Energy Conservation via Electron-Transferring Flavoprotein in Anaerobic Bacteria

Energy conservation in chemotrophic organisms is generally coupled to redox reactions in catabolic pathways. In the oxi- dative part or branch, "energy-rich" compounds are formed, from which ATP is generated via substrate-level phosphoryla- tion (SLP). In the reductive branch the electron carriers are reoxidized by a terminal acceptor; in this way an electrochem- ical ion gradient (H or Na) at the cytoplasmic mem- brane 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 phos- phorylation (ETP). Bacterial fermentations are considered ap- parent 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-methyl- asparate by the closely related anaerobic bacteria Clostridium tetani, Clostridium tetanomorphum, and Clostridium pascui to ammonia, acetate, butyrate, and molecular hydrogen accord- ing to equation 1 (Fig. 1) (8, 16, 51): 5 glutamate 6H2O 2H 3 5N H 4 5C O 2