The β and γ classes of carbonic anhydrase

There are currently five (α,β,γ,δ,ζ) classes of carbonic anhydrases (CA's) of which the α-class from mammalian sources has been studied to a much greater extent compared to the other four classes. Yet, CA's other than the α-class are widely distributed in Nature and play impouarit roles in human health, the global carbon cycle, and industrial applications. In aerobic prokaryotes, β-class CA's are implicated in maintaining internal pH and CO₂/bicarbonate balances required for biosynthetic reactions. In anaerobic prokaryotes, β-class CA's are implicawd in the transport of CO₂ and bicarbonate across the cytoplasmic membrane that regulates pH and facilitates acquisition of substrates and product removal required for growth. In phototrophic organisms, β-class CA's are particularly important for transport and concentration of CO₂ and bicarbonate for photosynthesis. The δ- and ζ-classes are proposed to function in marine diatoms to concentrate CO₂ for photosynthesis. Physiological roles for he γ-class are not as well documented; however, the active site arhitecture and catalytic mechanism is well understood as are patterns of inhibition by sulfonamides and anions.