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Citric Acid Cycle – Function, Steps and Importance

The citric acid cycle is a central metabolic pathway in the cell that releases energy from nutrients. It takes place in the mitochondria and is essential for cellular energy production.

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Things worth knowing about "Citric Acid Cycle"

The citric acid cycle is a central metabolic pathway in the cell that releases energy from nutrients. It takes place in the mitochondria and is essential for cellular energy production.

What Is the Citric Acid Cycle?

The citric acid cycle – also known as the tricarboxylic acid cycle (TCA cycle) or Krebs cycle (named after biochemist Hans Krebs, who discovered it in 1937) – is a biochemical metabolic pathway that takes place in the mitochondria of body cells. It forms the core of aerobic energy metabolism and is indispensable for the energy supply of all cells in the body.

The citric acid cycle is a series of eight chemical reactions in which acetyl-CoA – a breakdown product of carbohydrates, fats, and proteins – is completely oxidized to carbon dioxide (CO₂). In the process, energy-rich electron carriers (NADH and FADH₂) are generated, which are then used in the electron transport chain to produce ATP (adenosine triphosphate), the universal energy currency of the cell.

How the Citric Acid Cycle Works

The cycle begins when acetyl-CoA (a two-carbon molecule) condenses with oxaloacetate (a four-carbon molecule) to form citrate (a six-carbon molecule) – hence the name citric acid cycle. In the subsequent steps, citrate is gradually transformed, releasing carbon dioxide and generating energy carriers, until oxaloacetate is regenerated and the cycle can begin again.

Overview of the Eight Reaction Steps

Step 1: Condensation of acetyl-CoA and oxaloacetate to form citrate (by citrate synthase)
Step 2: Conversion of citrate to isocitrate (by aconitase)
Step 3: Oxidative decarboxylation to alpha-ketoglutarate + CO₂ + NADH
Step 4: Oxidative decarboxylation to succinyl-CoA + CO₂ + NADH
Step 5: Formation of succinate + GTP (energy gain)
Step 6: Oxidation of succinate to fumarate + FADH₂
Step 7: Hydration of fumarate to malate
Step 8: Oxidation of malate to oxaloacetate + NADH (regeneration of the starting molecule)

Each turn of the citric acid cycle yields: 3 NADH, 1 FADH₂, 1 GTP, and 2 CO₂. Since one glucose molecule provides two acetyl-CoA units, the cycle runs twice per glucose molecule.

Importance of the Citric Acid Cycle

The citric acid cycle fulfills several critical functions in the human body:

Energy production: The electron carriers NADH and FADH₂ generated in the cycle are used in the electron transport chain to synthesize ATP, the main energy carrier of the cell.
Provision of biosynthetic precursors: Intermediates of the citric acid cycle serve as starting materials for the synthesis of amino acids, fatty acids, porphyrins (components of hemoglobin), and other important biomolecules.
Integration of macronutrient metabolism: Carbohydrates, fats, and proteins are all fed into the citric acid cycle via acetyl-CoA or other intermediates, making it the central hub of energy metabolism.

Regulation of the Citric Acid Cycle

The citric acid cycle is regulated by various mechanisms to meet the energy demands of the cell:

Substrate availability: The availability of acetyl-CoA and oxaloacetate directly influences cycle activity.
Allosteric regulation: High concentrations of ATP, NADH, and succinyl-CoA inhibit certain cycle enzymes (e.g., isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase), while ADP and NAD⁺ act as activators.
Calcium ions: In muscle cells, calcium ions (Ca²⁺) stimulate several enzymes of the citric acid cycle, thereby coupling muscle activity with energy metabolism.

Clinical Relevance

Disruptions in the citric acid cycle can cause serious diseases. Clinically relevant associations include:

Mitochondrial diseases: Defects in mitochondrial enzymes of the citric acid cycle can lead to severe metabolic disorders, frequently affecting muscles and the nervous system.
Cancer research: Mutations in enzymes such as isocitrate dehydrogenase (IDH) are found in certain tumors (e.g., gliomas, acute myeloid leukemia) and are considered therapeutic targets.
Diabetes mellitus: In the presence of insufficient insulin action, increased acetyl-CoA is derived from fatty acids, which can alter citric acid cycle activity and lead to the formation of ketone bodies.
Vitamin deficiency: Several B vitamins (e.g., thiamine/B1, riboflavin/B2, niacin/B3, pantothenic acid/B5) are essential coenzymes for cycle enzymes. A deficiency can impair energy production.

References

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

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