The γ class of carbonic anhydrases

Homologs of the gamma class of carbonic anhydrases, one of five independently evolved classes, are found in the genomic sequences of diverse species from all three domains of life. The archetype (Cam) from the Archaea domain is a homotrimer of which the crystal structure reveals monomers with a distinctive left-handed parallel beta-helix fold. Histidines from adjacent monomers ligate the three active site metals surrounded by residues in a hydrogen bond network essential for activity. Cam is most active with iron, the physiologically relevant metal. Although the active site residues bear little resemblance to the other classes, kinetic analyses indicate a two-step mechanism analogous to all carbonic anhydrases investigated. Phylogenetic analyses of Cam homologs derived from the databases show that Cam is representative of a minor subclass with the great majority belonging to a subclass (CamH) with significant differences in active site residues and apparent mechanism from Cam. A physiological function for any of the Cam and CamH homologs is unknown, although roles in transport of carbon dioxide and bicarbonate across membranes has been proposed. (C) 2009 Elsevier B.V. All rights reserved.