Discovery of a Ferredoxin:NAD⁺‐Oxidoreductase (Rnf) in Acetobacterium woodii

Abstract

Acetogens use the Wood-Ljungdahl pathway for reduction of carbon dioxide to acetate. This pathway not only allows reoxidation of reducing equivalents during heterotrophic growth but also supports chemolithoautotrophic growth on H(2) + CO(2). The latter argues for this pathway being a source for net energy conservation, but the mechanism involved remains unknown. In addition to CO(2), acetogens can use alternative electron acceptors, such as nitrate or caffeate. Caffeate respiration in the model acetogen Acetobacterium woodii is coupled to energy conservation via a chemiosmotic mechanism, with Na(+) as coupling ion. The pathway and its bioenergetics were solved in some detail very recently. This review focuses on the regulation of caffeate respiration, describes the enyzmes involved, summarizes the evidence for a potential Na(+)-translocating ferredoxin:NAD(+)-oxidoreductase (Rnf complex) as a new coupling site, and hypothesizes on the role of this Rnf complex in the Wood-Ljungdahl pathway.

Alternative Pyrimidine Biosynthesis Protein ApbE Is a Flavin Transferase Catalyzing Covalent Attachment of FMN to a Threonine Residue in Bacterial Flavoproteins

“…It has been proposed that RNF catalyzes electron transfer from ferredoxin to NAD ϩ (12, 13) or methanophenazine (14) and, depending on the direction of the catalyzed redox reaction, can act as a ⌬ Na ϩ generator (forward reaction) (13) or consumer (reverse reaction) (12). The RNF complex is also formed by six subunits, five of which are homologous to the corresponding NqrA-E subunits.…”

Microbial dark matter ecogenomics reveals complex synergistic networks in a methanogenic bioreactor

“…As for syntrophs who do not encode Rnf and Hdr-Ifo or ECHyd, this survey reveals other reverse electron transport and electron bi(con)furcation mechanisms that syntrophs may employ. For acetogenesis, we identify that acetogens require (Schmidt et al, 2009) and encode at least one energy-conserving electron transfer between physiological electron carriers (Rnf, NAD(P)H transhydrogenase and/or NADH-dependent Fd red :NADP þ oxidoreductase) (Hugenholtz and Ljungdahl, 1989;Muller et al, 2008;Huang et al, 2012;Tremblay et al, 2012); often possess a putative clostridial sensory hydrogenase (Calusinska et al, 2010); and utilize a wide variety of hydrogenases and formate dehydrogenases. In addition, we identify five genotypes of acetogen MetF (methylenetetrahydrofolate reductase) essential for driving the homoacetogenic Wood-Ljungdahl pathway, but also postulate that undiscovered MetF homologs and genotypes must exist.…”

“…Based on the known metabolic diversity of methanogenic environments (Schink, 1997;McInerney et al, 2009), uncultivated secondary degraders and scavengers must perform fermentative, syntrophic or acetogenic metabolism. To investigate the ecological roles and metabolic capabilities of the uncultivated taxa, this study used an 'ecogenomics' approach synthesizing single-cell genomics (Marcy et al, 2007), metagenomics (Tyson et al, 2004), metatranscriptomics (Frias-Lopez et al, 2008) and metabolic reconstruction assisted by comparative genomics of acetogen and syntroph energy conservation pathways (McInerney et al, 2007;Muller et al, 2008;Sieber et al, 2010Sieber et al, , 2012Nobu et al, 2014). In doing so, we provide the first look into the genomes and metabolism of many uncultivated taxa and propose how these organisms metabolically interact to achieve holistic carbon flux from TA to CH 4 and CO 2 in the methanogenic reactor.…”

Structure of the V. cholerae Na+-pumping NADH:quinone oxidoreductase

“…Genes for the Na 1 NQR were identified in many pathogenic bacteria and have no eukaryotic homologue, des ignating the enzyme as a probable drug target in antibacterial therapy. Interestingly, four out of six subunits, NqrB, NqrC, NqrD and NqrE, are homologous to subunits of the so called RNF (Rhodobacter nitrogen fix ation) complex present in widely different groups of prokaryotes 6 (Ex tended Data Figs 1, 2). The high similarity over four subunits suggests that the core machinery for Na 1 translocation is identical in both res piratory complexes.…”