A 13C nuclear-magnetic-resonance study of CO2-HCO3-exchange catalyzed by human carbonic anhydrase C at chemical equilibrium.

The effects of human carbonic anhydrase C on the 13C nuclear magnetic resonance spectra of equilibrium mixtures of 13CO2 and NaH13CO3 were measured at 67.89 MHz. Enzyme-catalyzed CO2-HCO-3 exchange rates were estimated from the linewidths of the resonances. The results show that: (a) the maximal exchange rates are larger than the maximal turnover rates; (b) the exchange is equally rapid with 1H2O or with 2H2O as solvents; (c) the exchange is equally rapid in the presence or in the absence of added buffers; (d) the apparent substrate binding is weaker than predicted if steady-state Km values are assumed to represent substrate dissociation constants. The main conclusion concerning the catalytic mechanism of the enzyme is that the proton-transfer processes which limit turnover rates in the steady state are not directly involved in CO2-HCO-3 exchange. In addition, the results suggest that CO2-HCO-3 interconversion takes place by a nucleophilic mechanism, such as a reversible reaction of zinc-coordinated OH- with CO2.