Sour Cells Sense Carbonation

An enzyme on the surface of sour-sensing cells is the key to tasting the carbonation in your favorite fizzy drink, reports a group led by Charles S. Zuker of Columbia University (Science 2009, 326, 443). Mammals have multiple sensory systems that respond to the CO₂, but the molecular mechanisms of those systems are poorly understood. Zuker and colleagues blocked the function of selective classes of taste receptor cells to try to pin down the mechanism for tasting CO₂. They found that sour-sensing cells are also the ones that respond to CO₂. The group then used gene-expression profiling to narrow in on an extracellular carbonic anhydrase as the essential enzyme. Carbonic anhydrase converts CO₂ to HCO₃ ^– and H⁺. Further investigation revealed that it is likely the H⁺ in particular that signals CO₂ reception to taste cells. The authors suggest that the carbonic anhydrase may principally work to maintain the pH balance within taste buds, and its function as a carbonation detector may be merely “an accidental consequence.”