Sodium Ion Pumps and Hydrogen Production in Glutamate Fermenting Anaerobic Bacteria

Abstract

Anaerobic bacteria ferment glutamate via two different pathways to ammonia, carbon dioxide, acetate, butyrate and molecular hydrogen. The coenzyme B₁₂-dependent pathway in Clostridium tetanomorphum via 3-methylaspartate involves pyruvate:ferredoxin oxidoreductase and a novel enzyme, a membrane-bound NADH:ferredoxin oxidoreductase. The flavin- and iron-sulfur-containing enzyme probably uses the energy difference between reduced ferredoxin and NADH to generate an electrochemical Na⁺ gradient, which drives transport processes. The other pathway via 2-hydroxyglutarate in Acidaminococcus fermentans and Fusobacterium nucleatum involves glutaconyl-CoA decarboxylase, which uses the free energy of decarboxylation to generate also an electrochemical Na⁺ gradient. In the latter two organisms, similar membrane-bound NADH:ferredoxin oxidoreductases have been characterized. We propose that in the hydroxyglutarate pathway these oxidoreductases work in the reverse direction, whereby the reduction of ferredoxin by NADH is driven by the Na⁺ gradient. The reduced ferredoxin is required for hydrogen production and the activation of radical enzymes. Further examples show that reduced ferredoxin is an agent, whose reducing energy is about 1 ATP ‘richer’ than that of NADH.

Dissection of the Caffeate Respiratory Chain in the Acetogen Acetobacterium woodii : Identification of an Rnf-Type NADH Dehydrogenase as a Potential Coupling Site

“…A potential candidate was an Rnf complex since this enzyme catalyzes the reduction of NAD ϩ by ferredoxin and, in addition, oxidation of NADH with artificial electron acceptors. In addition, oxidation of NADH with artificial electron acceptors is catalyzed (5,28). Indeed, membranes of A. woodii catalyzed NADH oxidation with an activity of 86 or 74 U/mg using potassium hexacyanoferrate or benzylviologen as the acceptor.…”

“…Oxidation of reduced ferredoxin with concomitant reduction of NAD ϩ has a free energy change of Ϫ20 kJ/mol, and this energy could be used to drive the electrogenic transport of Na ϩ out of the cell. An enzyme that could potentially catalyze such a reaction is the Rnf-type NADH dehydrogenase found recently in bacteria and archaea (5,31). The Rnf complex of R. capsulatus is thought to contain six subunits (48).…”

Hydrogen is the central free intermediate during lignocellulose degradation by termite gut symbionts

“…A similar situation occurs in the termite gut spirochete T. azotonutricium . The observed hydrogen formation from unfavorable electron donors may be driven by reverse electron transport via a membrane-bound NADH:ferredoxin oxidoreductase (Boiangiu et al, 2005), or directly by a modified complex I, as proposed for hydrogenosomes of Trichomonas vaginalis (Dyall et al, 2004). Complex I is proposed to be related to energyconverting hydrogenases (Hedderich and Forzi, 2005).…”

The genome ofClostridium kluyveri, a strict anaerobe with unique metabolic features

“…Reaction 5 principally resembles the coenzyme Q cycle in that twice the amount of electrons required for substrate reduction is fed into the cycle in which the flow of electrons is divided to react both with a high potential acceptor (crotonyl-CoA) and a low potential acceptor (ferredoxin). The reduced ferredoxin generated in reaction 5 is then used to regenerate NADH in reaction 6 catalyzed via a membrane-associated, energy-converting NADH: ferredoxin oxidoreductase (RnfA-G) (34,35 It is the combination of these reactions that allows explaining H 2 production by C. kluyveri and the generation of a proton motive force. The Bcd/EtfAB complex from C. kluyveri has been purified from the soluble cell fraction and was shown to catalyze reaction 5 (36).…”