Citric acid cycle

The citric acid cycle, also known as the Krebs cycle is a key metabolic pathway that occurs in the mitochondrial matrix of cells.

• Purpose:

  • It's the final common pathway for the oxidation of fuel molecules (carbohydrates, fats, and proteins), releasing energy stored in their chemical bonds.

• Location:

  • Occurs in the mitochondrial matrix in eukaryotes, or the cytoplasm in prokaryotes.

• Process overview:

  • Acetyl-CoA (2-carbon molecule) enters the cycle

  • Goes through a series of 8 enzyme-catalyzed reactions

  • Produces 2 CO2 molecules, 1 ATP (or GTP), 3 NADH, and 1 FADH2 per turn

• Key steps:

  • Acetyl-CoA combines with oxaloacetate to form citrate (6-carbon molecule)

  • Through subsequent steps, citrate is oxidized back to oxaloacetate

  • CO2 is released at two points in the cycle

• Energy production:

  • Directly produces 1 ATP (or GTP) per cycle

  • Generates reduced coenzymes (NADH and FADH2) that feed into the electron transport chain for ATP production

• Cycle nature:

  • Oxaloacetate is regenerated at the end, allowing the cycle to continue

  • For each glucose molecule, the cycle turns twice

• Regulation:

  • Tightly controlled by allosteric regulation and product inhibition

  • Key regulatory enzymes: citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase

• Anaplerotic reactions:

  • "Fill-up" reactions that replenish intermediates of the cycle

• Amphibolic nature:

  • Functions in both catabolism (breaking down molecules) and anabolism (building up molecules)

• Connection to other pathways:

  • Links to glycolysis, fatty acid oxidation, and amino acid metabolism

  • Provides precursors for biosynthesis of various molecules

The citric acid cycle is crucial for aerobic respiration, playing a central role in energy production and metabolic integration in cells.