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

• 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.

• 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.

• Outcomes of Glycolysis

  • Glycolysis starts with one molecule of glucose and ends with two pyruvate (pyruvic acid) molecules, a total of four ATP molecules, and two molecules of NADH .
  • If the cell cannot catabolize the pyruvate molecules further (via the citric acid cycle or Krebs cycle), it will harvest only two ATP molecules from one molecule of glucose.
  • Glycolysis, or the aerobic catabolic breakdown of glucose, produces energy in the form of ATP, NADH, and pyruvate, which itself enters the citric acid cycle to produce more energy.

• Connecting Lipids to Glucose Metabolism

  • Like sugars and amino acids, the catabolic pathways of lipids are also connected to the glucose catabolism pathways.
  • Triglycerides, a form of long-term energy storage in animals, are made of glycerol and three fatty acids.
  • Animals can make most of the fatty acids they need.
  • Fatty acids are catabolized in a process called beta-oxidation that takes place in the matrix of the mitochondria and converts their fatty acid chains into two carbon units of acetyl groups, while producing NADH and FADH2.
  • The acetyl groups are picked up by CoA to form acetyl CoA that proceeds into the citric acid cycle as it combines with oxaloacetate.

• ATP Yield

  • Moreover, the five-carbon sugars that form nucleic acids are made from intermediates in glycolysis.
  • Certain nonessential amino acids can be made from intermediates of both glycolysis and the citric acid cycle.
  • Lipids, such as cholesterol and triglycerides, are also made from intermediates in these pathways, and both amino acids and triglycerides are broken down for energy through these pathways.

• Connecting Other Sugars to Glucose Metabolism

  • This happens because all of the catabolic pathways for carbohydrates, proteins, and lipids eventually connect into glycolysis and the citric acid cycle pathways.
  • Like sugars and amino acids, the catabolic pathways of lipids are also connected to the glucose catabolism pathways.

• Lipid-Derived, Amino Acid-Derived, and Peptide Hormones

  • The amino acid-derived hormones are relatively small molecules derived from the amino acids tyrosine and tryptophan .
  • If a hormone is amino acid-derived, its chemical name will end in "-ine".
  • The pineal gland in the brain makes and secretes melatonin, which regulates sleep cycles.
  • The structure of peptide hormones is that of a polypeptide chain (chain of amino acids).
  • Amino acid-derived and polypeptide hormones are water-soluble and insoluble in lipids.