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Krebs Cycle – Function, Steps & Clinical Relevance

The Krebs cycle is a central metabolic pathway in human cells that generates energy from nutrients. It takes place in the mitochondria and is essential for life.

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The Krebs cycle is a central metabolic pathway in human cells that generates energy from nutrients. It takes place in the mitochondria and is essential for life.

What is the Krebs Cycle?

The Krebs cycle – also known as the citric acid cycle or tricarboxylic acid (TCA) cycle – is a fundamental biochemical pathway that takes place in the mitochondria of all aerobic (oxygen-dependent) cells. It was discovered in the 1930s by the German-British biochemist Hans Adolf Krebs, who was awarded the Nobel Prize in Physiology or Medicine in 1953 for this achievement. The cycle is named in his honor.

The Krebs cycle is the biochemical cornerstone of cellular energy metabolism and plays a central role in converting nutrients into usable energy in the form of ATP (adenosine triphosphate), the universal energy carrier of the body.

Biological Significance and Function

The Krebs cycle serves two essential roles in metabolism:

Energy production: The cycle completely oxidizes acetate (bound as acetyl-CoA) to carbon dioxide (CO₂), capturing energy-rich electrons in the carrier molecules NADH and FADH₂. These are later used in the electron transport chain to synthesize ATP.
Provision of biosynthetic precursors: Intermediates of the cycle serve as starting materials for the synthesis of amino acids, fatty acids, glucose, and other vital molecules.

Steps of the Krebs Cycle

The Krebs cycle consists of eight enzymatically catalyzed reaction steps that follow each other in a cyclic manner. The starting molecule is acetyl-CoA, which is derived mainly from the breakdown of carbohydrates (glycolysis), fatty acids (beta-oxidation), and amino acids.

Key Steps at a Glance

Step 1 – Condensation: Acetyl-CoA (2 carbon atoms) combines with oxaloacetate (4 carbon atoms) to form citrate (6 carbon atoms), catalyzed by the enzyme citrate synthase.
Steps 2–3 – Isomerization: Citrate is converted via aconitate to isocitrate.
Step 4 – First oxidation: Isocitrate is oxidized to alpha-ketoglutarate; CO₂ is released and NADH is formed.
Step 5 – Second oxidation: Alpha-ketoglutarate is converted to succinyl-CoA; CO₂ is again released and NADH is formed.
Step 6 – Substrate-level phosphorylation: Succinyl-CoA is converted to succinate; one molecule of energy-rich GTP (equivalent to ATP) is directly produced.
Step 7 – Dehydrogenation: Succinate is oxidized to fumarate; FADH₂ is produced.
Step 8 – Hydration: Fumarate is converted via malate back to oxaloacetate; NADH is produced and the cycle begins again.

Energy Yield

Per turn of the Krebs cycle (i.e., per acetyl-CoA molecule), the following are produced:

3 NADH molecules
1 FADH₂ molecule
1 GTP (equivalent to 1 ATP)
2 CO₂ molecules (exhaled)

The energy stored in NADH and FADH₂ is subsequently used in the electron transport chain (oxidative phosphorylation) to produce large amounts of ATP. In total, the complete aerobic breakdown of one glucose molecule via glycolysis, the Krebs cycle, and the electron transport chain yields approximately 30–32 ATP molecules.

Regulation of the Krebs Cycle

The Krebs cycle is tightly regulated to precisely meet the energy demands of the cell. Key regulatory points include:

Citrate synthase: inhibited by high levels of ATP and NADH
Isocitrate dehydrogenase: activated by ADP, inhibited by ATP and NADH
Alpha-ketoglutarate dehydrogenase: inhibited by succinyl-CoA and NADH

When the cell requires more energy (high ADP levels), the cycle speeds up. When sufficient energy is available (high ATP and NADH levels), it slows down.

Clinical Relevance

Disruptions of the Krebs cycle can have serious consequences for the organism. Relevant clinical associations include:

Mitochondrial diseases: Genetic defects in Krebs cycle enzymes can lead to severe metabolic disorders, primarily affecting organs with high energy demands such as the brain, heart, and skeletal muscle.
Cancer research: Tumor cells frequently show mutations in Krebs cycle enzymes (e.g., IDH1/IDH2 mutations in gliomas and leukemias), fundamentally altering cancer cell metabolism (the Warburg effect).
Diabetes mellitus: In the context of insulin deficiency or insulin resistance, the cycle can be impaired, contributing to altered energy metabolism and increased ketone body production.
Nutrient deficiencies: Vitamins such as thiamine (B1), riboflavin (B2), niacin (B3), and pantothenic acid (B5) are indispensable coenzymes for the Krebs cycle. Their deficiency can significantly impair cellular energy production.

References

Stryer L., Berg J.M., Tymoczko J.L.: Biochemistry. 9th edition, W.H. Freeman and Company, 2019.
Nelson D.L., Cox M.M.: Lehninger Principles of Biochemistry. 8th edition, W.H. Freeman and Company, 2021.
Krebs H.A., Johnson W.A.: The role of citric acid in intermediate metabolism in animal tissues. FEBS Letters, 1980; 117 Suppl: K1–K10. (Reprint of the 1937 original article)

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