EPR characterization of the new Qrc complex from sulfate reducing bacteria and its ability to form a supercomplex with hydrogenase and TpI c3

Genome and proteome analyses show the gaseous alkane degrader Desulfosarcina sp. strain BuS5 as an extreme metabolic specialist

The metabolic potential of the sulfate-reducing bacterium Desulfosarcina sp. strain BuS5, currently the only pure culture able to oxidize the volatile alkanes propane and butane without oxygen, was investigated via genomics, proteomics and physiology assays. Complete genome sequencing revealed that strain BuS5 encodes a single alkyl-succinate synthase, an enzyme which apparently initiates oxidation of both propane and butane. The formed alkyl-succinates are oxidized to CO₂ via beta oxidation and the oxidative Wood-Ljungdahl pathways as shown by proteogenomics analyses. Strain BuS5 conserves energy via the canonical sulfate reduction pathway and electron bifurcation. An ability to utilize long-chain fatty acids, mannose and oligopeptides, suggested by automated annotation pipelines, was not supported by physiology assays and in-depth analyses of the corresponding genetic systems. Consistently, comparative genomics revealed a streamlined BuS5 genome with a remarkable paucity of catabolic modules. These results establish strain BuS5 as an exceptional metabolic specialist, able to grow only with propane and butane, for which we propose the name Desulfosarcina aeriophaga BuS5. This highly restrictive lifestyle, most likely the result of habitat-driven evolutionary gene loss, may provide D. aeriophaga BuS5 a competitive edge in sediments impacted by natural gas seeps. Etymology: Desulfosarcina aeriophaga, aério (Greek): gas; phágos (Greek): eater; D. aeriophaga: a gas eating or gas feeding Desulfosarcina.

Modularity of membrane-bound charge-translocating protein complexes

Energy transduction is the conversion of one form of energy into another; this makes life possible as we know it. Organisms have developed different systems for acquiring energy and storing it in useable forms: the so-called energy currencies. A universal energy currency is the transmembrane difference of electrochemical potential (Δμ~). This results from the translocation of charges across a membrane, powered by exergonic reactions. Different reactions may be coupled to charge-translocation and, in the majority of cases, these reactions are catalyzed by modular enzymes that always include a transmembrane subunit. The modular arrangement of these enzymes allows for different catalytic and charge-translocating modules to be combined. Thus, a transmembrane charge-translocating module can be associated with different catalytic subunits to form an energy-transducing complex. Likewise, the same catalytic subunit may be combined with a different membrane charge-translocating module. In this work, we analyze the modular arrangement of energy-transducing membrane complexes and discuss their different combinations, focusing on the charge-translocating module.

The Ion-Translocating NrfD-Like Subunit of Energy-Transducing Membrane Complexes

Several energy-transducing microbial enzymes have their peripheral subunits connected to the membrane through an integral membrane protein, that interacts with quinones but does not have redox cofactors, the so-called NrfD-like subunit. The periplasmic nitrite reductase (NrfABCD) was the first complex recognized to have a membrane subunit with these characteristics and consequently provided the family's name: NrfD. Sequence analyses indicate that NrfD homologs are present in many diverse enzymes, such as polysulfide reductase (PsrABC), respiratory alternative complex III (ACIII), dimethyl sulfoxide (DMSO) reductase (DmsABC), tetrathionate reductase (TtrABC), sulfur reductase complex (SreABC), sulfite dehydrogenase (SoeABC), quinone reductase complex (QrcABCD), nine-heme cytochrome complex (NhcABCD), group-2 [NiFe] hydrogenase (Hyd-2), dissimilatory sulfite-reductase complex (DsrMKJOP), arsenate reductase (ArrC) and multiheme cytochrome c sulfite reductase (MccACD). The molecular structure of ACIII subunit C (ActC) and Psr subunit C (PsrC), NrfD-like subunits, revealed the existence of ion-conducting pathways. We performed thorough primary structural analyses and built structural models of the NrfD-like subunits. We observed that all these subunits are constituted by two structural repeats composed of four-helix bundles, possibly harboring ion-conducting pathways and containing a quinone/quinol binding site. NrfD-like subunits may be the ion-pumping module of several enzymes. Our data impact on the discussion of functional implications of the NrfD-like subunit-containing complexes, namely in their ability to transduce energy.

Redox loops in anaerobic respiration - The role of the widespread NrfD protein family and associated dimeric redox module

In prokaryotes, the proton or sodium motive force required for ATP synthesis is produced by respiratory complexes that present an ion-pumping mechanism or are involved in redox loops performed by membrane proteins that usually have substrate and quinone-binding sites on opposite sides of the membrane. Some respiratory complexes include a dimeric redox module composed of a quinone-interacting membrane protein of the NrfD family and an iron‑sulfur protein of the NrfC family. The QrcABCD complex of sulfate reducers, which includes the QrcCD module homologous to NrfCD, was recently shown to perform electrogenic quinone reduction providing the first conclusive evidence for energy conservation among this family. Similar redox modules are present in multiple respiratory complexes, which can be associated with electroneutral, energy-driven or electrogenic reactions. This work discusses the presence of the NrfCD/PsrBC dimeric redox module in different bioenergetics contexts and its role in prokaryotic energy conservation mechanisms.

Respiratory Membrane Protein Complexes Convert Chemical Energy

The invention of a biological membrane which is used as energy storage system to drive the metabolism of a primordial, unicellular organism represents a key event in the evolution of life. The innovative, underlying principle of this key event is respiration. In respiration, a lipid bilayer with insulating properties is chosen as the site for catalysis of an exergonic redox reaction converting substrates offered from the environment, using the liberated Gibbs free energy (ΔG) for the build-up of an electrochemical H⁺ (proton motive force, PMF) or Na⁺ gradient (sodium motive force, SMF) across the lipid bilayer. Very frequently , several redox reactions are performed in a consecutive manner, with the first reaction delivering a product which is used as substrate for the second redox reaction, resulting in a respiratory chain. From today's perspective, the (mostly) unicellular bacteria and archaea seem to be much simpler and less evolved when compared to multicellular eukaryotes. However, they are overwhelmingly complex with regard to the various respiratory chains which permit survival in very different habitats of our planet, utilizing a plethora of substances to drive metabolism. This includes nitrogen, sulfur and carbon compounds which are oxidized or reduced by specialized, respiratory enzymes of bacteria and archaea which lie at the heart of the geochemical N, S and C-cycles. This chapter gives an overview of general principles of microbial respiration considering thermodynamic aspects, chemical reactions and kinetic restraints. The respiratory chains of Escherichia coli and Vibrio cholerae are discussed as models for PMF- versus SMF-generating processes, respectively. We introduce main redox cofactors of microbial respiratory enzymes, and the concept of intra-and interelectron transfer. Since oxygen is an electron acceptor used by many respiratory chains, the formation and removal of toxic oxygen radicals is described. Promising directions of future research are respiratory enzymes as novel bacterial targets, and biotechnological applications relying on respiratory complexes.

An electrogenic redox loop in sulfate reduction reveals a likely widespread mechanism of energy conservation

The bioenergetics of anaerobic metabolism frequently relies on redox loops performed by membrane complexes with substrate- and quinone-binding sites on opposite sides of the membrane. However, in sulfate respiration (a key process in the biogeochemical sulfur cycle), the substrate- and quinone-binding sites of the QrcABCD complex are periplasmic, and their role in energy conservation has not been elucidated. Here we show that the QrcABCD complex of Desulfovibrio vulgaris is electrogenic, as protons and electrons required for quinone reduction are extracted from opposite sides of the membrane, with a H⁺/e⁻ ratio of 1. Although the complex does not act as a H⁺-pump, QrcD may include a conserved proton channel leading from the N-side to the P-side menaquinone pocket. Our work provides evidence of how energy is conserved during dissimilatory sulfate reduction, and suggests mechanisms behind the functions of related bacterial respiratory complexes in other bioenergetic contexts.

The bacterial complex QrcABCD plays a key role in the bioenergetics of sulfate respiration. Here, Duarte et al. show that this complex is electrogenic, with protons and electrons required for quinone reduction being extracted from opposite sides of the membrane.

The bioenergetics mechanisms and applications of sulfate-reducing bacteria in remediation of pollutants in drainage: A review

Sulfate-reducing bacteria (SRB), a group of anaerobic prokaryotes, can use sulfur species as a terminal electron acceptor for the oxidation of organic compounds. They not only have significant ecological functions, but also play an important role in bioremediation of contaminated sites. Although numerous studies on metabolism and applications of SRB have been conducted, they still remain incompletely understood and even controversial. Fully understanding the metabolism of SRB paves the way for allowing the microorganisms to provide more beneficial services in bioremediation. Here we review progress in bioenergetics mechanisms and application of SRB including: (1) electron acceptors and donors for SRB; (2) pathway for sulfate reduction; (3) electron transfer in sulfate reduction; (4) application of SRB for economical and concomitant treatment of heavy metal, organic contaminants and sulfates. Moreover, current knowledge gaps and further research needs are identified.

Differential proteomic analysis of the metabolic network of the marine sulfate-reducer Desulfobacterium autotrophicum HRM2

The marine sulfate-reducing bacterium Desulfobacterium autotrophicum HRM2 belongs to the deltaproteobacterial family Desulfobacteraceae and stands out for its capacity of facultative chemolithoautotrophic growth (next to heterotrophy). Here, proteomics-driven metabolic reconstruction was based on a combination of 2D-DIGE, shotgun proteomics, and analysis of the membrane protein enriched fraction applied to eight different substrate adaptation conditions (seven aliphatic compounds plus H₂ /CO₂ ). In total, 1344 different proteins were identified (∼27% of the 4947 genome-predicted), from which a complex metabolic network was reconstructed consisting of 136 proteins (124 detected; ∼91%). Peripheral degradation routes for organic substrates feed directly or via the methylmalonyl-CoA pathway into the Wood-Ljungdahl pathway (WLP) for terminal oxidation to CO₂ . Chemolithoautotrophic growth apparently involves the periplasmic [Ni/Fe/Se]-containing hydrogenase HysAB (H₂ oxidation), the reductively operating WLP (CO₂ fixation), and classical gluconeogenesis. Diverse soluble proteins (e.g., Hdr, Etf) probably establish a fine balanced cytoplasmic electron transfer network connecting individual catabolic reactions with the membrane menaquinone pool. In addition, multiple membrane protein complexes (Nqr, Qmo, Qrc, Rnf1, Rnf2, and Tmc) provide ample routes for interacting with the reducing equivalent pool and delivering electrons to dissimilatory sulfate reduction (both localized in the cytoplasm). Overall, this study contributes to the molecular understanding of the habitat-relevant Desulfobacteraceae.

Genome and catabolic subproteomes of the marine, nutritionally versatile, sulfate-reducing bacterium Desulfococcus multivorans DSM 2059

BACKGROUND

Sulfate-reducing bacteria (SRB) are key players of the carbon- and sulfur-cycles in the sediments of the world's oceans. Habitat relevant SRBs are often members of the Desulfosarcina-Desulfococcus clade belonging to the deltaproteobacterial family of Desulfobacteraceae. Despite this environmental recognition, their molecular (genome-based) physiology and their potential to contribute to organic carbon mineralization as well as to adapt to changing environmental conditions have been scarcely investigated. A metabolically versatile representative of this family is Desulfococcus multivorans that is able to completely oxidize (to CO2) a variety of organic acids, including fatty acids up to C14, as well as aromatic compounds.

RESULTS

In this study the complete 4.46 Mbp and manually annotated genome of metabolically versatile Desulfococcus multivorans DSM 2059 is presented with particular emphasis on a proteomics-driven metabolic reconstruction. Proteomic profiling covered 17 substrate adaptation conditions (6 aromatic and 11 aliphatic compounds) and comprised 2D DIGE, shotgun proteomics and analysis of the membrane protein-enriched fractions. This comprehensive proteogenomic dataset allowed for reconstructing a metabolic network of degradation pathways and energy metabolism that consists of 170 proteins (154 detected; ~91 % coverage). Peripheral degradation routes feed via central benzoyl-CoA, (modified) β-oxidation or methylmalonyl-CoA pathways into the Wood-Ljungdahl pathway for complete oxidation of acetyl-CoA to CO2. Dissimilatory sulfate reduction is fueled by a complex electron transfer network composed of cytoplasmic components (e.g., electron transfer flavoproteins) and diverse membrane redox complexes (Dsr, Qmo, Hmc, Tmc, Qrc, Nuo and Rnf). Overall, a high degree of substrate-specific formation of catabolic enzymes was observed, while most complexes involved in electron transfer appeared to be constitutively formed.

CONCLUSIONS

A highly dynamic genome structure in combination with substrate-specifically formed catabolic subproteomes and a constitutive subproteome for energy metabolism and electron transfer appears to be a common trait of Desulfobacteraceae members.

A Post-Genomic View of the Ecophysiology, Catabolism and Biotechnological Relevance of Sulphate-Reducing Prokaryotes

Dissimilatory sulphate reduction is the unifying and defining trait of sulphate-reducing prokaryotes (SRP). In their predominant habitats, sulphate-rich marine sediments, SRP have long been recognized to be major players in the carbon and sulphur cycles. Other, more recently appreciated, ecophysiological roles include activity in the deep biosphere, symbiotic relations, syntrophic associations, human microbiome/health and long-distance electron transfer. SRP include a high diversity of organisms, with large nutritional versatility and broad metabolic capacities, including anaerobic degradation of aromatic compounds and hydrocarbons. Elucidation of novel catabolic capacities as well as progress in the understanding of metabolic and regulatory networks, energy metabolism, evolutionary processes and adaptation to changing environmental conditions has greatly benefited from genomics, functional OMICS approaches and advances in genetic accessibility and biochemical studies. Important biotechnological roles of SRP range from (i) wastewater and off gas treatment, (ii) bioremediation of metals and hydrocarbons and (iii) bioelectrochemistry, to undesired impacts such as (iv) souring in oil reservoirs and other environments, and (v) corrosion of iron and concrete. Here we review recent advances in our understanding of SRPs focusing mainly on works published after 2000. The wealth of publications in this period, covering many diverse areas, is a testimony to the large environmental, biogeochemical and technological relevance of these organisms and how much the field has progressed in these years, although many important questions and applications remain to be explored.

New model for electron flow for sulfate reduction in Desulfovibrio alaskensis G20

To understand the energy conversion activities of the anaerobic sulfate-reducing bacteria, it is necessary to identify the components involved in electron flow. The importance of the abundant type I tetraheme cytochrome c3 (TpIc3) as an electron carrier during sulfate respiration was questioned by the previous isolation of a null mutation in the gene encoding TpIc3, cycA, in Desulfovibrio alaskensis G20. Whereas respiratory growth of the CycA mutant with lactate and sulfate was little affected, growth with pyruvate and sulfate was significantly impaired. We have explored the phenotype of the CycA mutant through physiological tests and transcriptomic and proteomic analyses. Data reported here show that electrons from pyruvate oxidation do not reach adenylyl sulfate reductase, the enzyme catalyzing the first redox reaction during sulfate reduction, in the absence of either CycA or the type I cytochrome c3:menaquinone oxidoreductase transmembrane complex, QrcABCD. In contrast to the wild type, the CycA and QrcA mutants did not grow with H2 or formate and sulfate as the electron acceptor. Transcriptomic and proteomic analyses of the CycA mutant showed that transcripts and enzymes for the pathway from pyruvate to succinate were strongly decreased in the CycA mutant regardless of the growth mode. Neither the CycA nor the QrcA mutant grew on fumarate alone, consistent with the omics results and a redox regulation of gene expression. We conclude that TpIc3 and the Qrc complex are D. alaskensis components essential for the transfer of electrons released in the periplasm to reach the cytoplasmic adenylyl sulfate reductase and present a model that may explain the CycA phenotype through confurcation of electrons.

Flexibility of syntrophic enzyme systems in Desulfovibrio species ensures their adaptation capability to environmental changes

The mineralization of organic matter in anoxic environments relies on the cooperative activities of hydrogen producers and consumers obligately linked by interspecies metabolite exchange in syntrophic consortia that may include sulfate reducing species such as Desulfovibrio. To evaluate the metabolic flexibility of syntrophic Desulfovibrio to adapt to naturally fluctuating methanogenic environments, we studied Desulfovibrio alaskensis strain G20 grown in chemostats under respiratory and syntrophic conditions with alternative methanogenic partners, Methanococcus maripaludis and Methanospirillum hungatei, at different growth rates. Comparative whole-genome transcriptional analyses, complemented by G20 mutant strain growth experiments and physiological data, revealed a significant influence of both energy source availability (as controlled by dilution rate) and methanogen on the electron transfer systems, ratios of interspecies electron carriers, energy generating systems, and interspecies physical associations. A total of 68 genes were commonly differentially expressed under syntrophic versus respiratory lifestyle. Under low-energy (low-growth-rate) conditions, strain G20 further had the capacity to adapt to the metabolism of its methanogenic partners, as shown by its differing gene expression of enzymes involved in the direct metabolic interactions (e.g., periplasmic hydrogenases) and the ratio shift in electron carriers used for interspecies metabolite exchange (hydrogen/formate). A putative monomeric [Fe-Fe] hydrogenase and Hmc (high-molecular-weight-cytochrome c3) complex-linked reverse menaquinone (MQ) redox loop become increasingly important for the reoxidation of the lactate-/pyruvate oxidation-derived redox pair, DsrC(red) and Fd(red), relative to the Qmo-MQ-Qrc (quinone-interacting membrane-bound oxidoreductase; quinone-reducing complex) loop. Together, these data underscore the high enzymatic and metabolic adaptive flexibility that likely sustains Desulfovibrio in naturally fluctuating methanogenic environments.

Microbial sulfite respiration

Despite its reactivity and hence toxicity to living cells, sulfite is readily converted by various microorganisms using distinct assimilatory and dissimilatory metabolic routes. In respiratory pathways, sulfite either serves as a primary electron donor or terminal electron acceptor (yielding sulfate or sulfide, respectively), and its conversion drives electron transport chains that are coupled to chemiosmotic ATP synthesis. Notably, such processes are also seen to play a general role in sulfite detoxification, which is assumed to have an evolutionary ancient origin. The diversity of sulfite conversion is reflected by the fact that the range of microbial sulfite-converting enzymes displays different cofactors such as siroheme, heme c, or molybdopterin. This chapter aims to summarize the current knowledge of microbial sulfite metabolism and focuses on sulfite catabolism. The structure and function of sulfite-converting enzymes and the emerging picture of the modular architecture of the corresponding respiratory/detoxifying electron transport chains is emphasized.

Variation among Desulfovibrio species in electron transfer systems used for syntrophic growth

Mineralization of organic matter in anoxic environments relies on the cooperative activities of hydrogen producers and consumers linked by interspecies electron transfer in syntrophic consortia that may include sulfate-reducing species (e.g., Desulfovibrio). Physiological differences and various gene repertoires implicated in syntrophic metabolism among Desulfovibrio species suggest considerable variation in the biochemical basis of syntrophy. In this study, comparative transcriptional and mutant analyses of Desulfovibrio alaskensis strain G20 and Desulfovibrio vulgaris strain Hildenborough growing syntrophically with Methanococcus maripaludis on lactate were used to develop new and revised models for their alternative electron transfer and energy conservation systems. Lactate oxidation by strain G20 generates a reduced thiol-disulfide redox pair(s) and ferredoxin that are energetically coupled to H(+)/CO(2) reduction by periplasmic formate dehydrogenase and hydrogenase via a flavin-based reverse electron bifurcation process (electron confurcation) and a menaquinone (MQ) redox loop-mediated reverse electron flow involving the membrane-bound Qmo and Qrc complexes. In contrast, strain Hildenborough uses a larger number of cytoplasmic and periplasmic proteins linked in three intertwining pathways to couple H(+) reduction to lactate oxidation. The faster growth of strain G20 in coculture is associated with a kinetic advantage conferred by the Qmo-MQ-Qrc loop as an electron transfer system that permits higher lactate oxidation rates under elevated hydrogen levels (thereby enhancing methanogenic growth) and use of formate as the main electron-exchange mediator (>70% electron flux), as opposed to the primarily hydrogen-based exchange by strain Hildenborough. This study further demonstrates the absence of a conserved gene core in Desulfovibrio that would determine the ability for a syntrophic lifestyle.

Unifying concepts in anaerobic respiration: insights from dissimilatory sulfur metabolism

Behind the versatile nature of prokaryotic energy metabolism is a set of redox proteins having a highly modular character. It has become increasingly recognized that a limited number of redox modules or building blocks appear grouped in different arrangements, giving rise to different proteins and functionalities. This modularity most likely reveals a common and ancient origin for these redox modules, and is obviously reflected in similar energy conservation mechanisms. The dissimilation of sulfur compounds was probably one of the earliest biological strategies used by primitive organisms to obtain energy. Here, we review some of the redox proteins involved in dissimilatory sulfur metabolism, focusing on sulfate reducing organisms, and highlight links between these proteins and others involved in different processes of anaerobic respiration. Noteworthy are links to the complex iron-sulfur molybdoenzyme family, and heterodisulfide reductases of methanogenic archaea. We discuss how chemiosmotic and electron bifurcation/confurcation may be involved in energy conservation during sulfate reduction, and how introduction of an additional module, multiheme cytochromes c, opens an alternative bioenergetic strategy that seems to increase metabolic versatility. Finally, we highlight new families of heterodisulfide reductase-related proteins from non-methanogenic organisms, which indicate a widespread distribution for these protein modules and may indicate a more general involvement of thiol/disulfide conversions in energy metabolism. This article is part of a Special Issue entitled: The evolutionary aspects of bioenergetic systems.

Hemoproteins in dissimilatory sulfate- and sulfur-reducing prokaryotes

Dissimilatory sulfate and sulfur reduction evolved billions of years ago and while the bacteria and archaea that use this unique metabolism employ a variety of electron donors, H(2) is most commonly used as the energy source. These prokaryotes use multiheme c-type proteins to shuttle electrons from electron donors, and electron transport complexes presumed to contain b-type hemoproteins contribute to proton charging of the membrane. Numerous sulfate and sulfur reducers use an alternate pathway for heme synthesis and, frequently, uniquely specific axial ligands are used to secure c-type heme to the protein. This review presents some of the types and functional activities of hemoproteins involved in these two dissimilatory reduction pathways.