Energy Conservation via Electron-Transferring Flavoprotein in Anaerobic Bacteria

Abstract

Energy conservation in chemotrophic organisms is generally coupled to redox reactions in catabolic pathways. In the oxidative part or branch, "energy-rich" compounds are formed, from which ATP is generated via substrate-level phosphorylation (SLP). In the reductive branch the electron carriers are reoxidized by a terminal acceptor; in this way an electrochemical ion gradient (⌬H ϩ or ⌬Na ϩ ) at the cytoplasmic membrane is established, which is used for ATP synthesis, transport across membranes, and motility. This second type of energy conservation is called respiration or electron transport phosphorylation (ETP). Bacterial fermentations are considered apparent exceptions to this generalization because they are thought to lack ETP (41). In these fermentations the substrate serves not only as an electron donor but also as a terminal acceptor, since oxygen, nitrate, fumarate, etc. are absent (44). An example is the fermentation of glutamate via 3-methylasparate by the closely related anaerobic bacteria Clostridium tetani, Clostridium tetanomorphum, and Clostridium pascui to ammonia, acetate, butyrate, and molecular hydrogen according to equation 1 (Fig. 1) (8,16,51):In the first part of this pathway, glutamate is converted to ammonia, acetate, and pyruvate, which is oxidized by ferredoxin and coenzyme A (CoA) to acetyl-CoA and CO 2 . In order to regenerate the oxidant, acetyl-CoA and protons are reduced to butyryl-CoA and hydrogen, respectively. From 2 butyrylCoA and 1 acetyl-CoA, 3 ATP are obtained via SLP. But the free enthalpy required to synthesize 1 ATP (Ϫ317/3 ϭ Ϫ106 kJ mol Ϫ1 ) is still much higher than that needed in other systems; i.e., the efficiency is low ( ϭ 42%). Usually, Ϫ75 Ϯ 5 kJ mol Ϫ1 ( ϭ 60%) is considered the minimum free enthalpy for ATP synthesis (44). It should be noted that without hydrogen formation the ATP yield would drop to 2.5 mol ATP (5 mol glutamate)Ϫ1 , because in order to maintain the redox balance, acetyl-CoA has to be completely reduced to butyrate. Thus, hydrogen formation increases SLP. If all the reducing equivalents dissipated as hydrogen and no butyrate was formed, the yield would rise to 5 mol ATP (5 mol glutamate)Ϫ1 . This, however, is thermodynamically not possible (Ϫ46 kJ mol glutamate Ϫ1 ). The oxidation of pyruvate derived from glutamate yields reduced ferredoxin (E 0 Ј Յ Ϫ420 mV), whereas NADH (E 0 Ј ϭ Ϫ320 mV) is the reductant in butyrate synthesis. The difference (Ն100 mV) could be used for additional energy conservation. Recently, we discovered the enzyme that catalyzes the reduction of NAD ϩ with reduced ferredoxin (Fig. 1) and characterized it as an Rnf-type NADH ferredoxin oxidoreductase localized in the membrane of C. tetanomorphum. Although it has not been demonstrated yet that this clostridial Rnf protein pumps protons or sodium ions, its cellular localization and the homology of four of its six subunits with four of the subunits of NADH-quinone reductase (Nqr) from Vibrio alginolyticus and Vibrio cholerae strongly suggest that this enzyme is involved in ener...

A Bacterial Electron-bifurcating Hydrogenase

“…The reduced ferredoxin is then used to produce hydrogen with a ferredoxin-dependent hydrogenase or to fuel the Rnf complex (21). Electron bifurcation in strictly anaerobic bacteria and archaea, as it is understood today, requires flavins.…”

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

“…31 and 32; see ref. 33 for a more detailed account of this hypothesis). 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).…”

Metabolic interdependencies between phylogenetically novel fermenters and respiratory organisms in an unconfined aquifer

“…Of the fermentative metabolic end-products, the production of hydrogen and ethanol was most strongly supported by our proteomic data (Figure 4). The reasons for this may include a selective advantage gleaned by an increase in ATP production for organisms capable of hydrogen over organic acid production (Herrmann et al, 2008).…”