Kinetic Model for the Reversible Hydration of Carbon Dioxide Catalyzed by Human Carbonic Anhydrase II

Four variants of the two-step Ping Pong mechanism for the reversible hydration of carbon dioxide to bicarbonate catalyzed by free human carbonic anhydrase II (HCA II) in solutions were derived and their goodness-of-fit to match measured initial hydration rates tested. The pseudo (i.e., no central complex) random Quad Quad Iso Ping Pong mechanism with one transitory complex was retained which implied a possible competitive intermolecular proton transfer step by the CO 2 /HCO 3 − pair with respect to external buffer. The model suggests this role could be emphasized in product inhibition conditions at high bicarbonate/buffer concentration ratios. A 4-parameter kinetic model was derived to complement the existing single- (intra- or intermolecular) limiting-step models for HCA II catalyzed hydration of CO 2 in which were accounted for altogether the enzyme isomerization and CO 2 /HCO 3 − proton transfer via a [CO 2 ]·[HCO 3 − ] coupling, the CO 2 /HCO 3 − proton transfer via [HCO 3 − ] 2 and [CO 2 ]·[HCO 3 − ] 2 couplings, and an enzyme−substrate transitory complex via [CO 2 ]·[HCO 3 − ]·[Buffer] coupling. This model may prove helpful for analysis of CO 2 capture reactor models subject to mixed (intra- or intermolecular) proton-transfer control, intermolecular proton transfer competition by the CO 2 /HCO 3 − pair, and large CO 2 conversions.