Table 1.

Characteristic features of basic six classes of carbonic anhydrases.

Characteristics	α CA	β CA	γ CA	δ CA	ζ CA	η CA
Occurrence	Predominantly in animal kingdom also found in protozoa, algae, green plants, and in some archaea, bacteria and fungi	Reported in all three domains of life	Eubacteria and Archaea Methanosarcina thermophila	Thalassiosira weissfogii	Thalassiosira weissfogii (marine diatom)	Plasmodium falciparum
Molecular weight	monomeric (Mw: 26-37 kDa)	dimer, can form various olgomeric structures 45 to 200 kDa	homotrimer 60 kDa (native)	monomer27 kDa	69 kDa	-
Active site amino acids	His – 94, His – 96, His – 119	Cys 38, His 96 and Cys 99	His 81, His 122 and His 117	-	-	His – 94, His – 96, His – 118
Function	pH homeostasis, secretion of ${\text{HCO}}_3^{-}$, ion exchange	cyanate degradation (E.coli), CO2 fixation (cyanobacteria), Overexpressed during pathogenesis of some pathogens (Salmonella)	acetate metabolism and conversion of acetate to methane (methanogens)	-	-	Overexpressed during pathogenesis
Other features	esterase activity, N – terminal signal peptide for periplasmic localisation or extracellular secretion	Basic component of Carbon Concentrating Mechanism (CCM)	Oldest form of CA	-	cambialistic