Xanthine dehydrogenase and 2-furoyl-coenzyme A dehydrogenase from Pseudomonas putida Fu1: two molybdenum-containing dehydrogenases of novel structural composition

Efficient chemoenzymatic valorization of biobased D-fructose into 2,5-bis(hydroxymethyl)furan with deep eutectic solvent Lactic acid:Betaine and Pseudomonas putida S12 whole cells

2,5-Bis(hydroxymethyl)furan (BHMF) is one kind of important upgraded derivatives of biobased 5-hydroxymethylfuran (5-HMF). This study verified the feasibility of one-pot chemoenzymatic conversion of biobased D-fructose to BHMF by cascade catalysis with deep eutectic solvent Lactic acid:Betaine (LA:B) and reductase biocatalyst in LA:B - H₂O. Using D-fructose (36.0 g/L) as feedstock, the yield of 5-HMF reached 91.6% in DES LA:B - H₂O (15:85, v:v) at 150 °C for 1.5 h. Using D-fructose (2 mol D-fructose/mol 5-HMF) as cosubstrate, commercial 5-HMF (125 mM) was converted into BHMF at 90.7% yield by whole-cells of Pseudomonas putida S12 within 24 h at 30 °C and pH 8.0. In addition, Pseudomonas Putida S12 could efficiently transform D-fructose-valorized 5-HMF into BHMF [98.4% yield, based on 5-HMF; 90.1% yield, based on substrate D-fructose] in DES LA:B - H₂O. An efficient chemoenzymatic valorization of D-fructose to BHMF was developed in a benign reaction system.

Production of Propionate by a Sequential Fermentation-Biotransformation Process via l-Threonine

Bio-based propionate is widely welcome in the food additive industry. The current anaerobic process by Propionibacteria endures low titers and a long fermentation time. In this study, a new route for propionate production from l-threonine was designed. 2-Ketobutyrate, deaminated from l-threonine, is cleaved into propionaldehyde and CO₂ and then be oxidized into propionic acid, which is neutralized by ammonia released from the first deamination step. This CoA-independent pathway with only CO₂ as a byproduct boosts propionate production from l-threonine with high productivity and purity. The key enzyme for 2-ketobutyrate decarboxylation was selected, and its expression was optimized. The engineered Pseudomonas putida strain, harboring 2-ketoisovalerate decarboxylase from Lactococcus lactis could produce 580 mM (43 g/L) pure propionic acid from 600 mM l-threonine in 24 h in the batch biotransformation process. Furthermore, a high titer of 62 g/L propionic acid with a productivity of 1.07 g/L/h and a molar yield of >0.98 was achieved in the fed-batch pattern. Finally, an efficient sequential fermentation-biotransformation process was demonstrated to produce propionate directly from the fermentation broth containing l-threonine, which further reduces the costs since no l-threonine purification step is required.

Selective Biosynthesis of Furoic Acid From Furfural by Pseudomonas Putida and Identification of Molybdate Transporter Involvement in Furfural Oxidation

Upgrading of furanic aldehydes to their corresponding furancarboxylic acids has received considerable interest recently. Herein we reported selective oxidation of furfural (FAL) to furoic acid (FA) with quantitative yield using whole-cells of Pseudomonas putida KT2440. The biocatalytic capacity could be substantially promoted through adding 5-hydroxymethylfurfural into media at the middle exponential growth phase. The reaction pH and cell dosage had notable impacts on both FA titer and selectivity. Based on the validation of key factors for FAL conversion, the capacity of P. putida KT2440 to produce FAL was substantially improved. In batch bioconversion, 170 mM FA was produced with selectivity nearly 100% in 2 h, whereas 204 mM FA was produced with selectivity above 97% in 3 h in fed-batch bioconversion. Particularly, the role of molybdate transporter in oxidation of FAL and 5-hydroxymethylfurfural was demonstrated for the first time. The furancarboxylic acids synthesis was repressed markedly by destroying molybdate transporter, which implied Mo-dependent enzyme/molybdoenzyme played pivotal role in such oxidation reactions. This research further highlights the potential of P. putida KT2440 as next generation industrial workhorse and provides a novel understanding of molybdoenzyme in oxidation of furanic aldehydes.

Valorization of Gelidium amansii for dual production of D-galactonic acid and 5-hydroxymethyl-2-furancarboxylic acid by chemo-biological approach

BACKGROUND

Marine macroalgae Gelidium amansii is a promising feedstock for production of sustainable biochemicals to replace petroleum and edible biomass. Different from terrestrial lignocellulosic biomass, G. amansii is comprised of high carbohydrate content and has no lignin. In previous studies, G. amansii biomass has been exploited to obtain fermentable sugars along with suppressing 5-hydroxymethylfurfural (HMF) formation for bioethanol production. In this study, a different strategy was addressed and verified for dual production of D-galactose and HMF, which were subsequently oxidized to D-galactonic acid and 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) respectively via Pseudomonas putida.

RESULTS

G. amansii biomass was hydrolyzed by dilute acid to form D-galactose and HMF. The best result was attained after pretreatment with 2% (w/w) HCl at 120 °C for 40 min. Five different Pseudomonas sp. strains including P. putida ATCC 47054, P. fragi ATCC 4973, P. stutzeri CICC 10402, P. rhodesiae CICC 21960, and P. aeruginosa CGMCC 1.10712, were screened for highly selective oxidation of D-galactose and HMF. Among them, P. putida ATCC 47054 was the outstanding suitable biocatalyst converting D-galactose and HMF to the corresponding acids without reduced or over-oxidized products. It was plausible that the pyrroloquinoline quinone-dependent glucose dehydrogenase and undiscovered molybdate-dependent enzyme(s) in P. putida ATCC 47054 individually played pivotal role for D-galactose and HMF oxidation. Taking advantage of its excellent efficiency and high selectivity, a maximum of 55.30 g/L D-galactonic acid and 11.09 g/L HMFCA were obtained with yields of 91.1% and 98.7% using G. amansii hydrolysates as substrate.

CONCLUSIONS

Valorization of G. amansii biomass for dual production of D-galactonic acid and HMFCA can enrich the product varieties and improve the economic benefits. This study also demonstrates the perspective of making full use of marine feedstocks to produce other value-added products.

Biocatalytic production of 2,5-furandicarboxylic acid: recent advances and future perspectives

2,5-Furandicarboxylic acid (FDCA) is attracting increasing attention because of its potential applications as a sustainable substitute to petroleum-derived terephthalic acid for the production of bio-based polymers, such as poly(ethylene 2,5-furandicarboxylate) (PEF). Many catalytic methods have been developed for the synthesis of FDCA, including chemocatalysis, biocatalysis, photocatalysis, and electrocatalysis. Biocatalysis is a promising approach with advantages that include mild reaction condition, lower cost, higher selectivity, and environment amity. However, the biocatalytic production of FDCA has hardly been reviewed. To fully understand the current research developments, this review comprehensively considers the research progress on toxic effects and biodegradation of furan aldehydes, and then summarizes the latest achievements concerning the synthesis of FDCA from 5-hydroxymethylfurfural and other chemicals, such as 2-furoic acid and 5-methoxymethylfurfural. Our primary focus is on biocatalytic methods, including enzymatic catalysis (in vitro) and whole-cell catalysis (in vivo). Furthermore, future research directions and general developmental trends for more efficient biocatalytic production of FDCA are also proposed.

Replacing Di(2-ethylhexyl) Terephthalate by Di(2-ethylhexyl) 2,5-Furandicarboxylate for PVC Plasticization: Synthesis, Materials Preparation and Characterization

The worldwide regulatory demand for the elimination of non-phthalate compounds for poly(vinyl chloride) (PVC) plasticization has intensified the search for alternatives. Concomitantly, sustainability concerns have highlighted sugar-based 2,5-furandicarboxylic acid as one key renewable-chemical for the development of several products, namely di(2-ethylhexyl) 2,5-furandicarboxylate (DEHF) plasticizer. This study addresses the use of DEHF under a realistic scenario of the co-existence of both DEHF and entirely fossil-based plasticizers. More precisely, original PVC blends using mixtures of non-toxic DEHF and di(2-ethylhexyl) terephthalate ester (DEHT) were designed. The detailed structural, thermal, and mechanical characterization of these materials showed that they all have a set of interesting properties that are compatible with those of commercial DEHT, namely a low glass transition (19.2-23.8 °C) and enhanced elongation at break (up to 330%). Importantly, migration tests under different daily situations, such as for example exudation from food/beverages packages and medical blood bags, reveal very low weight loss percentages. For example, in both distilled water and phosphate buffered saline (PBS) solution, weight loss does not exceed ca. 0.3% and 0.2%, respectively. Viability tests show, for the first time, that up to 500 μM of DEHF, a promising cytotoxic profile is observed, as well as for DEHT. Overall, this study demonstrates that the combination of DEHF and DEHT plasticizers result in a noticeable plasticized PVC with an increased green content with promising cytotoxic results.

A Case of Adaptive Laboratory Evolution (ALE): Biodegradation of Furfural by Pseudomonas pseudoalcaligenes CECT 5344

Pseudomonas pseudoalcaligenes CECT 5344 is a bacterium able to assimilate cyanide as a nitrogen source at alkaline pH. Genome sequencing of this strain allowed the detection of genes related to the utilization of furfurals as a carbon and energy source. Furfural and 5-(hydroxymethyl) furfural (HMF) are byproducts of sugars production during the hydrolysis of lignocellulosic biomass. Since they inhibit the yeast fermentation to obtain bioethanol from sugars, the biodegradation of these compounds has attracted certain scientific interest. P. pseudoalcaligenes was able to use furfuryl alcohol, furfural and furoic acid as carbon sources, but after a lag period of several days. Once adapted, the evolved strain (R1D) did not show any more prolonged lag phases. The transcriptomic analysis (RNA-seq) of R1D revealed a non-conservative punctual mutation (L261R) in BN5_2307, a member of the AraC family of activators, modifying the charge of the HTH region of the protein. The inactivation of the mutated gene in the evolved strain by double recombination reverted to the original phenotype. Although the bacterium did not assimilate HMF, it transformed it into value-added building blocks for the chemical industry. These results could be used to improve the production of cost-effective second-generation biofuels from agricultural wastes.

Highly Selective Oxidation of 5-Hydroxymethylfurfural to 5-Hydroxymethyl-2-Furancarboxylic Acid by a Robust Whole-Cell Biocatalyst

Value-added utilization of biomass-derived 5-hydroxymethylfurfural (HMF) to produce useful derivatives is of great interest. In this work, extremely radiation resistant Deinococcus wulumuqiensis R12 was explored for the first time as a new robust biocatalyst for selective oxidation of HMF to 5-hydroxymethylfuroic acid (HMFCA). Its resting cells exhibited excellent catalytic performance in a broad range of pH and temperature values, and extremely high tolerance to HMF and the HMFCA product. An excellent yield of HMFCA (up to 90%) was achieved when the substrate concentration was set to 300 mM under the optimized reaction conditions. In addition, 511 mM of product was obtained within 20 h by employing a fed-batch strategy, affording a productivity of 44 g/L per day. Of significant synthetic interest was the finding that the D. wulumuqiensis R12 cells were able to catalyze the selective oxidation of other structurally diverse aldehydes to their corresponding acids with good yield and high selectivity, indicating broad substrate scope and potential widespread applications in biotechnology and organic chemistry.

Biotransformation of 5-hydroxymethylfurfural by Acinetobacter oleivorans S27 for the synthesis of furan derivatives

Hydroxymethylfurfural (HMF) is an industrially important chemical which is a starting material in the production of plenty of platform chemicals. In this study, a complete biotransformation of HMF was achieved using a novel isolate, Acinetobacter oleivorans S27. This strain could tolerate up to 3000 mg/L of HMF concentration and convert to other furan derivatives. The conversion products includes high-value chemicals like 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), a known interleukin inhibitor and 2,5-furan dicarboxylic acid (FDCA), an alternate of terephthalic acid in polyester industries. The biotransformation efficiency was found to be 100%, as there is complete conversion of HMF to other chemicals. Most importantly, it is an environmental friendly process for the production of furan derivatives.

Modelling of growth kinetics of isolated Pseudomonas sp. and optimisation of parameters for enhancement of xanthine oxidoreductase production by statistical design of experiments

This report presents the substrate inhibitory effect of xanthine (XN) on microbial growth and optimisation of effective parameters to achieve high enzyme activity of xanthine oxidoreductase (XOR) through statistical design. Three efficient isolated strains (Pseudomonas aeruginosa CEBP1 and CEBP2, Pseudomonas sp. CEB1G) were screened for growth kinetic studies. Substrate inhibitory models (eg. Aiba, Edward) could explain the growth kinetics of CEBP1, CEBP2 and CEB1G very well with various initial [XN] (S₀), e.g., 0.1-35 g L^-1. Highest XOR activity was obtained at stationary phase when biomass yield was high. Highest catalytic efficiency (k_cat/K_M) of XOR was obtained by CEBP1 at optimum specific growth rate of 0.082 h^-1 and biomass yield of 0.196 g g^-1 at S₀ = 5 g L^-1. The effects of S₀, pH and temperature were studied by Box-Behnken experimental design to evaluate the interactive effects of the significant variables influencing XOR production by CEBP1. ANOVA with high correlation coefficient (R² > 0.99) and lower 'Prob > F'value (< 0.05) validated the second order polynomial model for the enzyme production. The highest XOR activity of 31.2 KU min^-1mg^-1 was achieved by CEBP1 under optimised conditions (35 °C; S₀=5 g L^-1; pH = 7.0) as compared to any report in literature. A sevenfold substrate affinity of the enzyme was observed after purification.

Conversion and assimilation of furfural and 5-(hydroxymethyl)furfural by Pseudomonas putida KT2440

The sugar dehydration products, furfural and 5-(hydroxymethyl)furfural (HMF), are commonly formed during high-temperature processing of lignocellulose, most often in thermochemical pretreatment, liquefaction, or pyrolysis. Typically, these two aldehydes are considered major inhibitors in microbial conversion processes. Many microbes can convert these compounds to their less toxic, dead-end alcohol counterparts, furfuryl alcohol and 5-(hydroxymethyl)furfuryl alcohol. Recently, the genes responsible for aerobic catabolism of furfural and HMF were discovered in Cupriavidus basilensis HMF14 to enable complete conversion of these compounds to the TCA cycle intermediate, 2-oxo-glutarate. In this work, we engineer the robust soil microbe, Pseudomonas putida KT2440, to utilize furfural and HMF as sole carbon and energy sources via complete genomic integration of the 12 kB hmf gene cluster previously reported from Burkholderia phytofirmans. The common intermediate, 2-furoic acid, is shown to be a bottleneck for both furfural and HMF metabolism. When cultured on biomass hydrolysate containing representative amounts of furfural and HMF from dilute-acid pretreatment, the engineered strain outperforms the wild type microbe in terms of reduced lag time and enhanced growth rates due to catabolism of furfural and HMF. Overall, this study demonstrates that an approach for biological conversion of furfural and HMF, relative to the typical production of dead-end alcohols, enables both enhanced carbon conversion and substantially improves tolerance to hydrolysate inhibitors. This approach should find general utility both in emerging aerobic processes for the production of fuels and chemicals from biomass-derived sugars and in the biological conversion of high-temperature biomass streams from liquefaction or pyrolysis where furfural and HMF are much more abundant than in biomass hydrolysates from pretreatment.

•

HMF and furfural are common microbial inhibitors in biomass conversion.

•

HMF and furfural gene cluster was isolated from Burkholderia phytofirmans..

•

We heterologously express the HMF/furfural gene cluster in Pseudomonas putida..

•

Expression enables cultivation on HMF and furfural as a sole carbon source.

•

Expression also enables enhanced conversion on lignocellulosic hydrolysate.

Identification, purification and characterization of furfural transforming enzymes from Clostridium beijerinckii NCIMB 8052

Generation of microbial inhibitory compounds such as furfural and 5-hydroxymethylfurfural (HMF) is a formidable roadblock to fermentation of lignocellulose-derived sugars to butanol. Bioabatement offers a cost effective strategy to circumvent this challenge. Although Clostridium beijerinckii NCIMB 8052 can transform 2-3 g/L of furfural and HMF to their less toxic alcohols, higher concentrations present in biomass hydrolysates are intractable to microbial transformation. To delineate the mechanism by which C. beijerinckii detoxifies furfural and HMF, an aldo/keto reductase (AKR) and a short-chain dehydrogenase/reductase (SDR) found to be over-expressed in furfural-challenged cultures of C. beijerinckii were cloned and over-expressed in Escherichia coli Rosetta-gami™ B(DE3)pLysS, and purified by histidine tag-assisted immobilized metal affinity chromatography. Protein gel analysis showed that the molecular weights of purified AKR and SDR are close to the predicted values of 37 kDa and 27 kDa, respectively. While AKR has apparent Km and Vmax values of 32.4 mM and 254.2 mM s(-1) respectively, using furfural as substrate, SDR showed lower Km (26.4 mM) and Vmax (22.6 mM s(-1)) values on the same substrate. However, AKR showed 7.1-fold higher specific activity on furfural than SDR. Further, both AKR and SDR were found to be active on HMF, benzaldehyde, and butyraldehyde. Both enzymes require NADPH as a cofactor for aldehydes reduction. Based on these results, it is proposed that AKR and SDR are involved in the biotransformation of furfural and HMF by C. beijerinckii.

Allopurinol-mediated lignocellulose-derived microbial inhibitor tolerance by Clostridium beijerinckii during acetone-butanol-ethanol (ABE) fermentation

In addition to glucans, xylans, and arabinans, lignocellulosic biomass hydrolysates contain significant levels of nonsugar components that are toxic to the microbes that are typically used to convert biomass to biofuels and chemicals. To enhance the tolerance of acetone-butanol-ethanol (ABE)-generating Clostridium beijerinckii NCIMB 8052 to these lignocellulose-derived microbial inhibitory compounds (LDMICs; e.g., furfural), we have been examining different metabolic perturbation strategies to increase the cellular reductant pools and thereby facilitate detoxification of LDMICs. As part of these efforts, we evaluated the effect of allopurinol, an inhibitor of NAD(P)H-generating xanthine dehydrogenase (XDH), on C. beijerinckii grown in furfural-supplemented medium and found that it unexpectedly increased the rate of detoxification of furfural by 1.4-fold and promoted growth, butanol, and ABE production by 1.2-, 2.5-, and 2-fold, respectively. Since NAD(P)H/NAD(P)(+) levels in C. beijerinckii were largely unchanged upon allopurinol treatment, we postulated and validated a possible basis in DNA repair to account for the solventogenic gains with allopurinol. Following the observation that supplementation of allopurinol in the C. beijerinckii growth media mitigates the toxic effects of nalidixic acid, a DNA-damaging antibiotic, we found that allopurinol elicited 2.4- and 6.7-fold increase in the messenger RNA (mRNA) levels of xanthine and hypoxanthine phosphoribosyltransferases, key purine-salvage enzymes. Consistent with this finding, addition of inosine (a precursor of hypoxanthine) and xanthine led to 1.4- and 1.7-fold increase in butanol production in furfural-challenged cultures of C. beijerinckii. Taken together, our results provide a purine salvage-based rationale for the unanticipated effect of allopurinol in improving furfural tolerance of the ABE-fermenting C. beijerinckii.

Engineering Sugar Utilization and Microbial Tolerance toward Lignocellulose Conversion

Production of fuels and chemicals through a fermentation-based manufacturing process that uses renewable feedstock such as lignocellulosic biomass is a desirable alternative to petrochemicals. Although it is still in its infancy, synthetic biology offers great potential to overcome the challenges associated with lignocellulose conversion. In this review, we will summarize the identification and optimization of synthetic biological parts used to enhance the utilization of lignocellulose-derived sugars and to increase the biocatalyst tolerance for lignocellulose-derived fermentation inhibitors. We will also discuss the ongoing efforts and future applications of synthetic integrated biological systems used to improve lignocellulose conversion.

Transcriptional analysis of Amorphotheca resinae ZN1 on biological degradation of furfural and 5-hydroxymethylfurfural derived from lignocellulose pretreatment

BACKGROUND

Furfural and 5-hydroxymethylfurfural (HMF) are the two major inhibitor compounds generated from lignocellulose pretreatment, especially for dilute acid, steam explosion, neutral hot water pretreatment methods. The two inhibitors severely inhibit the cell growth and metabolism of fermenting strains in the consequent bioconversion step. The biodetoxification strain Amorphotheca resinae ZN1 has demonstrated its extraordinary capacity of fast and complete degradation of furfural and HMF into corresponding alcohol and acid forms. The elucidation of degradation metabolism of A. resinae ZN1 at molecular level will facilitate the detoxification of the pretreated lignocellulose biomass and provide the metabolic pathway information for more powerful biodetoxification strain development.

RESULTS

Amorphotheca resinae ZN1 was able to use furfural or HMF as the sole carbon source for cell growth. During the detoxification process, A. resinae ZN1 firstly reduced furfural or HMF into furfuryl alcohol or HMF alcohol, and then oxidized into furoic acid or HMF acid through furan aldehyde as the intermediate at low concentration level. The cell mass measurement suggested that furfural was more toxic to A. resinae ZN1 than HMF. In order to identify the degradation mechanism of A. resinae ZN1, transcription levels of 137 putative genes involved in the degradation of furfural and HMF in A. resinae ZN1 were investigated using the real-time quantitative PCR (qRT-PCR) method under the stress of furfural and HMF, as well as the stress of their secondary metabolites, furfuryl alcohol and HMF alcohol. Two Zn-dependent alcohol dehydrogenase genes and five AKR/ARI genes were found to be responsible for the furfural and HMF conversion to their corresponding alcohols. For the conversion of the two furan alcohols to the corresponding acids, three propanol-preferring alcohol dehydrogenase genes, one NAD(P)(+)-depending aldehyde dehydrogenase gene, or two oxidase genes with free oxygen as the substrate were identified under aerobic condition.

CONCLUSIONS

The genes responsible for the furfural and HMF degradation to the corresponding alcohols and acids in A. resinae ZN1 were identified based on the analysis of the genome annotation, the gene transcription data and the inhibitor conversion results. These genetic resources provided the important information for understanding the mechanism of furfural and HMF degradation and modification of high tolerant strains used for biorefinery processing.

Extreme furfural tolerance of a soil bacterium Enterobacter cloacae GGT036

Detoxification process of cellular inhibitors including furfural is essential for production of bio-based chemicals from lignocellulosic biomass. Here we isolated an extreme furfural-tolerant bacterium Enterobacter cloacae GGT036 from soil sample collected in Mt. Gwanak, Republic of Korea. Among isolated bacteria, only E. cloacae GGT036 showed cell growth with 35 mM furfural under aerobic culture. Compared to the maximal half inhibitory concentration (IC50) of well-known industrial strains Escherichia coli (24.9 mM furfural) and Corynebacterium glutamicum (10 mM furfural) based on the cell density, IC50 of E. cloacae GGT036 (47.7 mM) was significantly higher after 24 h, compared to E. coli and C. glutamicum. Since bacterial cell growth was exponentially inhibited depending on linearly increased furfural concentrations in the medium, we concluded that E. cloacae GGT036 is an extreme furfural-tolerant bacterium. Recently, the complete genome sequence of E. cloacae GGT036 was announced and this could provide an insight for engineering of E. cloacae GGT036 itself or other industrially relevant bacteria.

Chemotaxis to furan compounds by furan-degrading Pseudomonas strains

Two Pseudomonas strains known to utilize furan derivatives were shown to respond chemotactically to furfural, 5-hydroxymethylfurfural, furfuryl alcohol, and 2-furoic acid. In addition, a LysR-family regulatory protein known to regulate furan metabolic genes was found to be involved in regulating the chemotactic response.

Microbial degradation of furanic compounds: biochemistry, genetics, and impact

Microbial metabolism of furanic compounds, especially furfural and 5-hydroxymethylfurfural (HMF), is rapidly gaining interest in the scientific community. This interest can largely be attributed to the occurrence of toxic furanic aldehydes in lignocellulosic hydrolysates. However, these compounds are also widespread in nature and in human processed foods, and are produced in industry. Although several microorganisms are known to degrade furanic compounds, the variety of species is limited mostly to Gram-negative aerobic bacteria, with a few notable exceptions. Furanic aldehydes are highly toxic to microorganisms, which have evolved a wide variety of defense mechanisms, such as the oxidation and/or reduction to the furanic alcohol and acid forms. These oxidation/reduction reactions constitute the initial steps of the biological pathways for furfural and HMF degradation. Furfural degradation proceeds via 2-furoic acid, which is metabolized to the primary intermediate 2-oxoglutarate. HMF is converted, via 2,5-furandicarboxylic acid, into 2-furoic acid. The enzymes in these HMF/furfural degradation pathways are encoded by eight hmf genes, organized in two distinct clusters in Cupriavidus basilensis HMF14. The organization of the five genes of the furfural degradation cluster is highly conserved among microorganisms capable of degrading furfural, while the three genes constituting the initial HMF degradation route are organized in a highly diverse manner. The genetic and biochemical characterization of the microbial metabolism of furanic compounds holds great promises for industrial applications such as the biodetoxifcation of lignocellulosic hydrolysates and the production of value-added compounds such as 2,5-furandicarboxylic acid.

Identification and characterization of the furfural and 5-(hydroxymethyl)furfural degradation pathways of Cupriavidus basilensis HMF14

The toxic fermentation inhibitors in lignocellulosic hydrolysates pose significant problems for the production of second-generation biofuels and biochemicals. Among these inhibitors, 5-(hydroxymethyl)furfural (HMF) and furfural are specifically notorious. In this study, we describe the complete molecular identification and characterization of the pathway by which Cupriavidus basilensis HMF14 metabolizes HMF and furfural. The identification of this pathway enabled the construction of an HMF and furfural-metabolizing Pseudomonas putida. The genetic information obtained furthermore enabled us to predict the HMF and furfural degrading capabilities of sequenced bacterial species that had not previously been connected to furanic aldehyde metabolism. These results pave the way for in situ detoxification of lignocellulosic hydrolysates, which is a major step toward improved efficiency of utilization of lignocellulosic feedstock.

Isolation and characterization of Cupriavidus basilensis HMF14 for biological removal of inhibitors from lignocellulosic hydrolysate

The formation of toxic fermentation inhibitors such as furfural and 5‐hydroxy‐2‐methylfurfural (HMF) during acid (pre‐)treatment of lignocellulose, calls for the efficient removal of these compounds. Lignocellulosic hydrolysates can be efficiently detoxified biologically with microorganisms that specifically metabolize the fermentation inhibitors while preserving the sugars for subsequent use by the fermentation host. The bacterium Cupriavidus basilensis HMF14 was isolated from enrichment cultures with HMF as the sole carbon source and was found to metabolize many of the toxic constituents of lignocellulosic hydrolysate including furfural, HMF, acetate, formate and a host of aromatic compounds. Remarkably, this microorganism does not grow on the most abundant sugars in lignocellulosic hydrolysates: glucose, xylose and arabinose. In addition, C. basilensis HMF14 can produce polyhydroxyalkanoates. Cultivation of C. basilensis HMF14 on wheat straw hydrolysate resulted in the complete removal of furfural, HMF, acetate and formate, leaving the sugar fraction intact. This unique substrate profile makes C. basilensis HMF14 extremely well suited for biological removal of inhibitors from lignocellulosic hydrolysates prior to their use as fermentation feedstock.

Purification and characterization of a furfural reductase (FFR) from Escherichia coli strain LYO1—An enzyme important in the detoxification of furfural during ethanol production

Furfural, an inhibitor of ethanol production from hemicellulose acid hydrolysates, is reductively detoxified to furfuryl alcohol by the ethanologenic bacterium Escherichia coli strain LYO1. Furfural reductase was purified 106-fold from this bacterium to approximately 50% homogeneity. It has a native molecular mass of 135 kDa, determined by gel filtration, and subunit molecular mass of approximately 68 kDa, determined by denaturing gel electrophoresis, indicating the holoenzyme is a dimer of two similar if not identical subunits. The enzyme shows strong activity from pH 4 to 8 (optimum pH 7.0), relatively high temperature tolerance (50-55 degrees C), and an apparent Km and Vmax for furfural of 1.5x10(-4)M and 28.5 micromol/min/mg of protein, respectively. It catalyzes the essentially irreversible reduction of furfural with NADPH, is specific for NADPH as cofactor, and is relatively specific for the reduction of furfural and benzaldehyde; 2-acetylfuran, xylose, and glucose were not reduced, while acetaldehyde was reduced at a rate 25-fold lower than furfural. This is the first description of a furfural reductase which, based upon size and substrate specificity, appears to represent a new type of alcohol-aldehyde oxido-reductase. The conversion of relatively toxic furfural to less toxic furfuryl alcohol suggests a beneficial role for this enzyme in mitigating furfural toxicity encountered during ethanol production from lignocellulosic biomass.

Xanthine dehydrogenase from Pseudomonas putida 86: specificity, oxidation-reduction potentials of its redox-active centers, and first EPR characterization

Xanthine dehydrogenase (XDH) from Pseudomonas putida 86, which was induced 65-fold by growth on hypoxanthine, was purified to homogeneity. It catalyzes the oxidation of hypoxanthine, xanthine, purine, and some aromatic aldehydes, using NAD+ as the preferred electron acceptor. In the hypoxanthine:NAD+ assay, the specific activity of purified XDH was 26.7 U (mg protein)(-1). Its activity with ferricyanide and dioxygen was 58% and 4%, respectively, relative to the activity observed with NAD+. XDH from P. putida 86 consists of 91.0 kDa and 46.2 kDa subunits presumably forming an alpha4beta4 structure and contains the same set of redox-active centers as eukaryotic XDHs. After reduction of the enzyme with xanthine, electron paramagnetic resonance (EPR) signals of the neutral FAD semiquinone radical and the Mo(V) rapid signal were observed at 77 K. Resonances from FeSI and FeSII were detected at 15 K. Whereas the observable g factors for FeSII resemble those of other molybdenum hydroxylases, the FeSI center in contrast to most other known FeSI centers has nearly axial symmetry. The EPR features of the redox-active centers of P. putida XDH are very similar to those of eukaryotic XDHs/xanthine oxidases, suggesting that the environment of each center and their functionality are analogous in these enzymes. The midpoint potentials determined for the molybdenum, FeSI and FAD redox couples are close to each other and resemble those of the corresponding centers in eukaryotic XDHs.

Tetrahydrofuran degradation by a newly isolated culture of Pseudonocardia sp. strain K1

An organism capable to grow aerobically on tetrahydrofuran as sole source of carbon and energy was isolated from a waste water treatment plant. The organism designated as strain K1 was identified as Pseudonocardia sp. by chemotaxonomic and morphological characteristics as well as analysis of the gene encoding the 16S rRNA. The highest binary sequence similarity value of 99.0% was obtained to Pseudonocardia sulfidoxydans and Pseudonocardia hydrocarbonoxydans. Optimal growth with a doubling time of 14 h was observed at a tetrahydrofuran concentration of 20 mM and pH 7.0 at 28 degrees C. Under these conditions the substrate was completely degraded within 72 h. In situ concentrations of up to 60 mM were tolerated by the organism without a significantly increased doubling time. The strain also grew on diethyl ether, polyethylene glycol and on gamma-butyrolactone and 4-hydroxybutyrate - two potential intermediates in tetrahydrofuran degradation - as sole carbon and energy source.

Selenium-containing xanthine dehydrogenase from Eubacterium barkeri

A specific dehydrogenase, different from nicotinic acid hydroxylase, was induced during growth of Eubacterium barkeri on xanthine. The protein designated as xanthine dehydrogenase was enriched 39-fold to apparent homogeneity using a three-step purification scheme. It exhibited an NADP-dependent specific activity of 164 micromol xanthine oxidized per min and per mg of protein. In addition it showed an NADPH-dependent oxidase and diaphorase activity. A molecular mass of 530 kDa was determined for the native enzyme and SDS/PAGE revealed three types of subunits with molecular masses of 17.5, 30 and 81 kDa indicating a dodecameric native structure. Molybdopterin was identified as the molybdenum-complexing cofactor using activity reconstitution experiments and fluorescence measurements after KI/I2 oxidation. The molecular mass of the cofactor indicated that it is of the dinucleotide type. The enzyme contained iron, acid-labile sulfur, molybdenum, tungsten, selenium and FAD at molar ratios of 17.5, 18.4, 2.3, 1.1, 0.95 and 2.8 per mol of native enzyme. Xanthine dehydrogenase was inactivated upon incubation with arsenite, cyanide and different purine analogs. Reconstitution experiments of xanthine dehydrogenase activity by addition of selenide and selenite performed with cyanide-inactivated enzyme and with chloramphenicol-treated cells, respectively, indicated that selenium is not attached to the protein in a covalently bound form such as selenocysteine.

Degradation of tetrahydrofurfuryl alcohol by Ralstonia eutropha is initiated by an inducible pyrroloquinoline quinone-dependent alcohol dehydrogenase

An organism tentatively identified as Ralstonia eutropha was isolated from enrichment cultures containing tetrahydrofurfuryl alcohol (THFA) as the sole source of carbon and energy. The strain was able to tolerate up to 200 mM THFA in mineral salt medium. The degradation was initiated by an inducible ferricyanide-dependent alcohol dehydrogenase (ADH) which was detected in the soluble fraction of cell extracts. The enzyme catalyzed the oxidation of THFA to the corresponding tetrahydrofuran-2-carboxylic acid. Studies with n-pentanol as the substrate revealed that the corresponding aldehyde was released as a free intermediate. The enzyme was purified 211-fold to apparent homogeneity and could be identified as a quinohemoprotein containing one pyrroloquinoline quinone and one covalently bound heme c per monomer. It was a monomer of 73 kDa and had an isoelectric point of 9.1. A broad substrate spectrum was obtained for the enzyme, which converted different primary alcohols, starting from C2 compounds, secondary alcohols, diols, polyethylene glycol 6000, and aldehydes, including formaldehyde. A sequence identity of 65% with a quinohemoprotein ADH from Comamonas testosteroni was found by comparing 36 N-terminal amino acids. The ferricyanide-dependent ADH activity was induced during growth on different alcohols except ethanol. In addition to this activity, an NAD-dependent ADH was present depending on the alcohol used as the carbon source.

Two-component flavin-dependent pyrrole-2-carboxylate monooxygenase from Rhodococcus sp

Pyrrole-2-carboxylate can serve as the sole source of carbon, nitrogen, and energy for a strain tentatively identified to belong to the genus Rhodococcus. An NADH-dependent oxygenase activity was detected in cell extracts that initiated the degradation of the substrate. During purification of the enzyme, this activity was separated into two protein components which were both purified to apparent homogeneity. A small monomeric 18.7-kDa protein designated as reductase, catalyzed in vitro the NADH and FAD-dependent reduction of cytochrome c and had an NADH-oxidase activity. The second component, a 54-kDa protein with a trimeric native structure had no enzymatic activity by itself, but exhibited a pyrrole-2-carboxylate-dependent oxygen consumption when it was complemented with the reductase component, FAD, and NADH. This indicated that the large protein referred to as oxygenase was responsible for the oxygen-dependent hydroxylation of the substrate. The rate of an uncoupled NADH oxidation without hydroxylation of the substrate was found to be strongly dependent on the molar ratio of both components. The uncoupling was nearly completely suppressed by a 5-7-fold molar excess of the oxygenase component. The small protein was N-terminally blocked. It was thus proteolytically digested and four of the resulting peptides were sequenced comprising 47 amino acids. The sequences of these fragments were similar to the sequences reported for the small component of different two-component flavin monooxygenases. Furthermore, the N-terminus of the oxygenase component showed high sequence similarity to the second, usually large subunit of these enzymes and to two single-component flavin monooxygenases. Thus, the enzyme from Rhodococcus sp. designated as pyrrole-2-carboxylate monooxygenase belongs to the recently discovered new class of two-component flavin aromatic monooxygenases. Some of the basic properties of both components were determined and their interaction during catalysis was investigated.

The FEMA GRAS assessment of furfural used as a flavour ingredient. Flavor and Extract Manufacturers' Association

The Expert Panel of the Flavor and Extract Manufacturers' Association (FEMA) has assessed the safety of furfural for its continued use as a flavour ingredient. The safety assessment takes into account the current scientific information on exposure, metabolism, pharmacokinetics, toxicology, carcinogenicity and genotoxicity. Furfural was reaffirmed as GRAS (GRASr) as a flavour ingredient under conditions of intended use based on: (1) its mode of metabolic detoxication in humans; (2) its low level of flavour use compared with higher intake levels as a naturally occurring component of food; (3) the safety factor calculated from results of subchronic and chronic studies, (4) the lack of reactivity with DNA; and (5) the conclusion that the only statistically significant finding in the 2-year NTP bioassays, an increased incidence of hepatocellular adenomas and carcinomas in the high-dose group of male mice, was secondary to pronounced hepatotoxicity. Taken together, these data do not indicate any risk to human health under conditions of use as a flavour ingredient. This evidence of safety is supported by the occurrence of furfural as a natural component of traditional foods, at concentrations in the diet resulting in a 'natural intake' that is at least 100 times higher than the intake of furfural from use as a flavour ingredient.

Characterization of xanthine dehydrogenase from the anaerobic bacterium Veillonella atypica and identification of a molybdopterin-cytosine-dinucleotide-containing molybdenum cofactor

The molybdenum-containing iron-sulfur flavoprotein xanthine dehydrogenase from the anaerobic bacterium Veillonella atypica has been purified approximately 800-fold with a yield of approximately 40% and a specific activity of approximately 70 micromol ferricyanide reduced x min(-1) x mg protein(-1) with xanthine as electron donor, which corresponds to approximately 30 micromol xanthine oxidized x min(-1) x mg protein(-1) with methylene blue as electron acceptor. The 129-kDa enzyme was a non-covalent heterotrimer with large (82.4 kDa), medium (28.5 kDa) and small (18.4 kDa) subunits. The N-termini of the small and medium polypeptides of V. atypica xanthine dehydrogenase and the corresponding domains of eukaryotic xanthine dehydrogenases were similar, whereas the N-terminus of the large polypeptide was unrelated to eukaryotic xanthine dehydrogenases. The enzyme contained 0.86 atoms Mo, 1.75 atoms Fe, 1.61 atoms acid-labile sulfur and 0.68 molecules FAD/molecule, which corresponds to a 1:2.0:1.9:0.8 molar ratio. Acid hydrolysis revealed 0.95 mol CMP and 0.80 mol AMP/mol xanthine dehydrogenase. After treatment of the enzyme with iodoacetamide, di(carboxamidomethyl)molybdopterin cytosine dinucleotide was identified, which indicates that molybdopterin cytosine dinucleotide is the organic portion of the V. atypica xanthine dehydrogenase molybdenum cofactor. The enzyme and its molybdenum cofactor occurred in a 1:1 molar ratio. Xanthine dehydrogenases from eukaryotic sources are characterized by a domain structure and the presence of duplicate copies of two types of [2Fe-2S) clusters. In contrast, the xanthine dehydrogenase from V. atypica had a heterotrimeric subunit structure and a single [2Fe-2S] cluster. In addition, the enzyme indicates the presence of a molybdopterin dinucleotide as a constituent of a xanthine dehydrogenase molybdenum cofactor.

Purification and characterization of a prokaryotic xanthine dehydrogenase from Comamonas acidovorans

Xanthine dehydrogenase (XDH) is induced in Comamonas acidovorans cells incubated in a limited medium with hypoxanthine as the only carbon and nitrogen source. The enzyme has been purified to homogeneity using standard techniques and characterized. It contains two subunits with M(r) values of 90 and 60 kDa. Gel filtration studies show the enzyme to have an alpha 2 beta 2 native structure. No precursor form of the enzyme is observed on Western blot analysis of cell extracts obtained at various stages of enzyme induction. Metal analysis of the purified enzyme shows 1.1 Mo, 4.0 Fe, and 3.6 phosphorus atoms per alpha beta protomer. Cofactor analysis shows the enzyme to contain a single molybdopterin mononucleotide and one FAD per alpha beta protomer. Electron spin resonance and circular dichroism spectral studies of the oxidized and reduced forms of the enzyme suggest the Fe centers to be two nonidentical [2Fe-2S] clusters. Electron spin resonance signals due to Mo(V) and neutral FAD radical are also observed in the reduced form of the enzyme. Purified enzyme preparations ranged from 70% to 100% functionality. The enzyme is irreversibly inactivated by CN- and is inhibited on incubation with allopurinol. With xanthine and NAD+ as substrates the enzyme has a specific activity of 50 units/mg, a kcat value of 120 s-1, an activity/flavin ratio of 1930, and respective Km values of 66 and 160 mM. Using 8-D-xanthine as substrate, a DV value of 1.8 is found with no change in Km. Thus, the Km and KD values of the enzyme for xanthine are equal. These data show Comamonas XDH to exhibit structural properties similar to bovine milk xanthine oxidase/dehydrogenase and to chicken liver xanthine dehydrogenase. Although the bacterial enzyme exhibits a 6-7-fold greater turnover rate than bovine or avian enzymes, the catalytic efficiencies (as measured by V/K) are similar for all three enzymes.

Hydroxylation of quinaldic acid: quinaldic acid 4-monooxygenase from Alcaligenes sp. F-2 versus quinaldic acid 4-oxidoreductases

The N-heterocycles quinaldic acid (quinoline 2-carboxylic acid), kynurenic acid (4-hydroxyquinoline 2-carboxylic acid), 2-oxo-1,2-dihydroquinoline, and xanthine are utilized by Alcaligenes sp. F-2 as sole source of carbon and energy. Although quinoline did not serve as growth substrate, 8-hydroxy-2-oxo-1,2-dihydroquinoline and 8-hydroxycoumarin, metabolites of the 'coumarin pathway' of quinoline catabolism, were isolated from the culture fluid during growth on 2-oxo-1,2-dihydroquinoline. Contrary to Serratia marcescens 2CC-1 and Pseudomonas sp. AK-2 (Sauter et al. (1993) Biol. Chem. Hoppe-Seyler 374, 1037-1046), which possess different molybdenum-containing hydroxylases catalysing the 4-hydroxylation of quinaldic acid to kynurenic acid with incorporation of oxygen derived from water and concomitant reduction of an electron acceptor, Alcaligenes sp. F-2 contains an inducible quinaldic acid 4-monooxygenase that catalyses the very same conversion in the presence of O2 and NADH. The activity of the monooxygenase was enhanced 1.5-fold by Fe2+ ions. The extremely thermolabile enzyme (apparent molecular mass: 155 kDa) exclusively accepted quinaldic acid as substrate. The 'pseudosubstrates' menadione, 8-hydroxyquinoline, and 8-hydroxy-2-oxo-1,2-dihydroquinoline effected consumption of NADH and oxygen without being hydroxylated. Quinaldic acid 4-monooxygenase was inhibited by sulfhydryl modifying and chelating agents, and by various divalent metal ions, whereas reducing agents did not affect enzymatic activity.

Quinaldine 4-oxidase from Arthrobacter sp. Rü61a, a versatile procaryotic molybdenum-containing hydroxylase active towards N-containing heterocyclic compounds and aromatic aldehydes

Quinaldine 4-oxidase from Arthrobacter sp. Rü61a, an inducible molybdenum-containing hydroxylase, was purified to homogeneity by an optimized five-step procedure. Molecular oxygen is proposed as physiological electron acceptor. Electrons are also transferred to artificial electron acceptors with E'o > -8 mV. The molybdo-iron/sulfur flavoprotein regiospecifically attacks its N-heterocyclic substrates: isoquinoline and phthalazine are hydroxylated adjacent to the N-heteroatom at Cl, whereas quinaldine, quinoline, cinnoline and quinazoline are hydroxylated at C4. Additionally, the aromatic aldehydes benzaldehyde, salicylaldehyde, vanillin and cinnamaldehyde are oxidized to the corresponding carboxylic acids, whereas short-chain aliphatic aldehydes are not. Quinaldine 4-oxidase is compared to the two molybdenum-containing hydroxylases quinoline 2-oxidoreductase from Pseudomonas putida 86 [Tshisuaka, B., Kappl, R., Hüttermann, J. & Lingens, F. (1993) Biochemistry 32, 12928-12934] and isoquinoline 1-oxidoreductase from Pseudomonas diminuta 7 [Lehmann, M., Tshisuaka, B., Fetzner, S., Röger, P. & Lingens, F. (1994) J. Biol. Chem. 269, 11254-11260] with respect to the substrates converted and the electron-acceptor specificities. These dehydrogenases hydroxylate their N-heterocyclic substrates exclusively adjacent to the heteroatom. Whereas the aldehydes tested are scarcely oxidized by quinoline 2-oxidoreductase, isoquinoline 1-oxidoreductase catalyzes the oxidation of the aromatic aldehydes, although being progressively inhibited. Neither quinoline 2-oxidoreductase nor isoquinoline 1-oxidoreductase transfer electrons to oxygen. Otherwise, the spectrum of electron acceptors used by quinoline 2-oxidoreductase and quinaldine 4-oxidase is identical. However, isoquinoline 1-oxidoreductase differs in its electron-acceptor specificity. Quinaldine 4-oxidase is unusual in its substrate and electron-acceptor specificity. This enzyme is able to function as oxidase or dehydrogenase, it oxidizes aldehydes, and it catalyzes the nucleophilic attack of N-containing heterocyclic compounds at two varying positions depending on the substrate.

A re-evaluation of the tissue distribution and physiology of xanthine oxidoreductase

Xanthine oxidoreductase is an enzyme which has the unusual property that it can exist in a dehydrogenase form which uses NAD+ and an oxidase form which uses oxygen as electron acceptor. Both forms have a high affinity for hypoxanthine and xanthine as substrates. In addition, conversion of one form to the other may occur under different conditions. The exact function of the enzyme is still unknown but it seems to play a role in purine catabolism, detoxification of xenobiotics and antioxidant capacity by producing urate. The oxidase form produces reactive oxygen species and, therefore, the enzyme is thought to be involved in various pathological processes such as tissue injury due to ischaemia followed by reperfusion, but its role is still a matter of debate. The present review summarizes information that has become available about the enzyme. Interpretations of contradictory findings are presented in order to reduce confusion that still exists with respect to the role of this enzyme in physiology and pathology.

Metabolism and excretion of [14C]furfural in the rat and mouse

The fate of furfural (2-furancarboxaldehyde) was investigated in male and female Fischer 344 (F344) rats given single oral doses of 1, 10 and 60 mg/kg and male and female CD1 mice given 1, 20 and 200 mg/kg [carbonyl-14C]furfural. There was a very high recovery (more than 90% of dose) of radioactivity in all dose groups in 72 hr. The major route of elimination was by the urine, with much smaller amounts present in the faeces and exhaled as 14CO2. The residue in the carcass after 72 hr was less than 1% of the administered dose. Furoylglycine and furanacryloylglycine were identified as the major urinary metabolites by high-performance thin-layer chromatography, radio-HPLC, gas chromatography-mass spectrometry and 1H-nuclear magnetic resonance spectroscopy, by comparison with synthetic reference compounds. There were only subtle differences in the metabolic profile as a function of dose size, sex and species. An additional minor polar metabolite was excreted by male rats and mice, and the parent acids of the glycine conjugates were excreted at the higher doses. The results are discussed in terms of the participation of xenobiotics in the chain elongation reactions of fatty acid biosynthesis.

Structural analysis and molybdenum-dependent expression of the pAO1-encoded nicotine dehydrogenase genes of Arthrobacter nicotinovorans

The genes of nicotine dehydrogenase (NDH) were identified, cloned and sequenced from the catabolic plasmid pAO1 of Arthrobacter nicotinovorans. In immediate proximity to this gene cluster is the beginning of the 6-hydroxy-L-niotine oxidase (6-HLNO) gene. NDH is composed of three subunits (A, B and C) of M(r) 30,011, 14,924 and 87,677. It belongs to a family of bacterial hydroxylases with a similar subunit structure; they have molybdopterin dinucleotide, FAD and Fe-S clusters as cofactors. Here the first complete primary structure of a bacterial hydroxylase is provided. Sequence alignments of each of the NDH subunits show similarities to the sequences of eukaryotic xanthine dehydrogenase (XDH) but not to other known molybdenum-containing bacterial enzymes. Based on alignment with XDH it is inferred that the smallest subunit (NDHB) carries an iron-sulphur cluster, that the middle-sized subunit (NDHA) binds FAD, and that the largest NDH subunit (NDHC) corresponds to the molybdopterin-binding domain of XDH. Expression of both the ndh and the 6-hino genes required the presence of nicotine and molybdenum in the culture medium. Tungsten inhibited enzyme activity but not the synthesis of the enzyme protein. The enzyme was found in A. nicotinovorans cells in a soluble form and in a membrane-associated form. In the presence of tungsten the fraction of membrane-associated NDH increased.

Formylmethanofuran dehydrogenases from methanogenic Archaea. Substrate specificity, EPR properties and reversible inactivation by cyanide of the molybdenum or tungsten iron-sulfur proteins

Formylmethanofuran dehydrogenases, which are found in methanogenic Archaea, are molybdenum or tungsten iron-sulfur proteins containing a pterin cofactor. We report here on differences in substrate specificity, EPR properties and susceptibility towards cyanide inactivation of the enzymes from Methanosarcina barkeri, Methanobacterium thermoautotrophicum and Methanobacterium wolfei. The molybdenum enzyme from M. barkeri (relative activity with N-formylmethanofuran = 100%) was found to catalyze, albeit at considerably reduced apparent Vmax, the dehydrogenation of N-furfurylformamide (11%), N-methylformamide (0.2%), formamide (0.1%) and formate (1%). The molybdenum enzyme from M. wolfei could only use N-furfurylformamide (1%) and formate (3%) as pseudosubstrates. The molybdenum enzyme from M. thermoautotrophicum and the tungsten enzymes from M. thermoautotrophicum and M. wolfei were specific for N-formylmethanofuran. The molybdenum formylmethanofuran dehydrogenases exhibited at 77 K two rhombic EPR signals, designated FMDred and FMDox, both derived from Mo as shown by isotopic substitution with 97Mo. The FMDred signal was only displayed by the active enzyme in the reduced form and was lost upon enzyme oxidation; the FMDox signal was displayed by an inactive form and was not quenched by O2. The tungsten isoenzymes were EPR silent. The molybdenum formylmethanofuran dehydrogenases were found to be inactivated by cyanide whereas the tungsten isoenzymes, under the same conditions, were not inactivated. Inactivation was associated with a characteristic change in the molybdenum-derived EPR signal. Reactivation was possible in the presence of sulfide.