Crystal structure of the catalytic domain of the tumor-associated human carbonic anhydrase IX

Carbonic anhydrase (CA) IX is a plasma membrane-assocd. member of the .alpha.-CA enzyme family, which is involved in solid tumor acidification. It is a marker of tumor hypoxia and a prognostic factor in several human cancers. An aberrant increase in CA IX expression in chronic hypoxia and during development of various carcinomas contributes to tumorigenesis through at least two mechanisms: pH regulation and cell adhesion control. Here, we report the X-ray structure of the catalytic domain of CA IX in complex with a classical, clin. used sulfonamide inhibitor, acetazolamide. The structure reveals a typical .alpha.-CA fold, which significantly differs from the other CA isoenzymes when the protein quaternary structure is considered. Thus, two catalytic domains of CA IX assoc. to form a dimer, which is stabilized by the formation of an intermol. disulfide bond. The active site clefts and the PG domains are located on one face of the dimer, while the C-termini are located on the opposite face to facilitate protein anchoring to the cell membrane. A correlation between the three-dimensional structure and the physiol. role of the enzyme is suggested, based on the measurement of the pH profile of the catalytic activity for the physiol. reaction, CO2 hydration to bicarbonate and protons. On the basis of the structural differences obsd. between CA IX and the other membrane-assocd. .alpha.-CAs, further prospects for the rational drug design of isoenzyme- specific CA inhibitors are proposed, given that inhibition of this enzyme shows antitumor activity both in vitro and in vivo.