Krebs cycle

Examples of Krebs cycle in the following topics:

• Importance of Glycolysis

  • Following the conversion of glucose to pyruvate, the glycolytic pathway is linked to the Krebs Cycle, where further ATP will be produced for the cell's energy needs.
  • It is followed by the Krebs cycle and oxidative phosphorylation to produce ATP.

• 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.
  • Additionally, the cycle is known as the Krebs cycle, named after Hans Krebs, who first identified the steps in the pathway in the 1930s in pigeon flight muscle.
  • Describe the fate of the acetyl CoA carbons in 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.

• Outcomes of Glycolysis

  • 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.

• Citric Acid Cycle

  • The last step in the citric acid cycle regenerates oxaloacetate by oxidizing malate.
  • Each turn of the cycle forms three NADH molecules and one FADH2 molecule.
  • One GTP or ATP is also made in each cycle.
  • 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).
  • Because the final product of the citric acid cycle is also the first reactant, the cycle runs continuously in the presence of sufficient reactants.

• The Carbon Cycle

• Regulator Molecules of the Cell Cycle

  • Cyclins regulate the cell cycle only when they are tightly bound to Cdks.
  • The second group of cell cycle regulatory molecules are negative regulators.
  • Negative regulators halt the cell cycle.
  • Rb halts the cell cycle by binding E2F.
  • The concentrations of cyclin proteins change throughout the cell cycle.

• The Lytic and Lysogenic Cycles of Bacteriophages

  • Bacteriophages, viruses that infect bacteria, may undergo a lytic or lysogenic cycle.
  • Bacteriophages may have a lytic cycle or a lysogenic cycle, and a few viruses are capable of carrying out both.
  • An example of a bacteriophage known to follow the lysogenic cycle and the lytic cycle is the phage lambda of E. coli.
  • A temperate bacteriophage has both lytic and lysogenic cycles.
  • In the lytic cycle, the phage replicates and lyses the host cell.

• Regulation of the Cell Cycle at Internal Checkpoints

  • The cell cycle is controlled by three internal checkpoints that evaluate the condition of the genetic information.
  • A checkpoint is one of several points in the eukaryotic cell cycle at which the progression of a cell to the next stage in the cycle can be halted until conditions are favorable (e.g. the DNA is repaired).
  • Cyclins are cell-signaling molecules that regulate the cell cycle.
  • The cell cycle is controlled at three checkpoints.
  • Explain the effects of internal checkpoints on the regulation of the cell cycle

• Proto-oncogenes

  • The genes that code for the positive cell cycle regulators are called proto-oncogenes.
  • Consider what might happen to the cell cycle in a cell with a recently-acquired oncogene.
  • The result is detrimental to the cell and will likely prevent the cell from completing the cell cycle; however, the organism is not harmed because the mutation will not be carried forward.
  • In addition to the cell cycle regulatory proteins, any protein that influences the cycle can be altered in such a way as to override cell cycle checkpoints.
  • An oncogene is any gene that, when altered, leads to an increase in the rate of cell cycle progression.