A wheel invented three times

The need for an enzyme to catalyze the slow conversion between carbon dioxide and bicarbonate enabled physiologists to predict the existence of carbonic anhydrase almost 70 years ago. Shortly thereafter the enzyme was purified from erythrocytes (Meldrum and Roughton, 1933; Stadie and O'Brien, 1933). The enzyme, now called α‐carbonic anhydrase (α‐CA), was found to contain a zinc ion (Keilin and Mann, 1940).A large number of α‐CA isoenzymes, primarily from mammalian species, have subsequently been identified and characterized, and several of their molecular structures have been elucidated, starting with that of human α‐CAII (Liljas et al ., 1972). When carbonic anhydrase from plants and certain bacteria was characterized and sequenced, it turned out to be oligomeric and its amino acid sequence had no similarity to the previously studied enzymes (Hewett‐Emmett and Tashian, 1996). This form is called β‐carbonic anhydrase (β‐CA). Subsequently, a carbonic anhydrase from archaea was identified (γ‐carbonic anhydrase, γ‐CA). In this case, the amino acid sequence was again strikingly different (Alber and Ferry, 1994). In spite of the differences in sequence, however, all forms of carbonic …