The Iron-Hydrogenase of Thermotoga maritima Utilizes Ferredoxin and NADH Synergistically: a New Perspective on Anaerobic Hydrogen Production

Abstract

The hyperthermophilic and anaerobic bacterium Thermotoga maritima ferments a wide variety of carbohydrates, producing acetate, CO 2 , and H 2 . Glucose is degraded through a classical Embden-Meyerhof pathway, and both NADH and reduced ferredoxin are generated. The oxidation of these electron carriers must be coupled to H 2 production, but the mechanism by which this occurs is unknown. The trimeric [FeFe]-type hydrogenase that was previously purified from T. maritima does not use either reduced ferredoxin or NADH as a sole electron donor. This problem has now been resolved by the demonstration that this hydrogenase requires the presence of both electron carriers for catalysis of H 2 production. The enzyme oxidizes NADH and ferredoxin simultaneously in an approximately 1:1 ratio and in a synergistic fashion to produce H 2 . It is proposed that the enzyme represents a new class of bifurcating [FeFe] hydrogenase in which the exergonic oxidation of ferredoxin (midpoint potential, ؊453 mV) is used to drive the unfavorable oxidation of NADH (E 0 ‫؍‬ ؊320 mV) to produce H 2 (E 0 ‫؍‬ ؊420 mV). From genome sequence analysis, it is now clear that there are two major types of [FeFe] hydrogenases: the trimeric bifurcating enzyme and the more well-studied monomeric ferredoxin-dependent [FeFe] hydrogenase. Almost one-third of the known H 2 -producing anaerobes appear to contain homologs of the trimeric bifurcating enzyme, although many of them also harbor one or more homologs of the simpler ferredoxin-dependent hydrogenase. The discovery of the bifurcating hydrogenase gives a new perspective on our understanding of the bioenergetics and mechanism of H 2 production and of anaerobic metabolism in general.

Phylogeny and physiology of candidate phylum ‘Atribacteria’ (OP9/JS1) inferred from cultivation-independent genomics

“…Sugar oxidation generates NADH and reduced ferredoxin (Fd red ) as reduced electron carriers and thus requires complementary pathways to dispose this reducing power (Figure 4a). These genomes encode pathways for reoxidation of NADH via acetyl-CoA reduction to ethanol (aldehyde and alcohol dehydrogenases) and H 2 production (NiFe hydrogenase, not shown in Figure 4) and concomitant re-oxidation of both NADH and Fd red by an electron-confurcating hydrogenase (Schut and Adams, 2009;Sieber et al, 2012;Figure 4b). In addition, they possess an NADH:Fd oxidoreductase (Rnf complex; Biegel et al, 2011) that may allow 'Ca.…”

Genomic and metagenomic surveys of hydrogenase distribution indicate H2 is a widely utilised energy source for microbial growth and survival

“…The group 3c [NiFe]-hydrogenase in functional complex with heterodisulphide reductase, for example, simultaneously reduces ferredoxin and heterodisulphide during H 2 oxidation (Kaster et al, 2011); these enzymes complete the recently elucidated Wolfe cycle of methanogenesis (Thauer, 2012), and are also distributed in some bacteria (for example, δ-Proteobacteria) (Figure 4). The group A3 [FeFe]-hydrogenases reversibly bifurcate electrons from H 2 to ferredoxin and NAD using trimeric or tetrameric complexes; in the reverse reaction, energy conserved during the oxidation of ferredoxin is used to drive the thermodynamically unfavourable production of H 2 from NADH (Schut and Adams, 2009;Schuchmann and Müller, 2012). A subtype of the group A4 [FeFe]-hydrogenases can also bifurcate electrons from H 2 to NADP and ferredoxin, and act physiologically in hexameric complexes with formate dehydrogenase (Wang et al, 2013).…”

“…In anaerobic systems, ultra-minimalistic hydrogenase-containing respiratory chains have been described that efficiently generate energy within oligotrophic environments (Kim et al, 2010;Lim et al, 2014). In parallel, the discovery of electron bifurcation has expanded our understanding of how energy is conserved in anaerobic processes such as cellulolytic fermentation, acetogenesis and methanogenesis (Schut and Adams, 2009;Kaster et al, 2011;Buckel and Thauer, 2013;Schuchmann and Muller, 2014). Other themes, including H 2 sensing within anaerobes (Zheng et al, 2014) and H 2 fermentation in aerobes , are emerging.…”

H2metabolism is widespread and diverse among human colonic microbes

“…These trimeric enzymes reversibly bifurcate electrons from H 2 to ferredoxin and NAD. 26,35 The reductant generated can be used to sustain anabolic processes, carbon-fixation (e.g., via reductive acetogenesis), or further fermentation (via [FeFe]-hydrogenases). However, based on recent models, it is likely that a large proportion is reoxidised through the respiratory Rnf complex, which generates sodium/protonmotive force by coupling Fd red oxidation to NAD C reduction.…”