The pK(a) of the zinc-bound water in carbonic anhydrase and its model compounds as studied by the AM1 calculation coupled with a reaction field theory

The AM1 calculation coupled with a reaction field theory was applied in order to investigate electronic properties of the zinc-bound water in carbonic anhydrase (CA). Here, several kinds of zinc complexes having macrocyclic polyamine ligands were selected as models of the active site of CA. First, the deprotonation energy of the zinc-bound waters in vacuo was calculated to provide insight into the intrinsic properties of these complexes. The calculated data indicated that the deprotonation energy of the zinc-bound water becomes lower as the valence shell of the zinc-bound nitrogen atoms changes from the sp(2) to sp(3) hybrid states. Subsequently, the dielectric constant dependence of the deprotonation energy was investigated by using the reaction field theory. As a whole, the calculation successfully reproduced the relative pK(a) shift observed for the model complexes. One of the important findings is that the pK, of the zinc-bound water increases with an increase in dielectric constant. On the basis of these results, it is concluded that the pK(a) of the zinc-bound water in CA is kept at a low value (7.5) if the dielectric constant of the surrounding protein matrix is ca. 4.