title: Molecular basis of the oxygen exchange from CO2 catalyzed by carbonic anhydrase III from bovine skeletal muscle.
abstract: The exchange of 18O from CO2 to H2O in aqueous solution is caused by the hydration-dehydration cycle and is catalyzed by the carbonic anhydrases. In our previous studies of 18O exchange at chemical equilibrium catalyzed by isozymes I and II of carbonic anhydrase, we observed simple first-order depletion of 18O from CO2 with the 18O distribution among the species C18O18O, C16O18O, and C16O16O described by the binomial expansion (i.e., a random distribution of 18O). Using membrane-inlet mass spectrometry, we have measured 18O exchange between CO2 and H2O catalyzed by native zinc-containing and cobalt(II)-substituted carbonic anhydrase III from bovine skeletal muscle near pH 7.5. The distributions of 18O in CO2 deviate from the binomial expansion and are accompanied by biphasic 18O-exchange patterns; moreover, we observed regions in which 18O loss from CO2 was faster than 18O loss from HCO3-. These data are interpreted in terms of a model that includes 18O loss from an enzyme-substrate or intermediate complex. We conclude that more than one 18O can be lost from CO2 per encounter with the active site of isozyme III, a process that requires scrambling of oxygens in a bicarbonate-enzyme complex and cycling between intermediate complexes. This suggests that the rate of dissociation of H2(18)O (or 18OH-) from isozyme III is comparable to or faster than substrate and product dissociation.
mesh_terms: D000818:Animals; D002245:Carbon Dioxide / Q000378:metabolism*; D002256:Carbonic Anhydrases / Q000378:metabolism*; D002417:Cattle; D007553:Isotope Labeling / Q000379:methods; D007700:Kinetics; D013058:Mass Spectrometry; D009132:Muscles / Q000201:enzymology*; D010103:Oxygen Isotopes
chemical_list: D010103:Oxygen Isotopes; D002245:Carbon Dioxide; D002256:Carbonic Anhydrases