Biochemistry and Molecular Biology - Belyasova, N. A. 2002

Metabolism. Processes Leading to Energy Storage
Respiration
Features of Anaerobic Respiration

As shown above, Oxidative Phosphorylation is a far more efficient energy-storage mechanism than substrate-level phosphorylation. It is hardly surprising, therefore, that evolution has enabled oxidative phosphorylation to occur under anaerobic conditions as well. In this case, the ultimate electron acceptor is not molecular oxygen, but rather various oxidized or partially oxidized compounds and ions, such as nitrates, sulfates, carbonates, fumarate, sulfur, and potentially ferric ions. Such processes are referred to as Anaerobic Respiration to emphasize that they involve electron transport through the Components of the Respiratory Chain without the involvement of molecular oxygen.

Anaerobic respiration occurs most frequently in bacterial Cells, although fumarate respiration (the reduction of fumarate within the Electron Transport Chain), for instance, has been detected in facultatively anaerobic worms and even in mammals. Fumarate respiration is often compared to Fermentation because both the electron Donors and acceptors in this process are organic substances. However, it should be kept in mind that the electrons reducing fumarate have already traversed part of the respiratory chain, thereby establishing a proton gradient across the membrane. Consequently, oxidative phosphorylation becomes feasible, and the reduction of fumarate is properly classified as anaerobic respiration.

All cells capable of anaerobic respiration possess a Cell/36.html">Respiratory Chain and typically contain Cytochromes. In most cases, inorganic electron acceptors enter the respiratory chain at the level of cytochrome b or c (owing to their redox potential values). As a result, phosphorylation generally does not take place at complex III, yielding less ATP than when O₂ is utilized.

Both organic and inorganic substrates can serve as electron donors in anaerobic respiration. Depending on the specific electron acceptor used, processes are distinguished as nitrate, sulfate, sulfur, carbonate, fumarate, or "iron" respiration. Table 12.2 lists the representatives of these respiration types along with their end products.

Class="center">Table 12.2. CHARACTERISTICS OF ANAEROBIC respiration processes

Most of the bacteria listed in Table 12.2 play a vital role in nature and human economic activity by participating in the nitrogen and sulfur cycles, determining soil fertility, and contributing to The formation of mineral and methane deposits. At the same time, the detrimental activities of many of these bacteria must be accounted for, including the accumulation of nitrites in soil and the anaerobic corrosion of iron, among others.