Carbonic Anhydrases (β‐Class)

Abstract

Carbonic anhydrase (CA) catalyzes the reversible hydration of CO 2 to HCO 3 − in aqueous solution. CAs are divided into three distinct classes (α‐CAs, β‐CAs, and γ‐CAs) that have little sequence homology to one another. β‐CA plays an essential role in photosynthesis, by concentrating CO 2 in the proximity of ribulose bisphosphate carboxylase/oxygenase for CO 2 fixation. Little is known about the physiological roles performed by β‐CA in nonphotosynthetic organisms. Two homologous motifs are repeated in the Porphyridium purpureum β‐CA polypeptide (55 kDa). Each motif is composed of two components: an α/β domain and an N‐terminal arm. The two motifs in a monomer are related by a pseudo twofold axis. The two monomers form a dimer with pseudo 222 symmetry. Escherichia coli β‐CA and Pisum sativum β‐CA are composed of a tetramer and an octamer respectively. The dimer of P. purpureum β‐CA resembles a tetramer of E. coli β‐CA. A dimer of P. sativum β‐CA, which resembles the monomer of P. purpureum β‐CA, is the basic building block of the octamer. Both β‐CAs from Methanobacterium thermoautotrophicum and E. coli have one zinc atom per monomer, while β‐CA from P. purpureum contains two zinc atoms per monomer. The catalytic zinc atom at each catalytic site is coordinated by four ligands in a tetrahedral manner. A possible catalytic mechanism of β‐CA based on the structure of P. purpureum CA is proposed.