Citric Acid Cycle

The Citric Acid Cycle, also referred to as the tricarboxylic acid cycle or Krebs cycle, is a central metabolic biochemical cycle that plays a key role in cellular energy production. It takes place in the mitochondrial matrix of almost all aerobic cells and serves the complete oxidation of energy-rich molecules. Characteristic of the Citric Acid Cycle is the stepwise oxidation of acetyl-CoA, which originates from carbohydrates, fats and amino acids. In this process, energy-rich reducing equivalents are formed that are used in the respiratory chain for ATP synthesis. The citric acid cycle thus represents a central hub of metabolism.

What functions does the Citric Acid Cycle fulfill in the body?

  • Central energy production: The citric acid cycle provides the basis for ATP production.
  • Linking of the metabolic pathways: Carbohydrate, fat and protein metabolism converge here.
  • Formation of reducing equivalents: NADH and FADH₂ are formed for the respiratory chain.
  • Provision of intermediates: Metabolites serve as starting materials for biosyntheses.
  • Support of the aerobic metabolism: The cycle depends on oxygen.
  • Regulation of the energy status of the cell: Its activity adapts to the energy demand.

How does the Citric Acid Cycle proceed in the body?

The sequence takes place in several enzymatic steps:

  1. Condensation of acetyl-CoA with oxaloacetate: Formation of citrate as the starting point of the cycle.
  2. Isomerization to isocitrate: Rearrangement for further processing.
  3. Oxidative decarboxylations: Release of carbon dioxide and formation of NADH.
  4. Substrate-level phosphorylation: Direct formation of GTP (ATP equivalent).
  5. Regeneration of oxaloacetate: Preparation for a new pass through the cycle.

How is the Citric Acid Cycle regulated?

The regulation takes place via key enzymes and energy signals:

  • ATP to ADP ratio
  • NADH/NAD⁺ level
  • Calcium ions (especially in muscle cells)

A high energy state inhibits the cycle, a low one activates it.

When is the Citric Acid Cycle particularly active?

High energy demand
  • Physical exertion
  • Growth and regeneration
Sufficient oxygen supply
  • Aerobic metabolic states
High substrate availability
  • Availability of acetyl-CoA

Conclusion

The Citric Acid Cycle is a central junction of the energy metabolism and essential for the efficient production of ATP under aerobic conditions. Through its linking of different metabolic pathways and the provision of important intermediates, it represents an indispensable basis for cellular metabolism.