Carbonic Anhydrase

 

Carbonic Anhydrase (CA)

Carbonic anhydrase is a zinc-containing enzyme that catalyzes the reversible conversion of carbon dioxide (CO₂) and water (H₂O) into carbonic acid (H₂CO₃), which then dissociates into bicarbonate (HCO₃⁻) and a proton (H⁺). This reaction is crucial for maintaining acid-base balance and gas exchange in living organisms.

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Reaction Catalyzed

$$\text{CO}_2 + \text{H}_2\text{O} \leftrightarrow \text{H}_2\text{CO}_3 \leftrightarrow \text{HCO}_3^- + \text{H}^+$$

This reaction occurs spontaneously but is dramatically accelerated (by over a million times) by carbonic anhydrase.

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Types of Carbonic Anhydrase

Carbonic anhydrase is found in different organisms, categorized into several isozymes based on structure and function:

1. α-Carbonic Anhydrases

• Found in mammals, birds, and some bacteria.
• Zinc-containing metalloenzymes.
• Examples:
  • CA I & CA II – Found in red blood cells, involved in respiration.
  • CA IV – Present in the kidneys, lungs, and eyes.
  • CA IX & CA XII – Involved in cancer cell metabolism.

2. β-Carbonic Anhydrases

• Found in bacteria, algae, and plants.
• Important for photosynthesis and CO₂ fixation.

3. γ-Carbonic Anhydrases

• Found in archaea and some bacteria.
• Evolutionarily distinct but functionally similar.

4. δ- & ζ-Carbonic Anhydrases

• Found in marine organisms such as diatoms.
• Help in carbon dioxide utilization in aquatic environments.

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Functions of Carbonic Anhydrase

1. Respiration & CO₂ Transport

   • Facilitates CO₂ transport in blood and lungs.
   • Converts CO₂ to bicarbonate for efficient excretion.

2. pH Regulation & Acid-Base Homeostasis

   • Helps maintain blood pH.
   • Involved in the buffering system of the body.

3. Urine Formation & Kidney Function

   • Regulates acid-base balance in renal tubules.

4. Eye Physiology

   • Controls aqueous humor production in the ciliary body of the eye.

5. Gastric Acid Secretion

   • Aids in hydrochloric acid (HCl) secretion in the stomach.

6. Bone Resorption

   • Plays a role in osteoclast activity (bone breakdown).

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Medical and Pharmaceutical Importance

• Carbonic Anhydrase Inhibitors (CAIs) are used to treat:
  • Glaucoma (reduces eye pressure, e.g., acetazolamide).
  • Altitude Sickness (helps prevent acidosis).
  • Epilepsy (used as adjunctive therapy).
  • Diuretics (increase urine production).
  • Cancer therapy (CA IX is a target in tumor treatment).

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Conclusion

Carbonic anhydrase is an essential enzyme for CO₂ transport, acid-base balance, and various physiological functions. Its inhibitors are widely used in medicine, particularly in treating glaucoma, epilepsy, and altitude sickness.

Would you like details on specific inhibitors or clinical applications? 😊