Fermentation electron transport chain

The microbes use two general strategies to synthesize ATP respiration and fermentation. A respiring microbe captures the energy released when electrons are transferred from a reduced species in the environment to an oxidized species (Fig. 18.1). The reduced species, the electron donor, sorbs to a complex of redox enzymes, or a series of such complexes, located in the cell membrane. The complex strips from the donor one or more electrons, which cascade through a series of enzymes and coenzymes that make up the electron transport chain to a terminal enzyme complex, also within the cell membrane. 

These energy-producing reactions are termed respiration processes. They require the presence of an external compound that can serve as the terminal electron acceptor of the electron transport chain. However, under anaerobic conditions, fermentation processes that do not require the participation of an external electron acceptor can also proceed. In this case, the organic substrate undergoes a balanced series of oxidative and reductive reactions, i.e., organic matter reduced in one step of the process is oxidized in another. 

In the case of fermentation, the carbon and energy source is broken down by a series of enzyme-mediated reactions that do not involve an electron transport chain. In aerobic respiration, the carbon and energy source is broken down by a series of enzyme-mediated reactions in which oxygen serves as an external electron acceptor. In anaerobic respiration, the carbon and energy source is broken down by a series of enzyme-mediated reactions in which sulfates, nitrates, and carbon dioxide serve... 

Fermentation, which differs from respiration in not having an electron transport chain. Yeasts produce ethanol from sugars by fermentation ... 

Organisms with anaerobic mitochondria can be divided into two different types those which perform anaerobic respiration and use an alternative electron acceptor present in the environment, such as nitrate or nitrite, and those which perform fermentation reactions using an endogenously produced, organic electron acceptor, such as fumarate (Martin et al. 2001 Tielens et al. 2002). An example of the first type is the nitrate respiration that occurs in several ciliates (Finlay et al. 1983), and fungi (Kobayashi et al. 1996 Takaya et al. 2003), which use nitrate and/or nitrite as the terminal electron acceptor of their mitochondrial electron-transport chain, producing nitrous oxide as... 

Fig. 11. Proposed function of electrochemical and Na potentials in energy conservation coupled to acetate fermentation to CH4 and CO2. The Na /H antiporter is involved in the generation of A/iH from A/iNa. CH3CO-S-C0A, acetyl-coenzyme A [CO], CO bound to carbon monoxide dehydrogenase CH3-H4MPT, methyl-tetrahydromethanopterin CH3-S-C0M, methyl-coenzyme M. The hatched boxes indicate membrane-bound electron transport chains or membrane-bound methyl-transferase catalyzing either IT or Na translocation (see Figs. 5, 6 and 12). It is assumed that enzyme-bound [CO] is energetically equal to free CO. ATP is synthesized via membrane-bound H -translocating ATP synthase. The stoichiometries of translocation were taken from refs. [107,234] n, X, y and z are unknown stoichiometric factors.

These compounds are common in soils and sediments as the products of anaerobic fermentation reactions. Metabolism of these compounds can directly consume O2 (through monooxygenase enzymes) or indirectly, through formation of NADH which is shuttled into oxidative phosphorylation and the electron transport chain. Oxidative metabolism of Ci compounds is an important microbial process in soils and sediments, consuming the methane produced by methanogenesis. 

In anaerobic glycolysis, electrons from NADH do not enter the electron transport chain (see here). Anaerobic glycolysis pathways include lactate fermentation and ethanol fermentation. 

---