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There are currently five (alpha, beta, gamma, delta, zeta) classes of carbonic anhydrases (CA's) of which the alpha-class from mammalian sources has been studied to a much greater extent compared to the other four classes. Yet, CA's other than the alpha-class are widely distributed in Nature and play important roles in human health, the global carbon cycle, and industrial applications. In aerobic prokaryotes, beta-class CA's are implicated in maintaining internal pH and CO(2)/bicarbonate balances required for biosynthetic reactions. In anaerobic prokaryotes, beta-class CA's are implicated in the transport of CO(2) and bicarbonate across the cytoplasmic membrane that regulates pH and facilitates acquisition of substrates and product removal required for growth. In phototrophic organisms, beta-class CA's are particularly important for transport and concentration of CO(2) and bicarbonate for photosynthesis. The delta- and zeta-classes are proposed to function in marine diatoms to concentrate CO(2) for photosynthesis. Physiological roles for the gamma-class are not as well documented; however, the active site architecture and catalytic mechanism is well understood as are patterns of inhibition by sulfonamides and anions.
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