title: X-ray structure of beta-carbonic anhydrase from the red alga, Porphyridium purpureum, reveals a novel catalytic site for CO(2) hydration.
abstract: The carbonic anhydrases (CAs) fall into three evolutionarily distinct families designated alpha-, beta-, and gamma-CAs based on their primary structure. beta-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 beta-CA from the red alga, Porphyridium purpureum, at 2.2-A 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 alpha/beta domain and three projecting alpha-helices. The motif is obviously distinct from that of either alpha- or gamma-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 beta-CAs. No water molecule is found in a zinc-liganding radius, indicating that the zinc-hydroxide mechanism in alpha-CAs, and possibly in gamma-CAs, is not directly applicable to the case in beta-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.
authors.affiliation: Marine Biotechnology Institute, Kamaishi Laboratories, Heita, Kamaishi, Iwate, 026-0001 Japan. mitsuhashi@kamaishi.mbio.co.jp
authors.forename: S
authors.initials: S
authors.lastname: Mitsuhashi
authors.forename: T
authors.initials: T
authors.lastname: Mizushima
authors.forename: E
authors.initials: E
authors.lastname: Yamashita
authors.forename: M
authors.initials: M
authors.lastname: Yamamoto
authors.forename: T
authors.initials: T
authors.lastname: Kumasaka
authors.forename: H
authors.initials: H
authors.lastname: Moriyama
authors.forename: T
authors.initials: T
authors.lastname: Ueki
authors.forename: S
authors.initials: S
authors.lastname: Miyachi
authors.forename: T
authors.initials: T
authors.lastname: Tsukihara
mesh_terms: D000595:Amino Acid Sequence; D002245:Carbon Dioxide / Q000378:metabolism; D002256:Carbonic Anhydrases / Q000737:chemistry*; D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D007527:Isoenzymes / Q000737:chemistry; D008956:Models, Chemical; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017510:Protein Folding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D000461:Rhodophyta / Q000201:enzymology*; D017386:Sequence Homology, Amino Acid; D015032:Zinc / Q000378:metabolism
chemical_list: D007527:Isoenzymes; D002245:Carbon Dioxide; D002256:Carbonic Anhydrases; D015032:Zinc