Cellular Respiration: Krebs Cycle, Electron Transport, and Fermentation
Krebs Cycle
The Krebs cycle is the common pathway in all aerobic cells for the complete oxidation of carbohydrates, fats, and proteins. It can also be the starting point of biosynthetic reactions. This is because there are intermediate metabolites, which may go to the cytosol and act as anabolic precursors. In this sense, we say that the Krebs cycle is amphibolic. The process involves the complete oxidation of acetyl-CoA, which is excreted as carbon dioxide. The e-/H+ obtained in the successive oxidations are used to form molecules of reducing power and chemical energy in the form of GTP. This formation of energy is known as substrate-level phosphorylation.
In summary, the Krebs cycle occurs as follows: acetyl-CoA joins oxaloacetic acid to form citric acid, liberating CoA. Then, a series of reactions are produced that will finally yield oxaloacetic acid again. In this sequence, the most important reactions that take place are two decarboxylations and four dehydrogenations, three with NAD and one with FAD, and then free energy in the form of GTP.
Electron Transport Chain (Respiratory Chain)
The electron transport chain corresponds to the final stage of respiration when the electrons torn from the molecules that are respired and stored in NADH and FADH2 will pass through a series of conveyors located in the mitochondrial cristae, forming three large enzyme complexes. The arrangement of conveyors allows electrons to hop from one to another, releasing a certain amount of energy used to form a high-energy bond between ADP and P, which results in a molecule of ATP. The final electron acceptor is molecular oxygen, and another consequence is the formation of water.
In the respiratory chain, we can observe: “For each generated NADH, 3 ATP are produced, and for each FADH2, 2 ATP are produced.”
Fermentation
Fermentation reactions are anaerobic energy reactions, essential to regenerate the NAD consumed in glycolysis from NADH2. Redox reactions occur, producing much less ATP than aerobic respiration. The final acceptor of electrons/protons is not oxygen but a simple organic molecule. There are two types of fermentation:
Lactic Acid Fermentation
Lactic acid fermentation occurs in many organisms and cells of higher organisms under anaerobic conditions to obtain energy from lactose in milk that is previously hydrolyzed to obtain glucose. This, after glycolysis, converts pyruvate to lactate (Lactobacillus bulgaris…). Industries such as cheese and yogurt are based on this process.
Alcoholic Fermentation
Alcoholic fermentation occurs in yeast and many other anaerobes. Pyruvate is decarboxylated and produces acetaldehyde, which is then reduced to ethanol. Acetaldehyde is the final electron acceptor.
Aerobic Respiration
Aerobic respiration reactions are carried out by aerobic cells. Energy is obtained through the transfer of e-/H+ from organic molecules to molecular oxygen fuels. It takes place in the mitochondria of eukaryotes and in the cytoplasm and membranous structures in prokaryotes. It is done in three phases:
- Oxidation of pyruvate and formation of acetyl-CoA
- The Krebs cycle, in which the remains of acetyl molecules are degraded, forming CO2 and reducing power, and energy is generated
- Electron transport: transport of H2 and e-/H+ from the molecules of reducing power to molecular oxygen. The flow of H2 and e-/H+ is coupled to the phosphorylation of ADP to ATP
Pyruvate Oxidation
Pyruvic acid is decarboxylated by pyruvate dehydrogenase, NAD, and CoA, producing acetyl-CoA. This reaction is inhibited when the amount of ATP in the cell is high. This is because its function is to provide fuel to obtain energy for the Krebs cycle. On the other hand, NAD is regenerated when NADH2 donates its e-/H+ to molecular oxygen in aerobic mitochondrial respiration. Acetyl-CoA is the entry point for anaerobic respiration. The largest amount of energy is extracted when glucose is completely oxidized to CO2 and H2O. Acetyl-CoA plays a central role in metabolism for two reasons: it originates in the degradation of different organic biomolecules and acts as a precursor for several biosynthetic pathways.
Chemiosmotic Hypothesis
The energy released by electron transport is used to pump protons from the matrix or intermembrane space from the stroma into the thylakoid. Proton pumping is performed through transporters located in enzyme complexes that exist in the membrane. This generates a proton electrochemical gradient that exerts a force called proton-motive force. When protons cross the inner membrane back down the gradient, they do so through the ATP synthase system found in these membranes, where proton-motive energy is transformed into binding energy in ATP molecules.
The Catabolism of Proteins
Protein has different missions than energetic ones. Three ways to distinguish amino acid oxidation:
- Transamination: The transfer of the amino group of an amino acid to a molecule called alpha-keto acid, which accepts it and transforms into another amino acid. Thus, one amino acid is degraded to allow another to be synthesized.
- Oxidative deamination: The direct release of the amino groups of amino acids in the form of NH4+, from alpha-keto acids.