Catalysis and Inhibition of Human Carbonic Anhydrase IV

Carbonic anhydrase IV (CA IV) is a membrane-bound form of carbonic anhydrase. We have characterized the catalytic activity and inhibition of recombinant human CA IV. CA IV is a high-activity isozyme in CO2 hydration with a pH-independent k cat value (1.1 × 106 s-1) comparable to that of CA II (8 × 105 s-1). Furthermore, CA IV is more active in HCO3 - dehydration than is CA II as illustrated by the nearly 3-fold increase in k cat/K M to 3 × 107 M-1 s-1. However, the esterase activity of CA IV is decreased 150-fold compared to CA II. The catalytic mechanisms of CA II and CA IV are nearly identical. Both isozymes show similar dependence on buffer concentration with the rate-limiting step at high buffer concentration being intramolecular proton transfer, although the intramolecular proton transfer for CA IV is 3 times faster than that observed with CA II. Additional positive charges in the active site of CA IV stabilize anions as indicated by a decreased pK a for the Zn-bound water compared to CA II (6.2 vs 6.9), as well as lower inhibition constants for a variety of anions, including halides, sulfate, formate, acetate, and bicarbonate. CA IV is also activated by low concentrations (