X-ray structure of β-carbonic anhydrase from the red alga, Porphyridium purpureum, reveals a novel catalytic site for CO₂ hydration

The carbonic anhydrases (CAs) fall into three evolutionarily distinct families designated α-, β-, and γ-CAs based on their primary structure. β-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 β-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 structures, being folded as a pair of fundamentally equivalent motifs of an α/β domain and three projecting α-helices. The motif is obviously distinct from that of either α- or γ-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 β-CAs. No water molecule is found in a zinc-liganding radius, indicating that the zinc-hydroxide mechanism in α- CAs, and possibly in γ-CAs, is not directly applicable to the case in β- CAs. Zinc coordination environments of the CAs provide an interesting example of the convergent evolution of distinct catalytic sites required for the same CO₂ hydration reaction.