Purification and kinetic analysis of recombinant CA XII, a membrane carbonic anhydrase overexpressed in certain cancers

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The National Academy of Sciences

RESUMO

Carbonic anhydrase XII (CA XII) is a transmembrane glycoprotein with an active extracellular CA domain that is overexpressed on cell surfaces of certain cancers. Its expression has been linked to tumor invasiveness. To characterize its catalytic properties, we purified recombinant secretory forms of wild-type and mutant CA XIIs. The catalytic properties of these enzymes in the hydration of CO2 were measured at steady state by stopped-flow spectrophotometry and at chemical equilibrium by the exchange of 18O between CO2 and water determined by mass spectrometry. The catalysis of CO2 hydration by soluble CA XII has a maximal value of kcat/Km at 34 μM−1⋅s−1, which is similar to those of the membrane-associated CA IV and to soluble CA I. The pH profiles of this catalysis and the catalyzed hydrolysis of 4-nitrophenylacetate indicate that the pKa of the zinc-bound water in CA XII is 7.1. His64 in CA XII acts as a proton shuttle residue, as evidenced by the reduced rate constant for proton transfer in the mutants containing the replacements His64 → Ala and His64 → Arg, as well as by the selective inhibition of the proton transfer step by cupric ions in wild-type CA XII. The catalytic rate of CO2 hydration by the soluble form of CA XII is identical with that of the membrane-bound enzyme. These observations suggest a role for CA XII in CO2/HCO3− homeostasis in cells in which it is normally expressed. They are also compatible with a role for CA XII in acidifying the microenvironment of cancer cells in which CA XII is overexpressed, providing a mechanism for CA XII to augment tumor invasiveness and suggesting CA XII as a potential target for chemotherapeutic agents.

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