citric acid cycle

Examples of citric acid cycle in the following topics:

• Acetyl CoA and the Citric Acid Cycle

  • The citric acid cycle is a key component of the metabolic pathway by which all aerobic organisms generate energy.
  • 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 name of this metabolic pathway is derived from citric acid, a type of tricarboxylic acid that is first consumed and then regenerated by this sequence of reactions to complete the cycle.
  • The citric acid cycle is a key component of the metabolic pathway by which all aerobic organisms generate energy.
  • The product of this reaction, acetyl-CoA, is the starting point for the citric acid 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.
  • The precursors include amino acids and reducing agents such as NADH.
  • Additional pathways that require precursors formed by the TCA include amino acid and nucleotide synthesis .

• Pyruvic Acid and Metabolism

  • Pyruvic acid (CH3COCOOH) is an organic acid, a ketone, and the simplest of the alpha-keto acids.
  • 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.
  • 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).

• Types of Catabolism

  • Finally, the acetyl group on the CoA is oxidized to water and carbon dioxide in the citric acid cycle and electron transport chain, releasing the energy that is stored by reducing the coenzyme nicotinamide adenine dinucleotide (NAD+) into NADH.
  • Pyruvate is an intermediate in several metabolic pathways, but the majority is converted to acetyl-CoA and fed into the citric acid cycle.
  • Although some more ATP is generated in the citric acid cycle, the most important product is NADH, which is made from NAD+ as the acetyl-CoA is oxidized.
  • The glycerol initiates glycolysis and the fatty acids are broken down by beta oxidation to release acetyl-CoA, which then is fed into the citric acid cycle.
  • Several of these keto acids are intermediates in the citric acid cycle, for example the deamination of glutamate forms α-ketoglutarate.

• 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.

• Fermentation Without Substrate-Level Phosphorylation

  • Homolactic fermentation is the production of lactic acid from pyruvate; alcoholic fermentation is the conversion of pyruvate into ethanol and carbon dioxide; and heterolactic fermentation is the production of lactic acid as well as other acids and alcohols.
  • However, more exotic compounds can be produced by fermentation, such as butyric acid and acetone.
  • Pyruvic acid can be made from glucose through glycolysis, converted back to carbohydrates (such as glucose) via gluconeogenesis, or to fatty acids through acetyl-CoA.
  • It can also be used to construct the amino acid alanine and be converted into ethanol.
  • 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).

• Citric Acid and Other Organic Compounds

  • Many organic compounds, like citric acid, are produced industrially by microorganisms.
  • Citric acid (citrate) is an important substance in the Krebs cycle.
  • For centuries, the source of citric acid were citrus fruits.
  • The microorganism makes more citric acid in the Krebs cycle than needed for the cell's metabolism and exports it outside the cell.
  • The citric acid is then precipitated out of solution and regenerated.

• 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.
  • Other organic compounds such as alcohols, acetic acid, or formic acid are used as alternative electron acceptors by methanogens.
  • Acetotrophic archaea also break down acetic acid into methane and carbon dioxide directly.
  • 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.
  • Here archaea were found living under highly acidic conditions, in the runoff from an iron mine.

• Amino Acid Synthesis

  • These polymers are linear and unbranched, with each amino acid within the chain attached to two neighboring amino acids.
  • Twenty-two amino acids are naturally incorporated into polypeptides and are called proteinogenic or natural amino acids.
  • All amino acids are synthesized from intermediates in glycolysis, the citric acid cycle, or the pentose phosphate pathway.
  • Amino acid synthesis depends on the formation of the appropriate alpha-keto acid, which is then transaminated to form an amino acid.
  • Nonstandard amino acids often occur as intermediates in the metabolic pathways for standard amino acids — for example, ornithine and citrulline occur in the urea cycle, part of amino acid catabolism.

• 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 .
  • A fatty acid is a carboxylic acid with a long aliphatic tail that may be either saturated or unsaturated.
  • The molecule shown here is the eight-carbon saturated fatty acid known as octanoic acid (or caprylic acid).