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

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Abstract
Carbonic anhydrases fall into three distinct evolutionary and structural classes: α, β, and γ. The β-class carbonic anhydrases (β-CAs) are widely distributed among higher plants, simple eukaryotes, eubacteria, and archaea. We have determined the crystal structure of ECCA, a β-CA from Escherichia coli, to a resolution of 2.0 Å. In agreement with the structure of the β-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 β-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 β-CA-deletion strain. The structural and biochemical characterizations of ECCA presented here and the comparisons with other β-CA structures suggest that ECCA can adopt two distinct conformations displaying widely divergent catalytic rates.