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