Carbonic anhydrases fall into three distinct evolutionary and structural cl
asses: alpha, beta, and gamma. The beta -class carbonic anhydrases (beta -C
As) are widely distributed among higher plants, simple eukaryotes, eubacter
ia, 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 wit
h the structure of the beta -CA from the chloroplast of the red alga Porphy
ridium purpureum, the active-site zinc in ECCA is tetrahedrally coordinated
by the side chains of four conserved residues, These results confirm the o
bservation 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 incons
istent with known mechanisms of CA activity, ECCA activity is highly pH-dep
endent 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.