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Carbon Dioxide Hydration Activity of Carbonic Anhydrase: Paradoxical Consequences of the Unusually Rapid Catalysis

Raja G. Khalifah
Proceedings of the National Academy of Sciences of the United States of America
Vol. 70, No. 7 (Jul., 1973), pp. 1986-1989
Stable URL: http://www.jstor.org/stable/62578
Page Count: 4
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Carbon Dioxide Hydration Activity of Carbonic Anhydrase: Paradoxical Consequences of the Unusually Rapid Catalysis
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Abstract

The kinetic parameters for carbon dioxide hydration catalysis by carbonic anhydrase (EC 4.2.1.1) present an apparent paradox. The assumption of H2CO3 as the hydration product requires the rate of recombination of H2CO3 with enzyme to be faster than the diffusion limit. The alternative assumption of HCO3 - as the product of hydration likewise requires active-site ionization rates to exceed the diffusion limit. We previously postulated the presence of special means for rapid active-site ionization. It is shown here that when proton transfer between enzyme and buffer species is taken into account, there is no need to invoke rates exceeding the diffusion limit. Bicarbonate ion thus appears as the most probable hydration product and dehydration substrate.

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