Krebs Cycle

Definition

The Krebs Cycle, also known as the citric acid cycle or tricarboxylic acid (TCA) cycle, is a series of chemical reactions that take place in the mitochondria of cells, responsible for converting carbohydrates, fats, and proteins into carbon dioxide, water, and energy. This cycle plays a crucial role in cellular metabolism and energy production by oxidizing acetyl-CoA, derived from these macronutrients, to generate ATP and reducing equivalents like NADH and FADH2, which are essential for ATP synthesis during oxidative phosphorylation.

5 Must Know Facts For Your Next Test

1. The Krebs Cycle takes place in the mitochondrial matrix and involves eight distinct enzymatic reactions that regenerate oxaloacetate at the end of the cycle.
2. Each turn of the Krebs Cycle produces three molecules of NADH, one molecule of FADH2, one molecule of ATP (or GTP), and two molecules of CO2.
3. The cycle is named after Hans Krebs, who identified it in 1937 and earned a Nobel Prize for his work in biochemistry.
4. The Krebs Cycle is considered amphibolic because it plays a role in both catabolism (breaking down molecules) and anabolism (building up molecules).
5. Regulation of the Krebs Cycle occurs at key enzymatic steps that respond to the cell's energy needs, particularly through feedback inhibition by ATP and NADH.

Review Questions

• How does the Krebs Cycle integrate different macronutrients for energy production?
  • The Krebs Cycle integrates carbohydrates, fats, and proteins through their conversion into acetyl-CoA, which serves as the primary substrate for the cycle. Carbohydrates are broken down into glucose, which is further metabolized into pyruvate and converted to acetyl-CoA. Fats undergo beta-oxidation to produce acetyl-CoA from fatty acids. Proteins are deaminated and converted into various intermediates that can also feed into the cycle. This allows the Krebs Cycle to be central in linking various metabolic pathways for efficient energy production.
• Discuss the importance of NADH and FADH2 produced during the Krebs Cycle in cellular respiration.
  • NADH and FADH2 are crucial electron carriers produced during the Krebs Cycle that feed into the electron transport chain during oxidative phosphorylation. Each NADH molecule can lead to the production of approximately 2.5 ATP molecules, while each FADH2 can generate about 1.5 ATP molecules when oxidized. This makes them essential for maximizing energy yield from nutrients processed by the Krebs Cycle, thus playing a vital role in cellular respiration and overall energy metabolism.
• Evaluate how alterations in the regulation of the Krebs Cycle could impact cellular metabolism and overall energy homeostasis.
  • Alterations in the regulation of the Krebs Cycle can significantly affect cellular metabolism and energy balance. For instance, if there is excessive accumulation of ATP or NADH due to inhibited enzymatic activity in the cycle, this could lead to reduced flux through it, ultimately decreasing ATP production. On the other hand, increased substrate availability or stimulation from low energy states could enhance cycle activity, promoting higher energy output. Such dysregulation may contribute to metabolic disorders or inefficiencies in energy use within cells, reflecting its crucial role in maintaining energy homeostasis.

Related terms

Acetyl-CoA: A central metabolite that enters the Krebs Cycle, formed from the breakdown of carbohydrates, fats, and proteins.

NADH:

A reduced coenzyme generated during the Krebs Cycle that carries electrons to the electron transport chain for ATP production.

Oxidative Phosphorylation: The final stage of cellular respiration where ATP is produced using energy derived from electrons transferred through the electron transport chain.