A LEFT-HANDED BETA-HELIX REVEALED BY THE CRYSTAL-STRUCTURE OF A CARBONIC-ANHYDRASE FROM THE ARCHAEON METHANOSARCINA-THERMOPHILA

A carbonic anhydrase from the thermophilic archaeon Methanosarcina the rmophila that exhibits no significant sequence similarity to known car bonic anhydrases has recently been characterized. Here we present the structure of this enzyme, which adopts a left-handed parallel beta-hel ix fold. This fold is of particular interest since it contains only le ft-handed crossover connections between the parallel beta-strands, whi ch so far have been observed very infrequently. The active form of the enzyme is a trimer with three zinc-containing active sites, each loca ted at the interface between two monomers. While the arrangement of ac tive site groups differs between this enzyme and the carbonic anhydras es from higher vertebrates, there are structural similarities in the z inc coordination environment, suggestive of convergent evolution dicta ted by the chemical requirements for catalysis of the same reaction. B ased on sequence similarities, the structure of this enzyme is the pro totype of a new class of carbonic anhydrases with representatives in a ll three phylogenetic domains of life.