Hydrogen-exchange study of the conformational stability of human carbonic-anhydrase B and its metallocomplexes.

In the range of pH 4.6--8.8, 25 degrees C, the apoenzyme of carbonic anhydrase B shows no evidence of any gross conformational changes, as studied by the hydrogen-deuterium exchange method. At pH 4.6 the addition of Co(II), Cd(II) or Mn(II) to the apoenzyme results in a destabilization of the native protein conformation, but in the range of pH 5.5--8.8 these metal ions, and Zn(II), slightly increase the conformational stability of the protein, in so far as they reduce the probability phi of solvent exposure of the peptide groups. In comparison with other proteins studied, native carbonic anhydrase is characterized by a rather compact conformation; for half of the peptide groups the probability of solvent exposure is less than 10(-4), corresponding to changes in standard free energy larger than 5.5 kcal mol-1 (23 kJ mol-1) following the conformational transitions by which these groups are exposed to the solvent.