Eta- and teta-carbonic anhydrase

Seven genetically distinct families of carbonic anhydrases (CAs) have been identified so far: alpha-, beta-, gamma-, delta-, zeta- eta-, and teta-class, with the latter two being described only very recently. Interestingly, whereas alpha, beta, gamma, delta, and zeta classes are very different from each other both for the overall fold and coordination of the catalytic metal ion, the last two identified classes present significant homologies with the alpha- and beta-classes, respectively. In particular, alpha- and eta-CAs share the same overall fold but a different coordination of the catalytic metal ion, whereas beta- and teta-CAs retain both the same fold and the same metal coordination but have completely different primary structures and catalytic properties. For both eta- and teta-CAs, only one member of the family has been so far characterized in depth in vitro. Despite the limited information available on these new CA classes, great interest in them has recently emerged due to their important biomedical and biotechnological applications. In particular, eta-CAs, so far described only in Plasmodia, have been proposed as potential targets for the development of new antimalarial drugs, whereas teta-CAs, so far identified only in photosynthetic aquatic microeukaryotes, have been recognized as important players in the CO2-concentrating mechanism (CCM) of these organisms. Thus, a detailed knowledge on the role that these enzymes play in the CCM may provide indications for improving the photosynthetic yield via genetic engineering strategies. In this chapter, we will summarize the literature data so far available on eta- and teta-CAs with particular attention to their biochemical and structural features and to the proposed roles that these enzymes play in the organisms in which they have been identified.