Fermentation Processes: Emerging and Conventional Technologies. Группа авторов

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Fermentation Processes: Emerging and Conventional Technologies - Группа авторов

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      1.2.2.2 Citric Acid Cycle

      Pyruvic acid is first decarboxylated (release of CO2) and then oxidized by a molecule of NAD+ (reduced to NADH + H+) to form the two carbon molecules of acetyl‐CoA. As two molecules of pyruvic acid are produced from a glucose molecule, two molecules of CO2, NADH + H+, and acetyl‐CoA are produced. In prokaryotic cells (e.g. bacteria), the citric acid cycle (or Krebs cycle) occurs in the cytosol, while in eukaryotic organisms (e.g. yeast), it takes place in the mitochondrial matrix.

      Overall, two molecules of CO2 are produced and three molecules of NAD+ are reduced to NADH + H+ during a turn of the citric acid cycle. Besides, two electrons and two protons released from pyruvic acid are used to reduce a molecule of flavin adenine dinucleotide (FAD) into FADH2, similar to that of NAD+. Finally, there is a release of chemical energy, which allows the synthesis of an ATP molecule. To summarize, for each molecule of glucose oxidized, four molecules of CO2, six molecules of NADH + H+, two molecules of FADH2, and two molecules of ATP are generated.

      1.2.2.3 Electron Transport Chain and Oxidative Phosphorylation

Schematic illustration of the electron transport chain showing the respiratory complexes.

      1.2.3 Anaerobic Respiration

      Some microbial species use anaerobic respiration to obtain their energy in the absence of O2. During this process, the electrons that are removed from organic nutrients such as glucose follow the same pathways as in aerobic respiration, except that the final acceptor is not O2, but another inorganic molecule (e.g. sulfate, nitrate, etc.). The sulfate ion is generally reduced to hydrogen sulfide, while nitrate ion can be reduced to nitrite, nitrogen oxide, or molecular nitrogen. Some bacteria reduce carbonate to methane. The number of ATP molecules produced by anaerobic respiration varies from one organism to another and from one metabolic pathway to another. This number is generally less than the 38 mol of ATP generated by aerobic respiration, the energy yield is lower, and anaerobic microorganisms usually grow slower than aerobic ones.

      1.2.4 Fermentation

      As shown in Figure 1.4, glucose is oxidized during glycolysis to form two molecules of pyruvate. The electrons and protons released during this pathway are captured by the NAD+ to be reduced to NADH + H+. As shown above, two molecules of ATP are produced during glycolysis. To regenerate the NAD+, the NADH + H+ must be reoxidized; otherwise, the oxidation of glucose will stop and glycolysis too. During this oxidation, electrons and protons are directly transferred to pyruvate or one of its derivatives. The reduction of these final electron acceptors results in the formation of many different compounds, which provide a great variety of types of fermentation. At the same time, the NAD+ is regenerated and can engage in another round of glycolysis. The goal is to provide an uninterrupted supply of NAD+, which allows uninterrupted oxidation of glucose.

      During fermentation, all ATP is produced solely by glycolysis, which implies a much lower energy yield compared to aerobic respiration (2 mol of ATP against 38 in prokaryotes). Considering that glucose oxidation is partial, a large part of the energy originally contained in glucose remains stored in the chemical bonds of the final fermentation product (e.g. ethanol, lactic acid, etc.). Fermentation microorganisms must, therefore, compensate for this shortfall by the oxidation of a larger quantity of substrate.

Schematic illustration of fermentation and energy generation.

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