waste was added to the bacterial culture after it has reached maximum growth and cyanide production during early stationary phase. Pure culture of C. violaceum showed the highest cyanide production, yielding maximum gold recovery of 11.3% at 0.5% w/v pulp density of ESM in two-step bioleaching. At the same pulp density of ESM, spent medium bioleaching using bacterial cell-free metabolites achieved gold recovery of 18%. Recovery increased to 30% when the pH of the spent medium was increased to shift the equilibrium in favor of cyanide ions production. It is demonstrated for the first time that pH modification of spent medium further improved metal solubilization and yielded higher metal recovery (compared to two-step bioleaching).

Enzymatic cyanide degradation by cell-free extract of Rhodococcus UKMP-5M

The cell-free extract of locally isolated Rhodococcus UKMP-5M strain was used as an alternative to develop greener and cost effective cyanide removal technology. The present study aims to assess the viability of the cell-free extract to detoxify high concentrations of cyanide which is measured through the monitoring of protein concentration and specific cyanide-degrading activity. When cyanide-grown cells were subjected to grinding in liquid nitrogen which is relatively an inexpressive and fast cell disruption method, highest cyanide-degrading activity of 0.63 mM min(-1) mg(-1) protein was obtained in comparison to enzymatic lysis and agitation with fine glass beads. The cell-free extracts managed to degrade 80% of 20 mM KCN within 80 min and the rate of cyanide consumption increased linearly as the concentration of protein was raised. In both cases, the addition of co-factor was not required which proved to be advantageous economically. The successful formation of ammonia and formate as endproducts indicated that the degradation of cyanide by Rhodococcus UKMP-5M proceeded via the activity of cyanidase and the resulting non-toxic products are safe for disposal into the environment. Further verification with SDS-PAGE revealed that the molecular weight of the active enzyme was estimated to be 38 kDa, which is consistent with previously reported cyanidases. Thus, the utilization of cell-free extracts as an alternative to live microbial in cyanide degradation offers numerous advantageous such as the potential to tolerate and degrade higher concentration of cyanide and total reduction in the overall cost of operation since the requirement for nutrient support is irrelevant.

Biodegradation of cyanide by acetonitrile-induced cells of Rhodococcus sp. UKMP-5M

A Rhodococcus sp. UKMP-5M isolate was shown to detoxify cyanide successfully, suggesting the presence of an intrinsic property in the bacterium which required no prior cyanide exposure for induction of this property. However, in order to promote growth, Rhodococcus sp. UKMP-5M was fully acclimatized to cyanide after 7 successive subcultures in 0.1 mM KCN for 30 days. To further shorten the lag phase and simultaneously increase the tolerance towards higher cyanide concentrations, the bacterium was induced with various nitrile compounds sharing a similar degradatory pathway to cyanide. Acetonitrile emerged as the most favored inducer and the induced cells were able to degrade 0.1 mM KCN almost completely within 18 h. With the addition of subsequent aliquots of 0.1 mM KCN a shorter period for complete removal of cyanide was required, which proved to be advantageous economically. Both resting cells and crude enzyme of Rhodococcus sp. UKMP-5M were able to biodegrade cyanide to ammonia and formate without the formation of formamide, implying the identification of a simple hydrolytic cyanide degradation pathway involving the enzyme cyanidase. Further verification with SDS-PAGE revealed that the molecular weight of the active enzyme was estimated to be 38 kDa, which is consistent with previously reported cyanidases. Since the recent advancement in the application of biological methods in treating cyanide-bearing wastewater has been promising, the discovery of this new bacterium will add value by diversifying the existing microbial populations capable of cyanide detoxification.

Review on Cyanogenic Bacteria for Gold Recovery from E-Waste

Electronic waste (E-waste) is recognized as a new emerging and fast-growing waste stream, and may be considered as a secondary ore for the recovery of some precious metals (such as gold). A number of control technologies have been conducted for gold recovery, and in which, cyanidation is widely used. In recent years, an alternate approach to the gold cyanidation process is being considered, in which the aim is to replace with the microorganism, specifically cyanogenic bacteria such asChromobacterium violaceum,Pseudomonas fluorescens,Pseudomonas aeruginosaandEscherichia coli.All these species can produce cyanide ions and dissolve gold in their metabolic processes. The mechanism is a combination of chemical knowledge (interaction of metals and cyanide) with microbiological principles (biological cyanide formation) regarding metal solubilization from waste printed circuit boards and the formation of water-soluble cyanide complexes. And the activity of cyanogenic bacteria is affected by many factors, such as pH, dissolved oxygen pulp density and nutriment, especially several metal ions, which can serve as the catalyst in the metabolism. Now researchers are devoting themselves to looking for the proper conditions, not only from the bacteria themselves, but also the combination of many methods, which can reinforce the cyanide generation and improve gold leaching efficiency. At present the reported leaching efficiency of gold with cyanogenic is approximately 70%. As the continuous optimization of conditions, the industrial application can be expected soon.

Draft genome sequence of the cyanide-utilizing bacterium Pseudomonas fluorescens strain NCIMB 11764

We report here the 6.97-Mb draft genome sequence of Pseudomonas fluorescens strain NCIMB 11764, which is capable of growth on cyanide as the sole nitrogen source. The draft genome sequence allowed the discovery of several genes implicated in enzymatic cyanide turnover and provided additional information contributing to a better understanding of this organism's unique cyanotrophic ability. This is the first sequenced genome of a cyanide-assimilating bacterium.

Alkaline cyanide degradation by Pseudomonas pseudoalcaligenes CECT5344 in a batch reactor. Influence of pH

Water containing cyanide was biologically detoxified with the bacterial strain Pseudomonas pseudoalcaligenes CECT5344 in a batch reactor. Volatilization of toxic hydrogen cyanide (HCN) was avoided by using an alkaline medium for the treatment. The operational procedure was optimized to assess cyanide biodegradation at variable pH values and dissolved oxygen concentrations. Using an initial pH of 10 without subsequent adjustment allowed total cyanide to be consumed at a mean rate of approximately 2.81 mg CN(-) L(-1) O.D.(-1) h(-1); however, these conditions posed a high risk of HCN formation. Cyanide consumption was found to be pH-dependent. Thus, no bacterial growth was observed with a controlled pH of 10; on the other hand, pH 9.5 allowed up to 2.31 mg CN(-) L(-1) O.D.(-1) h(-1) to be converted. The combination of a high pH and a low dissolved oxygen saturation (10%) minimized the release of HCN. This study contributes new basic knowledge about this biological treatment, which constitutes an effective alternative to available physico-chemical methods for the purification of wastewater containing cyanide or cyano-metal complexes.

Cyanide in industrial wastewaters and its removal: a review on biotreatment

Cyanides are produced by certain bacteria, fungi, and algae, and may be found in plants and some foods, such as lima beans and almonds. Although cyanides are present in small concentrations in these plants and microorganisms, their large-scale presence in the environment is attributed to the human activities as cyanide compounds are extensively used in industries. Bulk of cyanide occurrence in environment is mainly due to metal finishing and mining industries. Although cyanide can be removed and recovered by several processes, it is still widely discussed and examined due to its potential toxicity and environmental impact. From an economic standpoint, the biological treatment method is cost-effective as compared to chemical and physical methods for cyanide removal. Several microbial species can effectively degrade cyanide into less toxic products. During metabolism, they use cyanide as a nitrogen and carbon source converting it to ammonia and carbonate, if appropriate conditions are maintained. Biological treatment of cyanide under anaerobic as well as aerobic conditions is possible. The present review describes the mechanism and advances in the use of biological treatment for the removal of cyanide compounds and its advantages over other treatment processes. It also includes various microbial pathways for their removal.

Biodegradation of Cyanide by a White Rot Fungus, Trametes versicolor

The cyanide degradation abilities of three white rot fungi, Trametes versicolor ATCC 200801, Phanerochaete chrysosporium ME 496 and Pleurotus sajor-caju, were examined. T. versicolor was the most effective with 0.35 g dry cell/100 ml degrading 2 mM KCN (130 mg/l) over 42 h, at 30 degrees C, pH 10.5 with stirring at 150 rpm.

Bacterial cyanide oxygenase is a suite of enzymes catalyzing the scavenging and adventitious utilization of cyanide as a nitrogenous growth substrate

Cyanide oxygenase (CNO) from Pseudomonas fluorescens NCIMB 11764 catalyzes the pterin-dependent oxygenolytic cleavage of cyanide (CN) to formic acid and ammonia. CNO was resolved into four protein components (P1 to P4), each of which along with a source of pterin cofactor was obligately required for CNO activity. Component P1 was characterized as a multimeric 230-kDa flavoprotein exhibiting the properties of a peroxide-forming NADH oxidase (oxidoreductase) (Nox). P2 consisted of a 49.7-kDa homodimer that showed 100% amino acid identity at its N terminus to NADH peroxidase (Npx) from Enterococcus faecalis. Enzyme assays further confirmed the identities of both Nox and Npx enzymes (specific activity, 1 U/mg). P3 was characterized as a large oligomeric protein (approximately 300 kDa) that exhibited cyanide dihydratase (CynD) activity (specific activity, 100 U/mg). Two polypeptides of 38 kDa and 43 kDa were each detected in the isolated enzyme, the former believed to confer catalytic activity based on its similar size to other CynD enzymes. The amino acid sequence of an internal peptide of the 43-kDa protein was 100% identical to bacterial elongation factor Tu, suggesting a role as a possible chaperone in the assembly of CynD or a multienzyme CNO complex. The remaining P4 component consisted of a 28.9-kDa homodimer and was identified as carbonic anhydrase (specific activity, 2,000 U/mg). While the function of participating pterin and the roles of Nox, Npx, CynD, and CA in the CNO-catalyzed scavenging of CN remain to be determined, this is the first report describing the collective involvement of these four enzymes in the metabolic detoxification and utilization of CN as a bacterial nitrogenous growth substrate.

Utilization of the metal-cyano complex tetracyanonickelate (II) by Azotobacter vinelandii

AIMS

The ability of Azotobacter vinelandii, a N(2)-fixing bacterium, to biodegrade tetracyanonickelate (TCN) was evaluated.

METHODS AND RESULTS

The amounts of TCN were measured spectrophotometrically. Ammonia was determined colorimetrically by the indophenol method. The produced methane from TCN conversion by A. vinelandii was detected by gas chromatography. Results showed that A. vinelandii was able to biodegrade 1 mmol l(-1) of TCN. Ammonia and methane were detected during the process of TCN degradation. Effects of exogenous nitrogen sources on TCN degradation were addressed in this study. Results revealed that the addition of ammonia (1, 5 and 10 mmol l(-1)) into the reaction mixtures caused decrease of TCN degradation rate during a 24-h incubation period. This inhibition was also observed when nitrite (5 and 10 mmol l(-1)) was added, whereas TCN degradation still proceeded after the addition of nitrate at the same concentrations. Furthermore, the rate of TCN utilization was strikingly enhanced when 0.8% of glucose was added.

CONCLUSIONS

Azotobacter vinelandii can degrade 1 mmol l(-1) of TCN into ammonia and methane. However, the inhibitory effects of exogenous ammonia and nitrite on TCN degradation by this bacterium were found in this study.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first report defining the capability of A. vinelandii to degrade TCN. This bacterium might have potential value in applied strategies for removing metal-cyano wastes. Furthermore, these findings would be helpful in designing a practical system inoculated with A. vinelandii for the treatment of TCN.

Bacterial degradation of cyanide and its metal complexes under alkaline conditions

A bacterial strain able to use cyanide as the sole nitrogen source under alkaline conditions has been isolated. The bacterium was classified as Pseudomonas pseudoalcaligenes by comparison of its 16S RNA gene sequence to those of existing strains and deposited in the Coleccion Espanola de Cultivos Tipo (Spanish Type Culture Collection) as strain CECT5344. Cyanide consumption is an assimilative process, since (i) bacterial growth was concomitant and proportional to cyanide degradation and (ii) the bacterium stoichiometrically converted cyanide into ammonium in the presence of l-methionine-d,l-sulfoximine, a glutamine synthetase inhibitor. The bacterium was able to grow in alkaline media, up to an initial pH of 11.5, and tolerated free cyanide in concentrations of up to 30 mM, which makes it a good candidate for the biological treatment of cyanide-contaminated residues. Both acetate and d,l-malate were suitable carbon sources for cyanotrophic growth, but no growth was detected in media with cyanide as the sole carbon source. In addition to cyanide, P. pseudoalcaligenes CECT5344 used other nitrogen sources, namely ammonium, nitrate, cyanate, cyanoacetamide, nitroferricyanide (nitroprusside), and a variety of cyanide-metal complexes. Cyanide and ammonium were assimilated simultaneously, whereas cyanide strongly inhibited nitrate and nitrite assimilation. Cyanase activity was induced during growth with cyanide or cyanate, but not with ammonium or nitrate as the nitrogen source. This result suggests that cyanate could be an intermediate in the cyanide degradation pathway, but alternative routes cannot be excluded.

Enzymatic assimilation of cyanide via pterin-dependent oxygenolytic cleavage to ammonia and formate in Pseudomonas fluorescens NCIMB 11764

Utilization of cyanide as a nitrogen source by Pseudomonas fluorescens NCIMB 11764 occurs via oxidative conversion to carbon dioxide and ammonia, with the latter compound satisfying the nitrogen requirement. Substrate attack is initiated by cyanide oxygenase (CNO), which has been shown previously to have properties of a pterin-dependent hydroxylase. CNO was purified 71-fold and catalyzed the quantitative conversion of cyanide supplied at micromolar concentrations (10 to 50 micro M) to formate and ammonia. The specific activity of the partially purified enzyme was approximately 500 mU/mg of protein. The pterin requirement for activity could be satisfied by supplying either the fully (tetrahydro) or partially (dihydro) reduced forms of various pterin compounds at catalytic concentrations (0.5 micro M). These compounds included, for example, biopterin, monapterin, and neopterin, all of which were also identified in cell extracts. Substrate conversion was accompanied by the consumption of 1 and 2 molar equivalents of molecular oxygen and NADH, respectively. When coupled with formate dehydrogenase, the complete enzymatic system for cyanide oxidation to carbon dioxide and ammonia was reconstituted and displayed an overall reaction stoichiometry of 1:1:1 for cyanide, O(2), and NADH consumed. Cyanide was also attacked by CNO at a higher concentration (1 mM), but in this case formamide accumulated as the major reaction product (formamide/formate ratio, 0.6:0.3) and was not further degraded. A complex reaction mechanism involving the production of isocyanate as a potential CNO monooxygenation product is proposed. Subsequent reduction of isocyanate to formamide, whose hydrolysis occurs as a CNO-bound intermediate, is further envisioned. To our knowledge, this is the first report of enzymatic conversion of cyanide to formate and ammonia by a pterin-dependent oxygenative mechanism.

Biotransformation of cyanide to methane and ammonia by Klebsiella oxytoca

Klebsiella oxytoca, isolated from cyanide-containing industrial wastewater, was shown to be able to biodegrade cyanide to non-toxic endproducts using cyanide as the sole nitrogen source. In this study, ammonia was one of the detected endproduct of cyanide biodegradation by the concentrated resting cells of K. oxytoca. Moreover, cyanide has been shown to be biotransformed to methane through the actions of concentrated resting cells. Biodegradation of cyanide by cell-free extracts was not observed, which might be due to the inactivation of nitrogenase (an oxygen-labial enzyme) caused by the oxygen exposure after cell disruption. Results show that the cyanide consumption by resting cells of K. oxytoca was induced when the pretreatment of these cells with cyanide was conducted. However, the cyanide-degrading capability of resting cells pretreated with ammonia was inhibited. The inhibition of cyanide degradation by resting cells of K. oxytoca was affected by the ammonia concentration. This might result from the suppression of nitrogenase activity of K. oxytoca by ammonia since nitrogenase was suggested to be the sole cyanide-degrading enzyme during the cyanide degradation process. Results from this study also show that the processes of cyanide biodegradation and ammonia production by resting cells occurred simultaneously. This suggests that the utilization of cyanide as nitrogen source by K. oxytoca might proceed using ammonia as an assimilatory substrate.

Degradation of tetracyanonickelate (II) by Cryptococcus humicolus MCN2

A new yeast strain capable of degrading free and metallocyanides was isolated from coke-plant wastewater. The isolated strain designated MCN2 was identified as Cryptococcus humicolus by 26S rDNA sequencing and phylogenetic analysis. During growth of the isolate with KCN as a sole nitrogen source, formamide and formic acid were found as transient intermediates by [(13)C]nuclear magnetic resonance analysis and ammonia accumulated as a final product in the culture medium. The strain MCN2 could degrade high concentrations of tetracyanonickelate (II) (K(2)Ni(CN)(4), TCN) up to 65 mM CN within 60 h when a sufficient amount of glucose was supplied as a carbon source. The maximal degradation rate of TCN was 2.5 mM CN h(-1) at the initial concentration of 51 mM CN.

Pseudomonas aeruginosa RoxR, a response regulator related to Rhodobacter sphaeroides PrrA, activates expression of the cyanide-insensitive terminal oxidase

The facultative anaerobe Pseudomonas aeruginosa has multiple aerobic electron transport pathways, one of which is terminated by a cyanide-insensitive oxidase (CIO). This study characterizes a P. aeruginosa two-component system that regulates CIO production. The response regulator of this system (RoxR) has significant amino acid sequence similarity to PrrA of Rhodobacter sphaeroides and related proteins in other alpha-proteobacteria. In heterologous complementation analysis, R. sphaeroides PrrA rescued the growth defect of a P. aeruginosa mutant lacking RoxR, and RoxR enabled photosynthetic growth of an R. sphaeroides PrrA mutant. Also, RoxR could substitute for PrrA in activating transcription in vitro, demonstrating that these proteins are functional homologues. P. aeruginosa strains lacking RoxR or the sensor kinase (RoxS) were more sensitive than wild type to the respiratory inhibitors cyanide and azide. The phenotypes of these mutant strains correlated with reduced cyanide-insensitive O2 utilization and less cyanide-dependent expression of the locus encoding the CIO (cioAB). The ability of purified RoxR to bind to the cioAB promoter region also suggests that this protein acts directly to regulate cioAB transcription. Therefore, RoxR appears to play a role in regulating the transcription of loci for P. aeruginosa energy-generating enzymes similar to that of its homologues in alpha-proteobacteria.

Evidence that bacterial cyanide oxygenase is a pterin-dependent hydroxylase

The soluble cell-free fraction (150,000g high-speed supernatants [HSS]) of Pseudomonas fluorescens NCIMB 11764 contains putative cyanide oxygenase (CNO) responsible for initiating cyanide oxidation and assimilation as a nitrogenous growth substrate. CNO activity, assayed either by cyanide-dependent O(2) or NADH uptake, or by conversion of radioactive K(14)CN to (14)CO(2), was detected at micromolar concentrations (apparent half-saturation constant, 4 microM). Results demonstrating that CNO requires a protein-enriched cell fraction and a low MW redox factor (<500 Da) for which reduced biopterin could substitute are presented. The properties of CNO are consistent with those of a pterin hydroxylase.

Accumulation of alpha-keto acids as essential components in cyanide assimilation by Pseudomonas fluorescens NCIMB 11764

Pyruvate (Pyr) and alpha-ketoglutarate (alphaKg) accumulated when cells of Pseudomonas fluorescens NCIMB 11764 were cultivated on growth-limiting amounts of ammonia or cyanide and were shown to be responsible for the nonenzymatic removal of cyanide from culture fluids as previously reported (J.-L. Chen and D. A. Kunz, FEMS Microbiol. Lett. 156:61-67, 1997). The accumulation of keto acids in the medium paralleled the increase in cyanide-removing activity, with maximal activity (760 micromol of cyanide removed min-1 ml of culture fluid-1) being recovered after 72 h of cultivation, at which time the keto acid concentration was 23 mM. The reaction products that formed between the biologically formed keto acids and cyanide were unambiguously identified as the corresponding cyanohydrins by 13C nuclear magnetic resonance spectroscopy. Both the Pyr and alpha-Kg cyanohydrins were further metabolized by cell extracts and served also as nitrogenous growth substrates. Radiotracer experiments showed that CO2 (and NH3) were formed as enzymatic conversion products, with the keto acid being regenerated as a coproduct. Evidence that the enzyme responsible for cyanohydrin conversion is cyanide oxygenase, which was shown previously to be required for cyanide utilization, is based on results showing that (i) conversion occurred only when extracts were induced for the enzyme, (ii) conversion was oxygen and reduced-pyridine nucleotide dependent, and (iii) a mutant strain defective in the enzyme was unable to grow when it was provided with the cyanohydrins as a growth substrate. Pyr and alphaKg were further shown to protect cells from cyanide poisoning, and excretion of the two was directly linked to utilization of cyanide as a growth substrate. The results provide the basis for a new mechanism of cyanide detoxification and assimilation in which keto acids play an essential role.

Incorporation of Molecular Oxygen and Water during Enzymatic Oxidation of Cyanide by Pseudomonas fluorescens NCIMB 11764

Cell extracts (high-speed [150,000 x g] supernatants) from Pseudomonas fluorescens NCIMB 11764 catalyzed the oxidation of cyanide to CO(inf2) (and NH(inf3)). Conversion was both oxygen and NADH dependent, with 1 mol of each being consumed per mol of cyanide degraded. Analysis of (sup13)CO(inf2) by mass spectrometry indicated that one atom each of isotopically labelled oxygen 18 from molecular oxygen and water were incorporated during enzymatic conversion. The results confirm earlier reports of oxygenase-mediated cyanide conversion in this organism. A reaction pathway for cyanide oxidation involving initial monooxygenation followed by hydrolysis of a hypothetical oxygenated intermediate to CO(inf2) (and NH(inf3)) is proposed.

Cell-free extract(s) of Pseudomonas putida catalyzes the conversion of cyanides, cyanates, thiocyanates, formamide, and cyanide-containing mine waters into ammonia

Our isolate, Pseudomonas putida, is known to be capable of utilizing cyanides as the sole source of carbon (C) and nitrogen (N) both in the form of free cells and cells immobilized in calcium alginate. In the present study, the cell-free extract(s) were prepared from the cells of P. putida grown in the presence of sodium cyanide. The ability of enzyme(s) to convert cyanides, cyanates, thiocyanates, formamide and cyanide-containing mine waters into ammonia (NH3) was studied at pH 7.5 and pH 9.5. The kinetic analysis of cyanide and formamide conversion into NH3 at pH 7.5 and pH 9.5 by the cell-free extract(s) of P. putida was also studied. The Km and Vmax values for cyanide/formamide were found to be 4.3/8 mM and 142/227 mumol NH3 released mg protein-1 min-1 respectively at pH 7.5 and 5/16.67 mM and 181/434 mumol NH3 released mg protein-1 h-1 respectively at pH 9.5. The study thus concludes that the cell-free extract(s) of P. putida is able to metabolize not only cyanides, cyanates, thiocyanates, and formamide but also cyanide-containing mine waters to NH3.