Crystal structure of the dimeric extracellular domain of human carbonic anhydrase XII, a bitopic membrane protein overexpressed in certain cancer tumor cells

Overexpression of the zinc enzyme carbonic anhydrase (CA: EC 4.2.1.1) XII i s observed in certain human cancers. This bitopic membrane protein contains an N-terminal extracellular catalytic domain, a membrane-spanning a-helix, and a small intracellular C-terminal domain. We have determined the three- dimensional structure of the extracellular catalytic domain of human CA XII by x-ray crystallographic methods at 1.55-Angstrom resolution. The structu re reveals a prototypical CA fold; however, two CA XII domains associate to form an isologous dimer, an observation that is confirmed by studies of th e enzyme in solution. The identification of signature GXXXG and GXXXS motif s in the transmembrane sequence that facilitate helix-helix association is additionally consistent with dimeric architecture. The dimer interface is s ituated so that the active site clefts of each monomer are clearly exposed on one face of the dimer, and the C-termini are located together on the opp osite face of the dimer to facilitate membrane interaction. The amino acid composition of the active-site cleft closely resembles that of the other CA isozymes in the immediate vicinity of the catalytic zinc ion, but differs in the region of the nearby a-helical "130's segment." The structure of the CA Xii-acetazolamide complex is also reported at 1.50-Angstrom resolution, and prospects for the design of CA XII-specific inhibitors of possible che motherapeutic value are discussed.