Carbonic anhydrases (CA) are zinc metalloenzymes which catalyze the reversible hydration of carbon dioxide. In Escherichia coli, CA (gene cynT) is involved in recycling carbon dioxide formed in the bicarbonate-dependent decomposition of cyanate by cyanase (gene cynS). By this action, it prevents the depletion of cellular bicarbonate. In photosynthetic bacteria and plant chloroplast, CA is essential to inorganic carbon fixation. Prokaryotic and plant chloroplast CA are structurally and evolutionary related and form a family distinct from the one which groups the many different forms of eukaryotic CA's.