Snapshot Issue 36 August 2007
We order sparkling water for the fizz. The fizz comes from the bubbles of gas. But not any old gas: carbon dioxide. Other drinks bulge with other types of bubbles; bubbles of nitrogen, for instance, which also produce a sparkle but one that might go unnoticed. A little like a dampened firework. In sparkling water, or beverages of its kind, carbon dioxide is released and subsequently hydrated on your tongue, to produce carbonic acid. And it is believed that this is the reaction which gives off the familiar tingle:
H2O + CO2 <--- ---> HCO3- + H+
The hydration of carbon dioxide (CO2) to produce carbonic acid (HCO3-) and a proton (H+) – or indeed the opposite reaction – takes place whenever and wherever the correct chemical components are present. However, as is the case for many spontaneous chemical processes, when an enzyme can do the job, it can do it much faster. Carbonic anhydrase is at the heart of the reversible carbon dioxide hydration and carbon acid dehydration reactions and, as a result, can multiply the ping pong conversion as much as one million times!
This reversible swap of chemicals is known to many – bacteria, chicken and even the garden pea – and is found in numerous different kinds of tissue. In humans, thirteen isozymes are currently known, which are either cytosolic or membrane-bound, and one even turned out to be secreted in our saliva. Their role is to supply hydrogen, carbon dioxide and carbonic acid for their subsequent transport. Involved, as they are, in the supply of such important molecules, it is hardly surprising that carbonic anhydrases take part in a number of critical physiological processes such as respiration, pH homeostasis, gluconeogenesis and bone resorption for instance.
As a consequence, if – for one reason or another – carbonic anhydrase is dysfunctional, although the reaction occurs spontaneously, the rate at which it occurs would be insufficient. So we would either suffocate following carbon dioxide poisoning, for example, or suffer from conditions such as cerebral calcification, muscle rigidity or renal dysfunction. In some instances, we could even be affected by the lack of perception of sparkle; a feature which makes a beverage like champagne – say – what it really is. Although, granted, it is the least of discomforts.
- Carbonic anhydrase 1, Homo sapiens (Human): P00915
- Carbonic anhydrase 2, Homo sapiens (Human): P00918
- Carbonic anhydrase 3, Homo sapiens (Human): P07451
- Carbonic anhydrase 4, Homo sapiens (Human): P22748
- Carbonic anhydrase 5A, Homo sapiens (Human): P35218
- Carbonic anhydrase 5B, Homo sapiens (Human): Q9Y2D0
- Carbonic anhydrase 6, Homo sapiens (Human): P23280
- Carbonic anhydrase 7, Homo sapiens (Human): P43166
- Carbonic anhydrase 9, Homo sapiens (Human): Q16790
- Carbonic anhydrase 12, Homo sapiens (Human): O43570
- Carbonic anhydrase 13, Homo sapiens (Human): Q8N1Q1
- Carbonic anhydrase 14, Homo sapiens (Human): Q9ULX7
L'édition française de cette chronique est disponible dans l'Instantanés du mois de Prolune.
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