Carbonic Anhydrase - Enzyme , Structure, Function, And FAQs

Carbonic anhydrase is defined as an enzyme. It is found in red blood cells, pancreatic cells, gastric mucosa, and the renal tubules that catalyze the interconversion of carbonic acid (H2CO3) and carbon dioxide (CO2).

Carbonic anhydrase plays an essential role in respiration by influencing CO2 transport in the blood. This enzyme also functions in the hydrochloric acid formation by the stomach.

Carbonic anhydrase enzyme helps to maintain acid-base homeostasis, fluid balance and regulate pH. Based on its location, the enzyme role changes slightly. For example, carbonic anhydrase forms acid in the stomach lining.

Structure and Function

Many forms of carbonic anhydrase take place in nature. The zinc ion can be coordinated by the imidazole rings of three histidine residues, His94, His96, and His119, in the best-studied, -carbonic anhydrase shape, which is present in animals.

The major enzyme function in animals is to interconvert the bicarbonate and carbon dioxide to maintain acid-base balance in the blood and other tissues and also to help transport carbon dioxide out of the tissues.

At least, there are 14 different isoforms in mammals. At the same time, plants have a different form, known as β-carbonic anhydrase, that from an evolutionary standpoint, which is a distinct enzyme. However, it participates in a similar reaction and also uses a zinc ion in its active site. Carbonic anhydrase in plants helps to increase the carboxylation rate of the RuBisCO enzyme by increasing the concentration of CO2 within the chloroplast. This reaction integrates CO2 into the organic carbon sugars during the photosynthesis process and can use only CO2 form of carbon, but not carbonic acid or bicarbonate.

Reaction

The following reaction illustrates the catalysis of carbonic anhydrase in our tissues:

CO2 + H2O → H2CO3 → H+ + HCO-3

Catalyzation of the carbonic anhydrase in the lungs is represented by:

H+ + HCO-3 → H2CO3 → CO2 + H2O

The reaction's reason being in the opposite directions for lungs and tissues is because of the variable pH levels found in them. Without the carbonic anhydrase catalyst, however, this reaction is much slower; with the catalyst, the reaction is 107 times faster.

The reaction, which is catalyzed by the carbonic anhydrase is given by:

HCO-3 + H+ ⇋ CO2 + H2O

Since carbonic acid has a pKa of up to 6.36 (depending on the medium), a lower percentage of the bicarbonate can be protonated at pH 7.

Carbonic anhydrase is the fastest enzyme, and its rate is normally limited by the rate at which its substrates diffuse. The typical catalytic rates of the various forms of this enzyme range from 104 -106 reactions per second.

The uncatalyzed reverse reaction is said to be relatively slow (in the range of 15-second for kinetics). This is the reason why a carbonated drink does not instantly degas while opening the container when it comes into contact with the carbonic anhydrase found in saliva, however, it quickly degasses in the mouth.

An anhydrase can be defined as an enzyme, which catalyzes the water molecule removal from a compound, and so, it is the "reverse" reaction, which gives the carbonic anhydrase its name because it removes water a molecule from the carbonic acid.

Carbonic anhydrase in the lungs transforms bicarbonate into carbon dioxide, which is ideal for exhalation.

Mechanism

A zinc prosthetic group, which exists in the enzyme, can be coordinated in 3 positions by histidine side-chains. The 4th coordination position is taken place by the water. A 4th histidine is close to the water ligand by facilitating the formation of the Zn-OH centre, which binds CO2 to produce zinc bicarbonate. An example of general acid is the construct, which is a general base catalysis. Also, the active site features a pocket suited for carbon dioxide by bringing it close to a hydroxide group.

It is an enzyme of carbonic anhydrase in red blood cells or carbonic anhydrase in RBC.

The active site of human carbonic anhydrase II is depicted in the diagram below, with three histidine residues and one hydroxyl group coordinating (dashed lines) the zinc ion in the middle. From PDB: 1CA2​.

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Cadmium-Containing Carbonic Anhydrase

Marine diatoms have been found to express the new form of ζ carbonic anhydrase. The T. weissflogii, which is a species of phytoplankton common to several marine ecosystems, was found to have carbonic anhydrase with a cadmium ion in place of zinc. In previous days, it had been believed that cadmium was one of the toxic metals with zero biological function whatsoever.

However, this phytoplankton species appears to have adapted to the low zinc levels in the ocean using cadmium when there is no presence of enough zinc. Since the concentration of cadmium in seawater is poor (up to 1x1016 molar), there is an environmental gain of being able to use any metal depending on how much is available at the time. Therefore, this type of carbonic anhydrase is cambialistic, which means that it can interchange the metal in its active site with the other metals, namely, cadmium and zinc.