Molecule of the Month: Carbonic Anhydrase
Carbonic anhydrase solubilizes carbon dioxide gas so we can breathe it out
Breathing In, Breathing Out
Green Plants and Corals
Mammalian carbonic anhydrases occur in about 10 slightly different forms depending upon the tissue or cellular compartment they are located in. These isozymes have some sequence variations leading to specific differences in their activity. Thus isozymes found in some muscle fibers have low enzyme activity compared to that secreted by salivary glands. While most carbonic anhydrase isozymes are soluble and secreted, some are bound to the membranes of specific epithelial cells. For a deeper look at carbonic anhydrase from a genomic perspective, please visit the Protein of the Month feature at the European Bioinformatics Institute.
Carbonic Anhydrase in Health and Disease
When there is a build up of fluid that maintains the shape of our eyes, the fluid often presses on the optic nerve in the eye and may damage it. This condition is called glaucoma. In recent years, inhibitors of carbonic anhydrase are being used to treat glaucoma. Blocking this enzyme shifts the fluid balance in the eyes of the patient to reduce fluid build up thereby relieving pressure. The structure of PDB entry 1cnw shows how one such inhibitor (a sulfonamide), colored green in the figure, is bound to human carbonic anhydrase (isozyme II). Note that this inhibitor binds near the active site and disrupts the interactions of the water bound to the zinc ion, blocking the enzyme action. Unfortunately, prolonged use of this drug can affect the same enzyme present in other tissues and lead to side effects like kidney and liver damage.
Exploring the Structure
Carbonic Anhydrase in Action
The alpha carbonic anhydrase enzymes have been well studied, leading to an understanding of how they work. Three structures are shown here, showing three steps in the process. The structure at the top (PDB entry 5dsj) is the empty enzyme. The active site includes a zinc ion (magenta), which is held by three histidines (purple). The zinc also coordinates a hydroxide ion (red). At bottom left (PDB entry 5dsi), carbon dioxide (red and gray) is bound and a glutamate and a threonine (green) assist with the reaction. At bottom right (PDB entry 1cam), the hydroxide has been added, forming carbonic acid. The threonine has been mutated to alanine to help capture the complex. The histidine at upper left (green), which was seen in two different conformations in the structure, assists with guiding one of the nearby water molecules (orange) to the zinc ion, and converting it to a hydroxyl. To explore these three structures in more detail, click on the image for an interactive JSmol.
Topics for Further Discussion
- Carbonic anhydrase is an example of a "perfect" enzyme, which performs its reaction as fast as carbon dioxide molecules can diffuse to it. You can find other diffusion-limited enzymes in the PDB archive, including catalase, superoxide dismutase, and triosephosphate isomerase.
January 2004, Shuchismita Dutta, David Goodselldoi:10.2210/rcsb_pdb/mom_2004_1