A LEFT-HANDED [[beta]]-HELIX REVEALED BY THE CRYSTAL STRUCTURE OF A CARBONIC ANHYDRASE FROM AN ARCHAEON. C. Kisker, H. Schindelin, D. C. Rees, Division of Chemistry and Chemical Engineering, 147-75CH, California Institute of Technology, Pasadena, CA 91125, USA

Carbonic anhydrases are Zn²⁺ containing enzymes catalyzing the reversible hydration of CO₂. These metalloenzymes are among the fastest enzymes described so far having turnover numbers of up to 10⁶ s^-1. Until recently, two classes of carbonic anhydrases have been recognized: the "eukaryotic" class including seven isozymes from various higher vertebrates and two isozymes from Chlamydomonas reinhardtii and the "prokaryotic" class represented by chloroplast carbonic anhydrases and two bacterial enzymes. A carbonic anhydrase (CAM) that exhibits no significant sequence similarity to known carbonic anhydrases has been characterized from the thermophilic archaeon Methanosarcina thermophila. The crystal structure of this enzyme [1] revealed that it mainly consists of a left-handed parallel [[beta]]-helix, a domain motif that has been observed for the first time in this enzyme and in the recently determined structure of UDP-N-acetylglucosamine acyltransferase [2]. The [[beta]]-helix in CAM consists of seven complete turns and each turn contains three short [[beta]]-strands. As a consequence, the [[beta]]-helix contains three parallel [[beta]]-sheets that are essentially flat. This fold is of particular interest since it contains only left-handed crossover connections between the parallel [[beta]]-strands, which have so far been very infrequently observed. The active form of the enzyme is a trimer with three zinc-containing active sites, each located at the interface between two monomers. While the arrangement of active site groups differs between this enzyme and the carbonic anhydrases from higher vertebrates, there are structural similarities in the zinc coordination environment, suggestive of convergent evolution dictated by the chemical requirements for catalysis of the same reaction. Based on sequence similarities, the structure of this enzyme is the prototype of a new class of carbonic anhydrases with representatives in all three phylogenetic domains of life.

[1] C. Kisker, H. Schindelin, B. E. Alber, J. G. Ferry and D. C. Rees, EMBO J., in press.

[2] C. R. H. Raetz and S. L. Roderick, Science 270, 997-1000 (1995).