Formate Hydrogenlyase Complex Research Articles

Isolation and characterization of Enterococcus faecalis isolate VT-H1: A highly efficient hydrogen-producing bacterium from palm oil mill effluent (POME)

Facultative hydrogen-producing bacteria hold immense potential for fermentative hydrogen and valuable metabolite production. In this study, a mesophilic hydrogen-producing bacterium was isolated from palm oil mill effluent (POME) and identified as Enterococcus faecalis isolate VT-H1 using hybrid sequencing technologies. Our investigation encompassed the growth of the isolate and its capacity for hydrogen and volatile fatty acid (VFA) production. The results revealed that optimal growth occurred when glucose was utilized at 37 °C and initial pH of 7.0, resulting in a final OD600 of 2.58. The isolate demonstrated exceptional hydrogen production capabilities, achieving a maximum cumulative hydrogen yield of 2.22 mol-H2/mol-glucose at 30 °C and pH 7.0, outperforming previously reported E. faecalis strains by 5.84-fold. Kinetic analysis revealed favorable parameters for hydrogen production at 30 °C, with maximum hydrogen production (Hm = 2.63 mol-H2/mol-glucose), maximum hydrogen production rate (Rm = 0.03 mol-H2/mol-glucose·h), and lag-phase time (LPT = 18.34 h). Additionally, E. faecalis isolate VT-H1 exhibited significant butyric acid production (63.0 mM) alongside acetic acid (7.0 mM). Genome analysis unveiled the presence of key enzymes associated with hydrogen production, including formate hydrogen lyase (FHL) complex with formate dehydrogenase subunit alpha (FDH-α), bidirectional [NiFe] hydrogenase, NADH-dependent oxidoreductase subunit E, and NADH-quinone oxidoreductase subunit F. Moreover, genes associated with butyric acid production were also identified. These findings highlight the potential of E. faecalis isolate VT-H1 as a promising candidate for sustainable hydrogen and VFA production. Its unique characteristics, distinct genetic profile, and exceptional hydrogen and VFA production capabilities bring exciting opportunities for future biotechnological applications and further research to optimize and scale-up its potential for industrial use.

Comparative Physiology and Genomics of Hydrogen-Producing Vibrios.

The Hyf-type formate hydrogen lyase (FHL) complex was first proposed based on sequence comparisons in Escherichia coli in 1997 (Andrews et al. in Microbiology 143:3633-3647, 1997). The hydrogenase in the Hyf-type FHL was estimated to be a proton-translocating energy-conserving [NiFe]-hydrogenase. Although the structure of FHL is similar to that of complex I, silent gene expression in E. coli has caused delays in unveiling the genetic and biochemical features of the FHL. The entire set of genes required for Hyf-type FHL synthesis has also been found in the genome sequences of Vibrio tritonius in 2015 (Matsumura et al. in Int J Hydrog Energy 40:9137-9146, 2015), which produces more hydrogen (H2) than E. coli. Here we investigate the physiological characteristics, genome comparisons, and gene expressions to elucidate the genetic backgrounds of Hyf-type FHL, and how Hyf-type FHL correlates with the higher H2 production of V. tritonius. Physiological comparisons among the seven H2-producing vibrios reveal that V. porteresiae and V. tritonius, grouped in the Porteresiae clade, show greater capacity for H2 production than the other species. The structures of FHL-Hyp gene clusters were closely related in both Porteresiae species, but differed from those of the other species with the presence of hupE, a possible nickel permease gene. Interestingly, deeper genome comparisons revealed the co-presence of nickel ABC transporter genes (nik) with the Hyf-type FHL gene only on the genome of the Porteresiae clade species. Therefore, active primary Ni transport might be one of the key factors characterizing higher H2 production in V. tritonius. Furthermore, the expression of FHL gene cluster was significantly up-regulated in V. tritonius cells stimulated with formate, indicating that formate is likely to be a control factor for the gene expression of V. tritonius FHL in a similar way to the formate regulon encoding the E. coli FHL.

FocA and its central role in fine-tuning pH homeostasis of enterobacterial formate metabolism.

During enterobacterial mixed-acid fermentation, formate is generated from pyruvate by the glycyl-radical enzyme pyruvate formate-lyase (PflB). In Escherichia coli, especially at low pH, formate is then disproportionated to CO2 and H2 by the cytoplasmically oriented, membrane-associated formate hydrogenlyase (FHL) complex. If electron acceptors are available, however, formate is oxidized by periplasmically oriented, respiratory formate dehydrogenases. Formate translocation across the cytoplasmic membrane is controlled by the formate channel, FocA, a member of the formate-nitrite transporter (FNT) family of homopentameric anion channels. This review highlights recent advances in our understanding of how FocA helps to maintain intracellular formate and pH homeostasis during fermentation. Efflux and influx of formate/formic acid are distinct processes performed by FocA and both are controlled through protein interaction between FocA's N-terminal domain with PflB. Formic acid efflux by FocA helps to maintain cytoplasmic pH balance during exponential-phase growth. Uptake of formate against the electrochemical gradient (inside negative) is energetically and mechanistically challenging for a fermenting bacterium unless coupled with proton/cation symport. Translocation of formate/formic acid into the cytoplasm necessitates an active FHL complex, whose synthesis also depends on formate. Thus, FocA, FHL and PflB function together to govern formate homeostasis. We explain how FocA achieves efflux of formic acid and propose mechanisms for pH-dependent uptake of formate both with and without proton symport. We propose that FocA displays both channel- and transporter-like behaviour. Whether this translocation behaviour is shared by other members of the FNT family is also discussed.

Structure of the membrane-bound formate hydrogenlyase complex from Escherichia coli

The prototypical hydrogen-producing enzyme, the membrane-bound formate hydrogenlyase (FHL) complex from Escherichia coli, links formate oxidation at a molybdopterin-containing formate dehydrogenase to proton reduction at a [NiFe] hydrogenase. It is of intense interest due to its ability to efficiently produce H2 during fermentation, its reversibility, allowing H2-dependent CO2 reduction, and its evolutionary link to respiratory complex I. FHL has been studied for over a century, but its atomic structure remains unknown. Here we report cryo-EM structures of FHL in its aerobically and anaerobically isolated forms at resolutions reaching 2.6 Å. This includes well-resolved density for conserved loops linking the soluble and membrane arms believed to be essential in coupling enzymatic turnover to ion translocation across the membrane in the complex I superfamily. We evaluate possible structural determinants of the bias toward hydrogen production over its oxidation and describe an unpredicted metal-binding site near the interface of FdhF and HycF subunits that may play a role in redox-dependent regulation of FdhF interaction with the complex.

Function and structure of the membrane arm of the formate-hydrogenlyase complex from Trabulsiella guamensis

Function and structure of the membrane arm of the formate-hydrogenlyase complex from Trabulsiella guamensis

Transcriptome-based design of antisense inhibitors potentiates carbapenem efficacy in CRE Escherichia coli

In recent years, the prevalence of carbapenem-resistant Enterobacteriaceae (CRE) has risen substantially, and the study of CRE resistance mechanisms has become increasingly important for antibiotic development. Although much research has focused on genomic resistance factors, relatively few studies have examined CRE pathogens through changes in gene expression. In this study, we examined the gene expression profile of a CRE Escherichia coli clinical isolate that is sensitive to meropenem but resistant to ertapenem to explore transcriptomic contributions to resistance and to identify gene knockdown targets for carbapenem potentiation. We sequenced total and short RNA to analyze the gene expression response to ertapenem or meropenem treatment and found significant expression changes in genes related to motility, maltodextrin metabolism, the formate hydrogenlyase complex, and the general stress response. To validate these findings, we used our laboratory's Facile Accelerated Specific Therapeutic (FAST) platform to create antisense peptide nucleic acids (PNAs), gene-specific molecules designed to inhibit protein translation. PNAs were designed to inhibit the pathways identified in our transcriptomic analysis, and each PNA was then tested in combination with each carbapenem to assess its effect on the antibiotics' minimum inhibitory concentrations. We observed significant PNA-antibiotic interaction with five different PNAs across six combinations. Inhibition of the genes hycA, dsrB, and bolA potentiated carbapenem efficacy in CRE E. coli, whereas inhibition of the genes flhC and ygaC conferred added resistance. Our results identify resistance factors and demonstrate that transcriptomic analysis is a potent tool for designing antibiotic PNA.

Susceptibility of the Formate Hydrogenlyase Reaction to the Protonophore CCCP Depends on the Total Hydrogenase Composition

Fermentative hydrogen production by enterobacteria derives from the activity of the formate hydrogenlyase (FHL) complex, which couples formate oxidation to H2 production. The molybdenum-containing formate dehydrogenase and type-4 [NiFe]-hydrogenase together with three iron-sulfur proteins form the soluble domain, which is attached to the membrane by two integral membrane subunits. The FHL complex is phylogenetically related to respiratory complex I, and it is suspected that it has a role in energy conservation similar to the proton-pumping activity of complex I. We monitored the H2-producing activity of FHL in the presence of different concentrations of the protonophore CCCP. We found an inhibition with an apparent EC50 of 31 µM CCCP in the presence of glucose, a higher tolerance towards CCCP when only the oxidizing hydrogenase Hyd-1 was present, but a higher sensitivity when only Hyd-2 was present. The presence of 200 mM monovalent cations reduced the FHL activity by more than 20%. The Na+/H+ antiporter inhibitor 5-(N-ethyl-N-isopropyl)-amiloride (EIPA) combined with CCCP completely inhibited H2 production. These results indicate a coupling not only between Na+ transport activity and H2 production activity, but also between the FHL reaction, proton import and cation export.

Transcriptome‐based design of PNA inhibitors re‐sensitizes CRE E. coli to carbapenems

Carbapenems are a powerful class of antibiotics, often used as a last‐resort treatment to eradicate multidrug‐resistant infections. In recent years, however, the incidence of carbapenem‐resistant Enterobacteriaceae (CRE) has risen substantially, and the study of bacterial resistance mechanisms has become increasingly important for antibiotic development. In this research, we use transcriptomics and antisense gene inhibitors both to explore the resistance profile of a CRE Escherichia coli clinical isolate and to engineer carbapenem re‐sensitization. The clinical isolate demonstrates resistance to the carbapenem ertapenem but sensitivity to meropenem, and, though genomic analysis identified thirteen antibiotic resistance genes (including two β‐lactamases), no dedicated carbapenemases were found. Transcriptomic analysis was performed to examine the strain’s short‐term (<1 hr) gene expression changes in response to ertapenem or meropenem. While we did not observe differential expression of any resistance gene, significant expression changes were found in genes related to motility, maltodextrin metabolism, the formate hydrogenlyase complex, and the general stress response. Using our lab’s Facile Accelerated Specific Therapeutics (FAST) platform, we designed sequence‐specific antisense peptide nucleic acids (PNA) to inhibit the translation of genes that were identified by our transcriptomic analysis. These PNA were tested in combination with each carbapenem, either to assess their ability to re‐sensitize the isolate to ertapenem or their ability to alter the minimum inhibitory concentration of meropenem. We observed significant interaction between PNA and carbapenem treatments with four different PNA (two specific to the ertapenem response, one to the meropenem response, and one important to both responses). These results identify gene expression‐based resistance factors, and confirm the utility of transcriptomic analysis in engineering antibiotic re‐sensitization.Support or Funding InformationThis work was funded by a University of Colorado Dean’s Graduate Research Grant as well as the W.M. Keck Foundation and DARPA Young Faculty Award (D17AP00024).(A) Overview of the sample collection protocol for RNA sequencing. (B) Hierarchical clustering of gene expression values in ertapenem and meropenemtreated E. coli CUS2B. (C) Intersections in significantly differentially expressed (DE) genes between stress conditions and timepoints. (D) Time course of gene expression changes for the 6 transcripts that were DE in ertapenem and meropenem at both 30 and 60 minutes. *: P < 0.05 vs. NT. NT = no treatment, ERT = ertapenem, MER = meropenem.Figure 1(A) α‐hycA (10 μM) combined with ertapenem (1 μg/mL); (B) α‐flhC (10 μM) combined with meropenem (0.2 μg/mL); (C) α‐bolA (10 μM) combined with ertapenem (1 μg/mL), and α‐bolA (15 μM) combined with meropenem (0.1 μg/mL); (D) α‐dsrB (10 μM) combined with ertapenem (1 μg/mL); (E) Summary of interaction significance with normalized S‐values. ERT = ertapenem, MER = meropenem. *: ANOVA interaction P‐value < 0.05.Figure 2

The N-terminal domains of the paralogous HycE and NuoCD govern assembly of the respective formate hydrogenlyase and NADH dehydrogenase complexes.

Formate hydrogenlyase (FHL) is the main hydrogen‐producing enzyme complex in enterobacteria. It converts formate to CO 2 and H2 via a formate dehydrogenase and a [NiFe]‐hydrogenase. FHL and complex I are evolutionarily related and share a common core architecture. However, complex I catalyses the fundamentally different electron transfer from NADH to quinone and pumps protons. The catalytic FHL subunit, HycE, resembles NuoCD of Escherichia coli complex I; a fusion of NuoC and NuoD present in other organisms. The C‐terminal domain of HycE harbours the [NiFe]‐active site and is similar to other hydrogenases, while this domain in NuoCD is involved in quinone binding. The N‐terminal domains of these proteins do not bind cofactors and are not involved in electron transfer. As these N‐terminal domains are separate proteins in some organisms, we removed them in E. coli and observed that both FHL and complex I activities were essentially absent. This was due to either a disturbed assembly or to complex instability. Replacing the N‐terminal domain of HycE with a 180 amino acid E. coli NuoC protein fusion did not restore activity, indicating that the domains have complex‐specific functions. A FHL complex in which the N‐ and C‐terminal domains of HycE were physically separated still retained most of its FHL activity, while the separation of NuoCD abolished complex I activity completely. Only the FHL complex tolerates physical separation of the HycE domains. Together, the findings strongly suggest that the N‐terminal domains of these proteins are key determinants in complex assembly.

Reversible and Selective Interconversion of Hydrogen and Carbon Dioxide into Formate by a Semiartificial Formate Hydrogenlyase Mimic.

The biological formate hydrogenlyase (FHL) complex links a formate dehydrogenase (FDH) to a hydrogenase (H2ase) and produces H2 and CO2 from formate via mixed-acid fermentation in Escherichia coli. Here, we describe an electrochemical and a colloidal semiartificial FHL system that consists of an FDH and a H2ase immobilized on conductive indium tin oxide (ITO) as an electron relay. These in vitro systems benefit from the efficient wiring of a highly active enzyme pair and allow for the reversible conversion of formate to H2 and CO2 under ambient temperature and pressure. The hybrid systems provide a template for the design of synthetic catalysts and surpass the FHL complex in vivo by storing and releasing H2 on demand by interconverting CO2/H2 and formate with minimal bias in either direction.

Dissection of the Hydrogen Metabolism of the Enterobacterium Trabulsiella guamensis: Identification of a Formate-Dependent and Essential Formate Hydrogenlyase Complex Exhibiting Phylogenetic Similarity to Complex I.

Trabulsiella guamensis is a nonpathogenic enterobacterium that was isolated from a vacuum cleaner on the island of Guam. It has one H2-oxidizing Hyd-2-type hydrogenase (Hyd) and encodes an H2-evolving Hyd that is most similar to the uncharacterized Escherichia coli formate hydrogenlyase (FHL-2 Ec ) complex. The T. guamensis FHL-2 (FHL-2 Tg ) complex is predicted to have 5 membrane-integral and between 4 and 5 cytoplasmic subunits. We showed that the FHL-2 Tg complex catalyzes the disproportionation of formate to CO2 and H2 FHL-2 Tg has activity similar to that of the E. coli FHL-1 Ec complex in H2 evolution from formate, but the complex appears to be more labile upon cell lysis. Cloning of the entire 13-kbp FHL-2 Tg operon in the heterologous E. coli host has now enabled us to unambiguously prove FHL-2 Tg activity, and it allowed us to characterize the FHL-2 Tg complex biochemically. Although the formate dehydrogenase (FdhH) gene fdhF is not contained in the operon, the FdhH is part of the complex, and FHL-2 Tg activity was dependent on the presence of E. coli FdhH. Also, in contrast to E. coli, T. guamensis can ferment the alternative carbon source cellobiose, and we further investigated the participation of both the H2-oxidizing Hyd-2 Tg and the H2-forming FHL-2 Tg under these conditions.IMPORTANCE Biological H2 production presents an attractive alternative for fossil fuels. However, in order to compete with conventional H2 production methods, the process requires our understanding on a molecular level. FHL complexes are efficient H2 producers, and the prototype FHL-1 Ec complex in E. coli is well studied. This paper presents the first biochemical characterization of an FHL-2-type complex. The data presented here will enable us to solve the long-standing mystery of the FHL-2 Ec complex, allow a first biochemical characterization of T. guamensis's fermentative metabolism, and establish this enterobacterium as a model organism for FHL-dependent energy conservation.

Insights Into the Redox Sensitivity of Chloroflexi Hup-Hydrogenase Derived From Studies in Escherichia coli: Merits and Pitfalls of Heterologous [NiFe]-Hydrogenase Synthesis.

The highly oxygen-sensitive hydrogen uptake (Hup) hydrogenase from Dehalococcoides mccartyi forms part of a protein-based respiratory chain coupling hydrogen oxidation with organohalide reduction on the outside of the cell. The HupXSL proteins were previously shown to be synthesized and enzymatically active in Escherichia coli. Here we examined the growth conditions that deliver active Hup enzyme that couples H2 oxidation to benzyl viologen (BV) reduction, and identified host factors important for this process. In a genetic background lacking the three main hydrogenases of E. coli we could show that additional deletion of genes necessary for selenocysteine biosynthesis resulted in inactive Hup enzyme, suggesting requirement of a formate dehydrogenase for Hup activity. Hup activity proved to be dependent on the presence of formate dehydrogenase (Fdh-H), which is typically associated with the H2-evolving formate hydrogenlyase (FHL) complex in the cytoplasm. Further analyses revealed that heterologous Hup activity could be recovered if the genes encoding the ferredoxin-like electron-transfer protein HupX, as well as the related HycB small subunit of Fdh-H were also deleted. These findings indicated that the catalytic HupL and electron-transferring HupS subunits were sufficient for enzyme activity with BV. The presence of the HupX or HycB proteins in the absence of Fdh-H therefore appears to cause inactivation of the HupSL enzyme. This is possibly because HupX or HycB aided transfer of electrons to the quinone pool or other oxidoreductase complexes, thus maintaining the HupSL heterodimer in a continuously oxidized state causing its inactivation. This proposal was supported by the observation that growth under either aerobic or anaerobic respiratory conditions did not yield an active HupSL. These studies thus provide a system to understand the redox sensitivity of this heterologously synthesized hydrogenase.

PH and a mixed carbon-substrate spectrum influence FocA- and FocB-dependent, formate-driven H2 production in Escherichia coli.

Escherichia coli encodes two formate channels, FocA and FocB, that either export formate or import it for further disproportionation by the formate hydrogenlyase (FHL) complex to H2 and CO2. We show that FocA/B appear to change their substrate-translocation direction depending on pH and electron donor. When cells were grown on glucose and glycerol at pH 7.5, formate accumulated in focB or focA-focB mutants when glucose or formate was used as electron donor because H2 production increased ∼2- and ∼1.5-fold, respectively. Moreover, addition of external formate to the growth medium increased H2 production in a focA-focB mutant. This indicates that in the wild type, formate is preferentially exported at pH 7.5 and that another FocA/B-independent uptake system exists. At pH 6.5 and 5.5, the formate channel mutants showed reduced H2 production, suggesting that formate is usually imported by them to produce H2 at acidic pH. Addition of formate to the growth medium increased H2 production at these pHs. Notably, glycerol failed to act as an effective electron donor for formate production. Taken together, our results suggest that regulation of formate translocation direction by FocA/FocB channels is important for maintaining internal pH and proton motive force by modulating H2 production.

The Ferredoxin-Like Proteins HydN and YsaA Enhance Redox Dye-Linked Activity of the Formate Dehydrogenase H Component of the Formate Hydrogenlyase Complex.

Formate dehydrogenase H (FDH-H) and [NiFe]-hydrogenase 3 (Hyd-3) form the catalytic components of the hydrogen-producing formate hydrogenlyase (FHL) complex, which disproportionates formate to H2 and CO2 during mixed acid fermentation in enterobacteria. FHL comprises minimally seven proteins and little is understood about how this complex is assembled. Early studies identified a ferredoxin-like protein, HydN, as being involved in FDH-H assembly into the FHL complex. In order to understand how FDH-H and its small subunit HycB, which is also a ferredoxin-like protein, attach to the FHL complex, the possible roles of HydN and its paralogue, YsaA, in FHL complex stability and assembly were investigated. Deletion of the hycB gene reduced redox dye-mediated FDH-H activity to approximately 10%, abolished FHL-dependent H2-production, and reduced Hyd-3 activity. These data are consistent with HycB being an essential electron transfer component of the FHL complex. The FDH-H activity of the hydN and the ysaA deletion strains was reduced to 59 and 57% of the parental, while the double deletion reduced activity of FDH-H to 28% and the triple deletion with hycB to 1%. Remarkably, and in contrast to the hycB deletion, the absence of HydN and YsaA was without significant effect on FHL-dependent H2-production or total Hyd-3 activity; FDH-H protein levels were also unaltered. This is the first description of a phenotype for the E. coli ysaA deletion strain and identifies it as a novel factor required for optimal redox dye-linked FDH-H activity. A ysaA deletion strain could be complemented for FDH-H activity by hydN and ysaA, but the hydN deletion strain could not be complemented. Introduction of these plasmids did not affect H2 production. Bacterial two-hybrid interactions showed that YsaA, HydN, and HycB interact with each other and with the FDH-H protein. Further novel anaerobic cross-interactions of 10 ferredoxin-like proteins in E. coli were also discovered and described. Together, these data indicate that FDH-H activity measured with the redox dye benzyl viologen is the sum of the FDH-H protein interacting with three independent small subunits and suggest that FDH-H can associate with different redox-protein complexes in the anaerobic cell to supply electrons from formate oxidation.

Genome Reduction in Psychromonas Species within the Gut of an Amphipod from the Ocean's Deepest Point.

Amphipods are the dominant scavenging metazoan species in the Mariana Trench, the deepest known point in Earth's oceans. Here the gut microbiota of the amphipod Hirondellea gigas collected from the Challenger and Sirena Deeps of the Mariana Trench were investigated. The 11 amphipod individuals included for analyses were dominated by Psychromonas, of which a nearly complete genome was successfully recovered (designated CDP1). Compared with previously reported free-living Psychromonas strains, CDP1 has a highly reduced genome. Genome alignment showed deletion of the trimethylamine N-oxide (TMAO) reducing gene cluster in CDP1, suggesting that the "piezolyte" function of TMAO is more important than its function in respiration, which may lead to TMAO accumulation. In terms of nutrient utilization, the bacterium retains its central carbohydrate metabolism but lacks most of the extended carbohydrate utilization pathways, suggesting the confinement of Psychromonas to the host gut and sequestration from more variable environmental conditions. Moreover, CDP1 contains a complete formate hydrogenlyase complex, which might be involved in energy production. The genomic analyses imply that CDP1 may have developed adaptive strategies for a lifestyle within the gut of the hadal amphipod H.gigas. IMPORTANCE As a unique but poorly investigated habitat within marine ecosystems, hadal trenches have received interest in recent years. This study explores the gut microbial composition and function in hadal amphipods, which are among the dominant carrion feeders in hadal habitats. Further analyses of a dominant strain revealed genomic features that may contribute to its adaptation to the amphipod gut environment. Our findings provide new insights into animal-associated bacteria in the hadal biosphere.

Multi-time series RNA-seq analysis of Enterobacter lignolyticus SCF1 during growth in lignin-amended medium.

The production of lignocellulosic-derived biofuels is a highly promising source of alternative energy, but it has been constrained by the lack of a microbial platform capable to efficiently degrade this recalcitrant material and cope with by-products that can be toxic to cells. Species that naturally grow in environments where carbon is mainly available as lignin are promising for finding new ways of removing the lignin that protects cellulose for improved conversion of lignin to fuel precursors. Enterobacter lignolyticus SCF1 is a facultative anaerobic Gammaproteobacteria isolated from tropical rain forest soil collected in El Yunque forest, Puerto Rico under anoxic growth conditions with lignin as sole carbon source. Whole transcriptome analysis of SCF1 during E.lignolyticus SCF1 lignin degradation was conducted on cells grown in the presence (0.1%, w/w) and the absence of lignin, where samples were taken at three different times during growth, beginning of exponential phase, mid-exponential phase and beginning of stationary phase. Lignin-amended cultures achieved twice the cell biomass as unamended cultures over three days, and in this time degraded 60% of lignin. Transcripts in early exponential phase reflected this accelerated growth. A complement of laccases, aryl-alcohol dehydrogenases, and peroxidases were most up-regulated in lignin amended conditions in mid-exponential and early stationary phases compared to unamended growth. The association of hydrogen production by way of the formate hydrogenlyase complex with lignin degradation suggests a possible value added to lignin degradation in the future.

The dual-function chaperone HycH improves assembly of the formate hydrogenlyase complex

The assembly of multi-protein complexes requires the concerted synthesis and maturation of its components and subsequently their co-ordinated interaction. The membrane-bound formate hydrogenlyase (FHL) complex is the primary hydrogen-producing enzyme in Escherichia coli and is composed of seven subunits mostly encoded within the hycA-I operon for [NiFe]-hydrogenase-3 (Hyd-3). The HycH protein is predicted to have an accessory function and is not part of the final structural FHL complex. In this work, a mutant strain devoid of HycH was characterised and found to have significantly reduced FHL activity due to the instability of the electron transfer subunits. HycH was shown to interact specifically with the unprocessed species of HycE, the catalytic hydrogenase subunit of the FHL complex, at different stages during the maturation and assembly of the complex. Variants of HycH were generated with the aim of identifying interacting residues and those that influence activity. The R70/71/K72, the Y79, the E81 and the Y128 variant exchanges interrupt the interaction with HycE without influencing the FHL activity. In contrast, FHL activity, but not the interaction with HycE, was negatively influenced by H37 exchanges with polar residues. Finally, a HycH Y30 variant was unstable. Surprisingly, an overlapping function between HycH with its homologous counterpart HyfJ from the operon encoding [NiFe]-hydrogenase-4 (Hyd-4) was identified and this is the first example of sharing maturation machinery components between Hyd-3 and Hyd-4 complexes. The data presented here show that HycH has a novel dual role as an assembly chaperone for a cytoplasmic [NiFe]-hydrogenase.

Adaptive evolution of a hyperthermophilic archaeon pinpoints a formate transporter as a critical factor for the growth enhancement on formate

Previously, we reported that the hyperthermophilic archaeon Thermococcus onnurineus NA1 could grow on formate and produce H2. Formate conversion to hydrogen was mediated by a formate-hydrogen lyase complex and was indeed a part of chemiosmotic coupling to ATP generation. In this study, we employed an adaptation approach to enhance the cell growth on formate and investigated molecular changes. As serial transfer continued on formate-containing medium at the serum vial, cell growth, H2 production and formate consumption increased remarkably. The 156 times transferred-strain, WTF-156T, was demonstrated to enhance H2 production using formate in a bioreactor. The whole-genome sequencing of the WTF-156T strain revealed eleven mutations. While no mutation was found among the genes encoding formate hydrogen lyase, a point mutation (G154A) was identified in a formate transporter (TON_1573). The TON_1573 (A52T) mutation, when introduced into the parent strain, conferred increase in formate consumption and H2 production. Another adaptive passage, carried out by culturing repeatedly in a bioreactor, resulted in a strain, which has a mutation in TON_1573 (C155A) causing amino acid change, A52E. These results implicate that substitution of A52 residue of a formate transporter might be a critical factor to ensure the increase in formate uptake and cell growth.

Differential effects of isc operon mutations on the biosynthesis and activity of key anaerobic metalloenzymes in Escherichia coli.

Escherichia coli has two machineries for the synthesis of FeS clusters, namely Isc (iron-sulfur cluster) and Suf (sulfur formation). The Isc machinery, encoded by the iscRSUA-hscBA-fdx-iscXoperon, plays a crucial role in the biogenesis of FeS clusters for the oxidoreductases of aerobic metabolism. Less is known, however, about the role of ISC in the maturation of key multi-subunit metalloenzymes of anaerobic metabolism. Here, we determined the contribution of each iscoperon gene product towards the functionality of the major anaerobic oxidoreductases in E. coli, including three [NiFe]-hydrogenases (Hyd), two respiratory formate dehydrogenases (FDH) and nitrate reductase (NAR). Mutants lacking the cysteine desulfurase, IscS, lacked activity of all six enzymes, as well as the activity of fumaratereductase, and this was due to deficiencies in enzyme biosynthesis, maturation or FeS cluster insertion into electron-transfer components. Notably, based on anaerobic growth characteristics and metabolite patterns, the activity of the radical-S-adenosylmethionine enzyme pyruvate formate-lyase activase was independent of IscS, suggesting that FeS biogenesis for this ancient enzyme has different requirements. Mutants lacking either the scaffold protein IscU, the ferredoxin Fdx or the chaperones HscA or HscB had similar enzyme phenotypes: five of the oxidoreductases were essentially inactive, with the exception being the Hyd-3 enzyme, which formed part of the H2-producing formate hydrogenlyase (FHL) complex. Neither the frataxin-homologue CyaY nor the IscX protein was essential for synthesis of the three Hyd enzymes. Thus, while IscS is essential for H2 production in E. coli, the other ISC components are non-essential.

Anaerobic Formate and Hydrogen Metabolism.

Numerous recent developments in the biochemistry, molecular biology, and physiology of formate and H2 metabolism and of the [NiFe]-hydrogenase (Hyd) cofactor biosynthetic machinery are highlighted. Formate export and import by the aquaporin-like pentameric formate channel FocA is governed by interaction with pyruvate formate-lyase, the enzyme that generates formate. Formate is disproportionated by the reversible formate hydrogenlyase (FHL) complex, which has been isolated, allowing biochemical dissection of evolutionary parallels with complex I of the respiratory chain. A recently identified sulfido-ligand attached to Mo in the active site of formate dehydrogenases led to the proposal of a modified catalytic mechanism. Structural analysis of the homologous, H2-oxidizing Hyd-1 and Hyd-5 identified a novel proximal [4Fe-3S] cluster in the small subunit involved in conferring oxygen tolerance to the enzymes. Synthesis of Salmonella Typhimurium Hyd-5 occurs aerobically, which is novel for an enterobacterial Hyd. The O2-sensitive Hyd-2 enzyme has been shown to be reversible: it presumably acts as a conformational proton pump in the H2-oxidizing mode and is capable of coupling reverse electron transport to drive H2 release. The structural characterization of all the Hyp maturation proteins has given new impulse to studies on the biosynthesis of the Fe(CN)2CO moiety of the [NiFe] cofactor. It is synthesized on a Hyp-scaffold complex, mainly comprising HypC and HypD, before insertion into the apo-large subunit. Finally, clear evidence now exists indicating that Escherichia coli can mature Hyd enzymes differentially, depending on metal ion availability and the prevailing metabolic state. Notably, Hyd-3 of the FHL complex takes precedence over the H2-oxidizing enzymes.