Krebs cycle

Examples of Krebs cycle in the following topics:

• Citric Acid and Other Organic Compounds

  • Citric acid (citrate) is an important substance in the Krebs cycle.
  • The Krebs cycle is key in the oxidation of sugars, proteins and fats to carbon dioxide and water.
  • Many of the cycle compounds are also needed for the synthesis of the cells' own proteins, carbohydrates, and fats.
  • The microorganism makes more citric acid in the Krebs cycle than needed for the cell's metabolism and exports it outside the cell.

• Pyruvic Acid and Metabolism

  • Pyruvic acid supplies energy to living cells through the citric acid cycle (also known as the Krebs cycle) when oxygen is present (aerobic respiration); when oxygen is lacking, it ferments to produce lactic acid.
  • Pyruvate is converted into acetyl-coenzyme A, which is the main input for a series of reactions known as the Krebs cycle.
  • Pyruvate is also converted to oxaloacetate by an anaplerotic reaction, which replenishes Krebs cycle intermediates; also, oxaloacetate is used for gluconeogenesis.
  • The cycle is also known as the citric acid cycle or tri-carboxylic acid cycle, because citric acid is one of the intermediate compounds formed during the reactions.
  • Pyruvic acid supplies energy to living cells through the citric acid cycle (also known as the Krebs cycle) when oxygen is present (aerobic respiration), and alternatively ferments to produce lactic acid when oxygen is lacking (fermentation).

• Respiration and Proton Motive Force

  • During aerobic conditions, the pyruvate enters the mitochondrion to be fully oxidized by the Krebs cycle.
  • The products of the Krebs cycle include energy in the form of ATP (via substrate level phosphorylation), NADH, and FADH2.
  • Both types of metabolism share the initial pathway of glycolysis, but aerobic metabolism continues with the Krebs cycle and oxidative phosphorylation.
  • A diagram of cellular respiration including glycolysis, Krebs cycle (AKA citric acid cycle), and the electron transport chain.

• Acetyl CoA and the Citric Acid Cycle

  • The citric acid cycle, shown in —also known as the tricarboxylic acid cycle (TCA cycle) or the Krebs cycle—is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetate—derived from carbohydrates, fats, and proteins—into carbon dioxide.
  • The NADH generated by the TCA cycle is fed into the oxidative phosphorylation pathway.
  • Components of the TCA cycle were derived from anaerobic bacteria, and the TCA cycle itself may have evolved more than once.
  • Theoretically there are several alternatives to the TCA cycle, however the TCA cycle appears to be the most efficient.
  • The citric acid cycle, or Krebs cycle, is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidization of acetate—derived from carbohydrates, fats, and proteins—into carbon dioxide.

• The Reverse TCA Cycle

  • The reverse TCA cycle utilizes carbon dioxide and water to form carbon compounds.
  • The citric acid cycle (TCA) or Krebs cycle, is a process utilized by numerous organisms to generate energy via the oxidation of acetate derived from carbohydrates, fats, and proteins into carbon dioxide .
  • However, there are numerous organisms that undergo reverse TCA or reverse Krebs cycles.
  • The chemical reactions that occur are the reverse of what is seen in the TCA cycle .
  • The following is a brief overview of the reverse TCA cycle.

• Substrates for Biosynthesis

  • The citric acid cycle, commonly referred to as the Krebs cycle, is characterized by the production of energy through the oxidation of acetate derived from carbohydrates, fats, and proteins into carbon dioxide.
  • The cycle is one of the major metabolic processes utilized to generate energy.
  • The citric acid cycle, comprised of a series of chemical reactions, provides precursors for additional biochemical pathways.

• Biosynthesis and Energy

  • The major pathways utilized to ensure fixation of carbon dioxide include: the Calvin cycle, the reductive TCA cycle, and the acetyl-CoA pathway.
  • The Calvin cycle involves utilizing carbon dioxide and water to form organic compounds.
  • The reductive TCA cycle, commonly referred to as the reverse Krebs cycle, also produces carbon compounds from carbon dioxide and water.

• Fermentation Without Substrate-Level Phosphorylation

  • Pyruvic acid supplies energy to living cells through the citric acid cycle (also known as the Krebs cycle) when oxygen is present (aerobic respiration), and alternatively ferments to produce lactic acid when oxygen is lacking (fermentation).

• Energy Conservation and Autotrophy in Archaea

  • For example, archaea use a modified form of glycolysis (the Entner–Doudoroff pathway) and either a complete or partial citric acid cycle.
  • This process involves either a highly modified form of the Calvin cycle or a recently discovered metabolic pathway called the 3-hydroxypropionate/4-hydroxybutyrate cycle.
  • In addition, the Crenarchaeota use the reverse Krebs cycle while the Euryarchaeota use the reductive acetyl-CoA pathway.

• Lipid Metabolism

  • These fatty acids can then enter a dedicated pathway that promotes step-wise lipid processing that ultimately yields acetyl-CoA, a critical metabolite that conveys carbon atoms to the TCA cycle (aka Krebs cycle or citric acid cycle) to be oxidized for energy production.
  • The acetyl-CoA molecule liberated by this process is eventually converted into ATP through the TCA cycle.
  • This cycle repeats until the fatty acid has been completely reduced to acetyl-CoA, which is fed through the TCA cycle to ultimately yield cellular energy in the form of ATP .