Structural study and catalytic activities of CafD, minor carbonic anhydrase of Aspergillus fumigatus

Abstract

Carbonic Anhydrases(CAs) are the zinc-metalloenzymes. CAs catalyze the interconversion reaction between carbon dioxide and bicarbonate. CAs distinct to a variety of subfamily to α-, β-, γ-, δ-, ζ-, η-, and θ. The zinc ion of β CAs was coordinated with two cysteines and one histidine residue. β-CAs of Aspergillus fumigatus have major and minor isoform. CafD is minor β-CA of Aspergillus fumigatus which have low catalytic activity. Here, I have determined the crystal structure of CafD to explain the reason for low activities. The overall crystal structure is homodimer and ‘closed’ form. The zinc-bound water molecule at active site was replaced by aspartic acid(D36). The arginine residue which was highly conserved the other β-CAs was replaced to glycine(G38) in CafD. The aspartic acid was unable to form D-R pair that makes the space for a zinc-bound water molecule. Hence, CafD active site cannot switches to active form. Furthermore, the dimer interface was observed ‘non-catalytic’ zinc ion which coordinates with two cysteine (C39, C39’) residues of each monomer and a water molecule that interacts two glutamic acids (E54, E54’) of each monomer. This zinc ion may stabilize the dimerization of CafD.