Carbonic Anhydrase

Carbonic anhydrase is a family of enzymes that speed up the interconversion of carbon dioxide and water with bicarbonate and a proton. It is one of the fastest known enzymes, with a single molecule able to convert up to a million CO₂ molecules a second.

At the center of the active site is a zinc ion held by three histidine residues. The zinc binds a water molecule and lowers its acidity so that it easily gives up a proton, leaving a hydroxide that attacks CO₂ to form bicarbonate. A nearby histidine shuttles protons to the surroundings so the cycle can repeat. It is an example of the speed-up described in catalysis. The enzyme was discovered in red blood cells in 1933 by Norman Meldrum and Francis Roughton.

In red blood cells, it converts CO₂ from tissues into bicarbonate for transport and reverses the reaction in the lungs, as part of carbonic acid chemistry. It helps to regulate pH, to make stomach acid and cerebrospinal fluid, and to form kidney urine. In corals and mollusks it supplies carbonate for shell building. Humans have about a dozen forms.

In photosynthetic organisms, the enzyme helps to concentrate CO₂ around Rubisco, in carboxysomes and in C4 plants. Inhibitors such as acetazolamide are used to treat glaucoma, to prevent altitude sickness and as diuretics. The enzyme evolved independently several times, with three unrelated families found across life, and it is a standard example in the study of enzymes.

• Carbonic Acid — the chemistry it catalyzes
• Carboxysome — a structure that houses it in bacteria
• Enzymes — the class of molecules it belongs to