Crystal structure of E-coli β-carbonic anhydrase, an enzyme with an unusual pH-dependent activity

Carbonic anhydrases fall into three distinct evolutionary and structural classes: alpha, beta, and gamma. The beta -class carbonic anhydrases (beta -CAs) are widely distributed among higher plants, simple eukaryotes, eubacteria, and archaea. We have determined the crystal structure of ECCA, a beta -CA from Escherichia coli, to a resolution of 2.0 Angstrom. In agreement with the structure of the beta -CA from the chloroplast of the red alga Porphyridium purpureum, the active-site zinc in ECCA is tetrahedrally coordinated by the side chains of four conserved residues, These results confirm the observation of a unique pattern of zinc ligation in at least some beta -CAs. The absence of a water molecule in the inner coordination sphere is inconsistent with known mechanisms of CA activity, ECCA activity is highly pH-dependent in the physiological range, and its expression in yeast complements an oxygen-sensitive phenotype displayed by a beta -CA-deletion strain. The structural and biochemical characterizations of ECCA presented here and the comparisons with other beta -CA structures suggest that ECCA can adopt two distinct conformations displaying widely divergent catalytic rates.