JBC, Vol. 254, Issue 8, 2588-2591, Apr, 1979
Role of hemoglobin in proton transfer to the active site of carbonic anhydrase
D. N. Silverman, L. Backman and C. Tu
The binding of bovine oxyhemoglobin to bovine carbonic anhydrase with a
dissociation constant between 10(-5) and 10(-7) M has been determined by
countercurrent distribution using aqueous, biphasic polymer systems. This
result provides an explanation for the very efficient proton transfer
between hemoglobin and carbonic anhydrase, a transfer which enhances the
catalytic activity of carbonic anhydrase as measured by 18O exchange
between bicarbonate and water at chemical equilibrium (Silverman, D. N.,
Tu, C. K., and Wynns, G. C. (1978) J. Biol. Chem, 253, 2563-2567). Two rate
constants describing 18O exchange activity of carbonic anhydrase at pH 7.5
show saturation behavior when plotted against hemoglobin concentration
consistent with a dissociation constant of 2.5 X 10(-6) M between bovine
hemoglobin and carbonic anhydrase. Interpretation of these rate constants
in terms of a two-step model for 18O exchange indicates that hemoglobin
enhances the rate of exchange from carbonic anhydrase of water containing
the oxygen abstracted from bicarbonate, but does not affect the catalytic
interconversion of CO2 and HCO3- at chemical equilibrium.