Carbonic Anhydrase – Definition, Functions, and Mechanism

Carbonic Anhydrase

The inter-conversion that occurs between water, carbon dioxide, and dissociated carbonic acid ions, is catalyzed by a group of enzymes that form by carbonic anhydrase, or carbonate dehydratase. The active site for most carbonic anhydrase is supposed to be a zinc ion, and it is as a result classified as metalloenzymes.

Other than helping in the transport of carbon dioxide, the enzyme also maintains acid-base balance, aids in the regulation of pH, and helps maintain fluid balance.

However, the exact role of the enzyme depends primarily on its location. For instance, in the stomach lining, carbonic anhydrase produces acid. Whereas in the kidney, due to the control of the bicarbonate ions the content of water in the cell gets uneasy. The bicarbonate ions also influence water content in the eyes; however, if there is a build-up of fluid it can cause glaucoma.

 

Definition of Carbonic Anhydrase

These are zinc-containing metalloenzymes that act as catalysts in the dehydration of bicarbonates, as well as the hydration of carbon dioxide.

The reversible reaction for this process is as follows:

CO₂ + H₂O <—–> HCO₃^– + H⁺

In spite of being one of the fast-reacting enzymes, the rate of carbonic anhydrase is inhibited by the rate of diffusion of the substrates.

Families of Carbonic Anhydrase

There are five recognized distinct carbonic anhydrase families. However, they do not have any similarity in their amino acid sequence and we believe it to be an example of convergent evolution. Carbonic anhydrase is classified as follows:

• Alpha-CA or α-CA
• Beta-CA or β-CA
• Gamma-CA or γ-CA
• Delta-CA or δ-CA
• Zeta-CA or ζ-CA

Functions of Carbonic Anhydrase

Reactions driven by carbonic anhydrases are extremely crucial in several tissues.

1. In the parietal cells in the stomach, there is a huge amount of acid secretion in the form of hydrogen ions and protons, in the lumen, as well as bicarbonate ions into the blood.
2. The cells in the pancreatic duct perform the same functions, except their main secretory product is bicarbonate instead of hydrogen.
3. As carbonic anhydrase converts bicarbonates to hydrogen ions, it helps in exhalation in the lungs.
4. The enzyme is also present in the renal tubules where it produces hydrogen ions which are essential for maintaining acid-base and fluid balance in the body.
5. It produces carbon dioxide in all cells by metabolism. Also, the red blood cells remove it from the body. With the help of the enzyme, the red blood cells convert a majority of it to bicarbonate for transport. After that, it again exhales the carbon dioxide again in the lungs.
6. We also use CA inhibitors for therapeutic purposes. We use acetazolamide as a prototype for the drugs which we occasionally use as a diuretic. It is quite useful in treating several conditions and for therapy for a few types of glaucoma.

 Mechanism of Carbonic Anhydrase

we can locate the zinc ion in the enzyme in a cavity shaped like a cone, with three histidine side chains, and in the fourth position, there is a solvent molecule. However, a fourth histidine is located near the water ligand which helps in the formation of the Zn- OH centre, where carbon dioxide attaches itself resulting in the formation of zinc bicarbonate.

Solved Questions for You

Q.Where is carbonic anhydrase not present?

1. Red blood cells
2. Gastric parietal cells
3. Plasma
4. Renal tubular cells 

A. The correct option is ‘c’. We can found carbonic anhydrase in the gastric parietal cells, red blood cells, as well as renal tubular cells.