Citric acid cycle

Definition

The citric acid cycle, also known as the Krebs cycle or TCA cycle, is a central metabolic pathway that takes place in the mitochondria of cells. It plays a crucial role in cellular respiration by oxidizing acetyl-CoA derived from carbohydrates, fats, and proteins to produce energy-rich molecules such as ATP, NADH, and FADH2, while also generating carbon dioxide as a waste product. This cycle connects various metabolic pathways, linking carbohydrate, lipid, and amino acid metabolism and contributing to cofactor regeneration.

5 Must Know Facts For Your Next Test

1. The citric acid cycle consists of eight enzyme-catalyzed steps that begin with the condensation of acetyl-CoA and oxaloacetate to form citrate.
2. During each turn of the citric acid cycle, three molecules of NADH and one molecule of FADH2 are produced, which are essential for ATP generation in oxidative phosphorylation.
3. Carbon dioxide is released as a byproduct during specific steps of the cycle when carbon atoms are oxidized, contributing to cellular respiration's overall carbon output.
4. The citric acid cycle is a key hub that integrates various metabolic pathways, allowing cells to derive energy from different macromolecules such as carbohydrates and lipids.
5. Regulation of the citric acid cycle occurs at several key enzymes, including citrate synthase and isocitrate dehydrogenase, ensuring metabolic balance based on cellular energy needs.

Review Questions

• How does the citric acid cycle integrate various macromolecule metabolism and contribute to energy production?
  • The citric acid cycle acts as a metabolic hub that connects the breakdown of carbohydrates, fats, and proteins into acetyl-CoA, which enters the cycle. Through its eight enzymatic steps, it oxidizes acetyl-CoA to produce energy-rich molecules like NADH and FADH2. These molecules then feed into oxidative phosphorylation to generate ATP, thus linking macromolecule metabolism to energy production within cells.
• Discuss the importance of cofactor regeneration within the context of the citric acid cycle and cellular respiration.
  • Cofactor regeneration is crucial for maintaining the flow of the citric acid cycle and overall cellular respiration. During the cycle, cofactors like NAD+ are reduced to NADH; however, they must be regenerated to sustain continued metabolic activity. The oxidation of NADH back to NAD+ occurs during oxidative phosphorylation. This regeneration ensures that essential cofactors are available for subsequent cycles and processes, allowing for efficient energy production.
• Evaluate how disruptions in the citric acid cycle could affect overall cellular metabolism and energy balance.
  • Disruptions in the citric acid cycle can significantly impair cellular metabolism and energy balance. If any key enzymes are inhibited or if there is an accumulation of intermediates due to poor regulation, it can lead to reduced production of NADH and FADH2. This diminishes ATP generation through oxidative phosphorylation and causes a backlog in metabolic pathways feeding into the cycle. Such imbalances can result in increased reliance on alternative energy sources or lead to metabolic diseases if prolonged.

Related terms

Acetyl-CoA: A key molecule that enters the citric acid cycle, derived from carbohydrates, fats, and proteins; it acts as a building block for the cycle.

NADH: A coenzyme that carries electrons and is generated during the citric acid cycle, playing a vital role in the electron transport chain to produce ATP.

Oxidative phosphorylation: The metabolic process following the citric acid cycle where ATP is produced using the energy from NADH and FADH2 through the electron transport chain.