citric acid cycle

Examples of citric acid cycle in the following topics:

• Acetyl CoA to CO2

  • The acetyl carbons of acetyl CoA are released as carbon dioxide in the citric acid cycle.
  • Acetyl CoA links glycolysis and pyruvate oxidation with the citric acid cycle.
  • In addition to the citric acid cycle, named for the first intermediate formed, citric acid, or citrate, when acetate joins to the oxaloacetate, the cycle is also known by two other names.
  • The TCA cycle is named for tricarboxylic acids (TCA) because citric acid (or citrate) and isocitrate, the first two intermediates that are formed, are tricarboxylic acids.
  • Describe the fate of the acetyl CoA carbons in the citric acid cycle

• 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.

• Citric Acid Cycle

  • Like the conversion of pyruvate to acetyl CoA, the citric acid cycle takes place in the matrix of the mitochondria.
  • If this transfer does not occur, the oxidation steps of the citric acid cycle also do not occur.
  • Note that the citric acid cycle produces very little ATP directly and does not directly consume oxygen.
  • The last step in the citric acid cycle regenerates oxaloacetate by oxidizing malate.
  • Several of the intermediate compounds in the citric acid cycle can be used in synthesizing non-essential amino acids; therefore, the cycle is amphibolic (both catabolic and anabolic).

• Connecting Proteins to Glucose Metabolism

  • When the amino group is removed from an amino acid, it is converted into ammonia through the urea cycle.
  • The keto acid can then enter the citric acid cycle.
  • When deaminated, amino acids can enter the pathways of glucose metabolism as pyruvate, acetyl CoA, or several components of the citric acid cycle.
  • For example, deaminated asparagine and aspartate are converted into oxaloacetate and enter glucose catabolism in the citric acid cycle.
  • The carbon skeletons of certain amino acids (indicated in boxes) are derived from proteins and can feed into pyruvate, acetyl CoA, and the citric acid cycle.

• Breakdown of Pyruvate

  • After glycolysis, pyruvate is converted into acetyl CoA in order to enter the citric acid cycle.
  • Acetyl CoA is a molecule that is further converted to oxaloacetate, which enters the citric acid cycle (Krebs cycle).
  • This molecule of acetyl CoA is then further converted to be used in the next pathway of metabolism, 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 .

• Control of Catabolic Pathways

  • Enzymes, proteins, electron carriers, and pumps that play roles in glycolysis, the citric acid cycle, and the electron transport chain tend to catalyze non-reversible reactions.
  • An increase in citrate concentration can occur because of a blockage in the citric acid cycle.
  • The citric acid cycle is controlled through the enzymes that catalyze the reactions that make the first two molecules of NADH .
  • A decrease in the rate of operation of the pathway at this point is not necessarily negative as the increased levels of the α-ketoglutarate not used by the citric acid cycle can be used by the cell for amino acid (glutamate) synthesis.
  • Enzymes, isocitrate dehydrogenase and α-ketoglutarate dehydrogenase, catalyze the reactions that make the first two molecules of NADH in the citric acid cycle.

• 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).

• 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.

• 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.