Carbonic anhydrase

By Jennifer McDowall

Carbon dioxide (CO₂) is a key metabolite in all living organisms. Carbon dioxide exists in equilibrium with bicarbonate (HCO₃^-), which is poorly soluble in lipid membranes compared to carbon dioxide; carbon dioxide can freely diffuse in and out of the cell, while bicarbonate must be transported. The conversion of bicarbonate to carbon dioxide facilitates its transport into the cell, while the conversion of carbon dioxide to bicarbonate helps trap the carbon dioxide in the cell. The interconversion of carbon dioxide and bicarbonate proceeds slowly at physiological pH, so organisms produce enzymes to speed up the process. Carbonic anhydrases are zinc-containing enzymes that catalyse the reversible reaction between carbon dioxide hydration and bicarbonate dehydration. Carbonic anhydrases have been found in all kingdoms of life. They have essential roles in facilitating the transport of carbon dioxide and protons in the intracellular space, across biological membranes and in the layers of the extracellular space; they are also involved in many other processes, from respiration and photosynthesis in eukaryotes to cyanate degradation in prokaryotes.

Mechanism of action

Carbonic anhydrase catalyses the following reaction:

H₂O + CO₂ ßà H⁺ + HCO₃^-

This reaction is ubiquitous in nature, involving the interchange of gaseous and ionic species crucial to a wide range of physiological and biochemical processes. The mechanism of action of the mammalian carbonic anhydrase has been studied in depth. The enzyme employs a two-step mechanism: in the first step, there is a nucleophilic attack of a zinc-bound hydroxide ion on carbon dioxide; in the second step, the active site is regenerated by the ionisation of the zinc-bound water molecule and the removal of a proton from the active site. The active site can exist in two forms: a high pH form that is active in the hydration of carbon dioxide and a low pH form that is active in the dehydration of bicarbonate.

A multitude of isozymes

Species can produce many different carbonic anhydrase isozymes, some of which act in the cytosol, while others are membrane-bound. For instance, in humans there are three cytosolic isozymes (I, II and III), five membrane-bound isozymes (IV, VII, IX, XII and XIV), a mitochondrial isozyme (V), and a secreted salivary isozyme (VI), as well as several related proteins that lack catalytic activity. Carbonic anhydrases are often arranged in clusters along membranes or localised in extracellular spaces, which may contribute to the ability of carbonic anhydrase to facilitate the intracellular diffusion of carbon dioxide and protons (H⁺). By increasing the movement of protons, carbonic anhydrase can dissipate intracellular pH gradients, thereby helping the cell to maintain a uniform cellular pH. The removal of protons is essential for several reactions within the cell, such as for the functioning of ATPases, which are inhibited by a build-up of protons; as a result, the inhibition of carbonic anhydrase could reduce muscle contractility and calcium handling. Carbonic anhydrases can also create localised gradients, which may aid in processes such as facilitated diffusion across a membrane.

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