Biochemistry and Molecular Biology - Belyasova N.A. 2002

Metabolism. Energy-Yielding Processes
Respiration
Utilization of Inorganic Electron Donors

In most organisms, Organic compounds serve as both electron Donors and carbon sources during cellular Respiration. However, certain groups of soil and aquatic prokaryotes can utilize inorganic reduced substances as electron donors, such as elemental sulfur, molecular hydrogen, carbon monoxide, as well as ammonium, nitrite, sulfite, thiosulfate, sulfide, and ferrous (iron(II)) ions. These Bacteria were first discovered and described by S.N. Winogradsky, who termed their metabolic pathway «METABOLISM/21.html">Chemosynthesis». Based on their nutritional type, these bacteria are classified as chemolithoautotrophs: they harness the energy of chemical bonds from chemical compounds, use inorganic substances as electron donors, and rely on CO₂ as a carbon source.

In lithotrophic bacteria, the number of Cytochromes in the Respiratory Chain is typically several times higher than in organotrophs. This is due to The properties of the electron donors they use, which in most cases have more positive E₀' values than NAD, meaning they cannot directly transfer their electrons to NAD⁺. The electrons split off from most inorganic substrates are usually fed into the respiratory chain at the level of flavin or cytochromes. For instance, cytochrome c acts as the primary electron acceptor during The oxidation of Fe²⁺ ions, whereas cytochrome a serves this role in the oxidation of NO₂^- ions. Consequently, in iron bacteria and nitrifying bacteria, only complexes III and IV—or sometimes solely complex IV—function within the Electron Transport Chain. Accordingly, the number of ADP phosphorylation events drops to two or even one, compared to three in organotrophic bacteria when electrons are transferred to NAD⁺. Hydrogen bacteria are an exception: for H₂, E₀' = -0.42 V, and the OXIDATION OF MOLECULAR hydrogen by O₂ drives electron transfer across all three coupling sites, resulting in the synthesis of 3 ATP molecules.

The presence of a shortened respiratory chain in most chemolithoautotrophs leads to inefficient energy storage during respiration, necessitating the Processing of massive amounts of substrate. As a consequence, these bacteria grow very slowly; for example, the time interval between Cell divisions in nitrifying bacteria can reach 5–10 hours. Furthermore, these bacteria experience an intracellular NADH deficiency, as most inorganic electron donors are incapable of reducing NAD⁺. To overcome this challenge, chemolithotrophs have evolved a mechanism for Reverse Electron Transport through the Components of the respiratory chain. This reversed flow of electrons is driven by ATP Hydrolysis.

Table 12.3 outlines several processes involving the utilization of inorganic electron donors. It is worth noting that chemolithoautotrophs include both obligate and facultative lithotrophs. The latter are also capable of a chemoorganoheterotrophic mode of Nutrition.

The oxidation of Inorganic Compounds can occur under both aerobic (utilizing O₂) and anaerobic conditions, i.e., during Anaerobic respiration (Table 12.3).

Chemolithoautotrophs are capable of fixing CO₂, with the majority utilizing The Calvin Cycle for this purpose.

Class="center">Table 12.3. Modes of oxidation of inorganic electron donors