Krebs Cycle or Citric Acid Cycle

The Krebs cycle, also called the citric acid cycle (CAC) or tricarboxylic acid (TCA) cycle, is a central metabolic pathway in cellular respiration. It occurs in the mitochondrial matrix of eukaryotic cells and the cytoplasm of prokaryotic cells. This cycle plays a vital role in energy production, biosynthetic precursor generation, and metabolic regulation by oxidizing acetyl-CoA into carbon dioxide and transferring high-energy electrons to NADH and FADH₂ for ATP synthesis.

Key Points About the Krebs Cycle

• The Krebs cycle is also called the citric acid cycle (CAC) or tricarboxylic acid (TCA) cycle due to its intermediates.
• The cycle produces energy, provides metabolites for other pathways, and connects protein, fat, and carbohydrate metabolism.
• It occurs in the mitochondrial matrix in eukaryotes and the cytoplasm in prokaryotes.
• The cycle starts with acetyl-CoA (2 carbons) combining with oxaloacetate (4 carbons) to form citrate (6 carbons).
• The cycle oxidizes acetyl-CoA, releasing 2 CO₂, generating 3 NADH, 1 FADH₂, and 1 GTP (or ATP) per turn.
• NADH and FADH₂ donate electrons to the electron transport chain (ETC) to drive ATP production.
• It is aerobic because its products fuel oxidative phosphorylation, requiring oxygen as the final electron acceptor.

History of the Discovery of the Krebs Cycle

• 19th Century: Early researchers suspected a cyclic process in metabolism.
• 1930s: Albert Szent-Györgyi identified key intermediates (fumarate, succinate, malate).
• 1937: Hans Krebs and William Arthur Johnson established the cyclic nature of the pathway, showing how acetyl-CoA feeds into a self-replenishing sequence of reactions. Albert Szent-Györgyi receives the Nobel Prize in Physiology or Medicine for discoveries concerning fumaric acid, a component of the cycle.
• 1953: Krebs received the Nobel Prize in Physiology or Medicine for his discovery (Johnson was a student at the time of the discovery).

Why Is It Called the Krebs Cycle?

The cycle takes its name after Hans Krebs, the German-British biochemist who first described it in 1937. It is also called the citric acid cycle because citrate is the first stable intermediate formed. The term tricarboxylic acid (TCA) cycle reflects the three carboxyl (-COOH) groups in citric acid.

However, despite its name, the biochemical reactions are not necessary a “cycle.” There are at least three alternative pathways for metabolites, so they don’t always follow a set route.

Functions of the Krebs Cycle

The Krebs cycle serves multiple essential functions in metabolism:

1. Energy Production – Generates high-energy electron carriers (NADH and FADH₂) that drive ATP synthesis in oxidative phosphorylation.
2. Carbon Source for Biosynthesis – Provides intermediates for the synthesis of amino acids, nucleotides, fatty acids, and heme.
3. Metabolic Regulation – Serves as a hub integrating glycolysis, fatty acid oxidation, and amino acid metabolism.
4. Waste Removal – Eliminates metabolic waste by oxidizing acetyl-CoA to CO₂.

Where Does the Krebs Cycle Occur?

• Eukaryotes: The Krebs cycle takes place in the mitochondrial matrix.
• Prokaryotes: The Krebs cycle occurs in the cytoplasm because prokaryotes lack mitochondria. The plasma membrane provides the proton gradient.

Overview of the Krebs Cycle

The Krebs or citric acid cycle consists of eight steps. Remember that each glucose molecule yields two acetyl-CoA, so there are two complete cycles per glucose molecule.

• Acetyl-CoA (2C) condenses with oxaloacetate (4C) to form citrate (6C).
• Citrate undergoes rearrangement and decarboxylation, forming α-ketoglutarate (5C), then succinyl-CoA (4C), releasing 2 CO₂.
• Energy is captured as NADH, FADH₂, and GTP (or ATP).
• Regeneration of oxaloacetate completes the cycle.

Table of the Krebs Cycle Steps

Products of the Krebs Cycle

Each turn of the cycle produces:

• 3 NADH
• 1 FADH₂
• 1 GTP (or ATP)
• 2 CO₂

Because the cycle occurs twice per glucose, the products at the end of two cycles are 6 NADF, 2 FADH₂, 2 GTP, and 4 CO₂.

Balanced Equation for One Turn of the Krebs Cycle

Acetyl-CoA + 3NAD⁺ + FAD + GDP + P_i + 2H₂O → 2CO₂ + 3NADH + 3H⁺ + FADH₂ + GTP + CoA

Overall Glucose Oxidation (Cellular Respiration)

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 36−38ATP

Efficiency of the Krebs Cycle

• Theoretical Maximum ATP Yield: ~36-38 ATP per glucose.
• The Krebs cycle itself produces 1 GTP (ATP equivalent) per turn, but most ATP comes from oxidative phosphorylation.

The actual ATP yield depends on several factors. For example using the glycerol phosphate shuttle rather than the malate-aspartate shuttle reduces ATP from 38 to 36. If protons leak across the mitochondrial membrane or other inefficiencies occur, the number reduces even more. Typically the actual total net production of ATP is 30.

Regulation of the Krebs Cycle

Allosteric inhibition of enzymes by metabolites and substrate availability are the key regulating factors of the Krebs or citric acid cycle. Tight regulation of the cycle is important because overproducing NADH and ATP is a huge waste of chemical energy.

Key Regulatory Enzymes:

1. Citrate Synthase – Inhibited by ATP, NADH, and citrate.
2. Isocitrate Dehydrogenase – Activated by ADP, inhibited by ATP and NADH.
3. α-Ketoglutarate Dehydrogenase – Inhibited by ATP, NADH, and succinyl-CoA.

Regulatory Mechanisms:

• Energy charge: High ATP/NADH inhibits; low ATP/AMP activates.
• Substrate availability: Acetyl-CoA, NAD⁺ levels control flux.
• Feedback inhibition: Citrate and succinyl-CoA inhibit upstream steps.

Intermediates as Precursors

Krebs cycle intermediates are vital in biosynthesis:

• Citrate → Fatty acids, cholesterol.
• α-Ketoglutarate → Amino acids, neurotransmitters.
• Succinyl-CoA → Heme, chlorophyll.
• Oxaloacetate → Glucose (via gluconeogenesis), amino acids.

Mnemonics for Memorization of the Krebs Cycle

Since the Krebs cycle involves several intermediates and enzymes, mnemonics help with recall. Here are some useful ones:

Mnemonics for the Intermediates

To remember the order of the cycle’s intermediates, you can use:

“Can I Keep Selling Substances For Money, Officer?”
Citrate → Isocitrate → Ketoglutarate (α-Ketoglutarate) → Succinyl-CoA → Succinate → Fumarate → Malate → Oxaloacetate

“Citrate Is Krebs’ Starting Substrate For Making Oxaloacetate”
(Citrate, Isocitrate, α-Ketoglutarate, Succinyl-CoA, Succinate, Fumarate, Malate, Oxaloacetate)

Mnemonics for the Enzymes

The enzymes catalyzing the reactions follow this order:

“Clever Is A Super Smart Fella, Man!”
Citrate synthase → Isocitrate dehydrogenase → Alpha-ketoglutarate dehydrogenase → Succinyl-CoA synthetase → Succinate dehydrogenase → Fumarase → Malate dehydrogenase

Mnemonic for the Products (NADH, FADH₂, GTP, CO₂)

To remember what is produced, you can use:

“No Fun And Games”
NADH, FADH₂, ATP (or GTP), GO₂ (CO₂)

Alternatively, if you want to recall where NADH, FADH₂, and ATP/GTP are made:
“Naughty Kids Suck Smelly Socks For Money”
(NADH: Isocitrate → α-Ketoglutarate → Succinyl-CoA,
ATP/GTP: Succinyl-CoA → Succinate,
FADH₂: Fumarate,
NADH: Malate)

Would you like me to refine these mnemonics or suggest others?

Real-World Applications of the Krebs Cycle

While the Krebs cycle is important in the context of cellular respiration, its principles apply to many areas of science and medicine.

1. Medical and Health Relevance

Cancer Metabolism (Warburg Effect)
Cancer cells often shift from aerobic respiration to aerobic glycolysis, reducing Krebs cycle activity. Understanding this shift helps in cancer treatment research.

Mitochondrial Diseases and Neurological Disorders
Dysfunction in the Krebs cycle links to neurodegenerative diseases (e.g., Alzheimer’s, Parkinson’s), as neurons rely on oxidative metabolism.

Diabetes and Metabolic Disorders
Defects in metabolic pathways involving the Krebs cycle contribute to insulin resistance and obesity-related disorders.

Lactic Acidosis and Anoxia
When oxygen is low (hypoxia), cells rely on glycolysis instead of the Krebs cycle, leading to lactic acid buildup, which occurs in shock, sepsis, and heart failure.

2. Biotechnology and Bioengineering

Biofuels Production
Microbial fermentation pathways that bypass the Krebs cycle optimize ethanol, butanol, and hydrogen gas production for renewable energy.

Pharmaceutical and Drug Development
Krebs cycle intermediates are targets for drug design in antibiotics, chemotherapy, and metabolic disease treatments.

3. Evolutionary and Environmental Biology

Evolutionary Adaptations in Extremophiles
Some bacteria and archaea use reverse Krebs cycles for carbon fixation. This helps them to thrive in extreme environments (e.g., deep-sea vents, hot springs).

Role in Photosynthesis (Calvin Cycle Parallels)
While the Krebs cycle is catabolic, the Calvin cycle (in plants) is its anabolic counterpart, fixing carbon for biosynthesis.

Frequently Asked Questions (FAQs) About the Krebs Cycle

1. Why is the Krebs cycle important?

The Krebs cycle is crucial because it generates NADH and FADH₂, which power ATP synthesis in the electron transport chain (ETC). It also provides biosynthetic intermediates for amino acids, nucleotides, fatty acids, and heme.

2. Where does the Krebs cycle occur?

• Eukaryotes: In the mitochondrial matrix.
• Prokaryotes: In the cytoplasm.

3. Is the Krebs cycle aerobic or anaerobic?

Aerobic. While the Krebs cycle itself does not use oxygen directly, it requires oxygen because NADH and FADH₂ must transfer electrons to the ETC, which relies on oxygen as the final electron acceptor.

4. What happens if the Krebs cycle is inhibited?

If the cycle is blocked or inhibited, cells cannot efficiently produce ATP and will rely more on anaerobic pathways like glycolysis and fermentation, leading to lactic acid accumulation (in animals) or ethanol production (in yeast).

5. How many ATP molecules are produced per turn of the Krebs cycle?

Each turn of the cycle produces 1 GTP (equivalent to 1 ATP). However, the real ATP gain comes from oxidative phosphorylation, where NADH and FADH₂ contribute to the production of ~9 ATP per cycle turn.

6. How many times does the Krebs cycle turn per glucose molecule?

Each glucose molecule produces two molecules of acetyl-CoA, so the cycle turns twice per glucose.

7. What are the main regulatory enzymes of the Krebs cycle?

The key enzymes regulating the cycle are:

• Citrate synthase (first step)
• Isocitrate dehydrogenase (rate-limiting step)
• α-Ketoglutarate dehydrogenase

These enzymes respond to ATP/NADH inhibition and ADP/AMP activation.

8. What are the main products of one turn of the Krebs cycle?

For one turn, the cycle produces:

• 3 NADH
• 1 FADH₂
• 1 GTP (or ATP)
• 2 CO₂

Since glucose fuels two turns, these numbers double per glucose molecule.

9. Why is the Krebs cycle cyclic?

The cycle begins and ends with oxaloacetate, which reacts with acetyl-CoA at the start and is regenerated at the end, allowing the process to continue indefinitely under normal conditions.

10. What molecules can enter the Krebs cycle besides acetyl-CoA?

• Amino acids (via deamination into intermediates like α-ketoglutarate, fumarate, and oxaloacetate).
• Fatty acids (via β-oxidation forming acetyl-CoA).
• Certain carbohydrates (via conversion to acetyl-CoA or intermediates).

11. Can the Krebs cycle run in reverse?

Yes! Some bacteria and archaea use a reductive citric acid cycle (reverse Krebs cycle) to fix CO₂ and synthesize organic molecules.

12. How is the Krebs cycle linked to the electron transport chain (ETC)?

The NADH and FADH₂ generated in the Krebs cycle donate electrons to the ETC, driving ATP production via oxidative phosphorylation.

13. What is the role of intermediates in metabolism?

Intermediates like citrate, α-ketoglutarate, and succinyl-CoA feed into biosynthesis of amino acids, lipids, nucleotides, and heme.

14. Why do some cells use fermentation instead of the Krebs cycle?

Cells switch to fermentation when oxygen is unavailable, preventing the ETC from functioning. Fermentation regenerates NAD⁺ so glycolysis can continue, but it produces much less ATP.

15. Do plants have a Krebs cycle?

Yes! Plants use the Krebs cycle in mitochondria for energy but also have modifications (e.g., glyoxylate cycle) to bypass CO₂ release and conserve carbon for biosynthesis.

References

• Berg, J.M.; Tymoczko, J.L.; Stryer, L. (2002). “The Citric Acid Cycle”. Biochemistry (5th ed.). W H Freeman. ISBN 0-7167-3051-0.
• Hui, S.; Ghergurovich, J.M.; et al. (2017). “Glucose feeds the TCA cycle via circulating lactate”. Nature. 551 (7678): 115–118. doi:10.1038/nature24057
• Krebs, H.A.; Johnson, W.A. (1937). “Metabolism of ketonic acids in animal tissues”. The Biochemical Journal. 31 (4): 645–60. doi:10.1042/bj0310645
• Lieberman, M. (2013). Marks’ Basic Medical Biochemistry : A Clinical Approach (4th ed.). Lippincott Williams & Wilkins. ISBN 978-1-60831-572-7.