Kinetics, anion binding and mechanism of Co(II)-substituted bovine muscle carbonic anhydrase

The binding of N3- to Co(II)-substituted bovine carbonic anhydrase III was measured at various pH values by spectrophotometric titrations. The apparent Ki values were found to increase with pH in the studied range between pH 5.8 and 8.9. The inhibition of CO2 hydration by N-3 was found to be essentially uncompetitive at all investigated pH values (pH 6.3-8.9). The Ki values for the inhibition of kcat are much smaller than those obtained in the spectrophotometric titrations indicating that an enzyme form with a high affinity for N-3, presumably having a metal-bound H2O, accumulates in the steady state at saturating CO2 concentrations. Assuming that the low pH limit of Ki = 9 microM for the inhibition of kcat represents the affinity of N-3 for the Co(II)-OH2 form, a pKa value near 5 can be estimated for Co(II)-bound water from the pH dependence of N-3 binding in the absence of CO2. Measurements of time-resolved absorption spectra during CO2 hydration in the presence of a low N-3 concentration showed the transient appearance of the characteristic spectrum of the enzyme-N-3 adduct clearly demonstrating the accumulation in the steady state of an enzyme form with a high affinity for N-3. In similar experiments without inhibitor the transient formation of a spectral form corresponding to a Co(II)-OH2 species has been demonstrated. This spectral form is rather featureless lacking the absorption maxima at 618 nm and 640 nm characteristic of the Co(II)-OH- species. Our results strongly support the hypothesis that the rate-limiting step in CO2 hydration catalyzed by carbonic anhydrase III is the protolysis of metal-bound water.

A comparison of the kinetic properties of native bovine muscle carbonic anhydrase and an activated derivative with modified thiol groups

Steady-state and equilibrium kinetic properties of native bovine carbonic anhydrase III (carbonate hydrolyase, EC 4.2.1.1) and a derivative modified with methyl methanethiosulfonate were investigated. The modified enzyme has a markedly increased CO2 hydration activity compared to the native form with a 3-times higher value of kcat and a 6-10-times higher value of kcat/Km. Qualitatively, the activated enzyme shows the same kinetic behavior as native isoenzyme III. This is reflected in similar pH dependences of the kinetic parameters for CO2 hydration, similar solvent hydrogen isotope effects on these parameters, similar deviations from Michaelis-Menten kinetics for the HCO3- dehydration reaction, and similar behavior of the kinetics of CO2/HCO3- exchange at chemical equilibrium as measured by a 13C-NMR magnetization transfer technique. It is concluded that the conversion of -SH groups to -S-S-CH3 moieties does not change the catalytic mechanism, but leads to an increased rate of CO2/HCO3- interconversion as well as to an increased rate of proton transfer between the active site and the reaction medium.