The Catalytic Properties of Human Carbonic Anhydrase IX

Abstract

Human carbonic anhydrase IX (CA IX) is an integral membrane protein and a member of the α class of carbonic anhydrases that includes the human and animal enzymes. We have prepared a truncated, recombinant form of human CA IX of 255 residues consistent with full-length human CA II, among the most efficient of the carbonic anhydrases. Catalysis by and inhibition of this form of human CA IX has been investigated using stopped-flow spectrophotometry and 18O exchange measured by mass spectrometry. In kinetic constants for the hydration of CO2, CA IX closely resembled CA II with maximal proton transfer-dependent 18O exchange near 1 μs−1 and kcat/Km near 55 μM−1 s−1. Human CA IX was very strongly inhibited by three classic sulfonamides and cyanate, with inhibition constants that are close to those for CA II.