The carbonic anhydrases (CAs) fall into three evolutionarily distinct families designated ␣-, ␤-, and ␥-CAs based on their primary structure. ␤-CAs are present in higher plants, algae, and prokaryotes, and are involved in inorganic carbon utilization. Here, we describe the novel x-ray structure of ␤-CA from the red alga, Porphyridium purpureum, at 2.2-Å resolution using intrinsic zinc multiwavelength anomalous diffraction phasing. The CA monomer is composed of two internally repeating

more »

... , being folded as a pair of fundamentally equivalent motifs of an ␣/␤ domain and three projecting ␣-helices. The motif is obviously distinct from that of either ␣or ␥-CAs. This homodimeric CA appears like a tetramer with a pseudo 222 symmetry. The active site zinc is coordinated by a Cys-Asp-His-Cys tetrad that is strictly conserved among the ␤-CAs. No water molecule is found in a zinc-liganding radius, indicating that the zinc-hydroxide mechanism in ␣-CAs, and possibly in ␥-CAs, is not directly applicable to the case in ␤-CAs. Zinc coordination environments of the CAs provide an interesting example of the convergent evolution of distinct catalytic sites required for the same CO 2 hydration reaction.