X-ray Structure of β-Carbonic Anhydrase from the Red Alga,Porphyridium purpureum, Reveals a Novel Catalytic Site for CO2Hydration

Satoshi Mitsuhashi, Tsunehiro Mizushima, Eiki Yamashita, Masaki Yamamoto, Takashi Kumasaka, Hideaki Moriyama, Tatzuo Ueki, Shigetoh Miyachi, Tomitake Tsukihara
2000 Journal of Biological Chemistry  
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.
doi:10.1074/jbc.275.8.5521 pmid:10681531 fatcat:t5n5pqr2njcvfbnwew2vampnk4