MOLECULAR-DYNAMICS SIMULATIONS OF HUMAN CARBONIC-ANHYDRASE II - INSIGHT INTO EXPERIMENTAL RESULTS AND THE ROLE OF SOLVATION

In this paper, the carbonic anhydrase II (CA II) enzyme active site is modeled using ab initio calculations and molecular dynamics simulatio ns to examine a number of important issues for the enzyme function. It is found that the Zn2+ ion is dominantly tetrahedrally coordinated, w hich agrees with X-ray crystallographic studies. However, a transient five-fold coordination with an extra water molecule is also found. Stu dies of His64 conformations upon a change in the protonation states of the Zn-bound water and the His64 residue also confirm the results of an X-ray study which suggest that the His64 conformation is quite flex ible. However, the degree of water solvation is found to affect this b ehavior. Water bridge formation between the Zn-bound water and the His 64 residue was found to involve a free energy barrier of 2-3 kcal/mol and an average lifetime of several picoseconds, which supports the con cept of a proton transfer mechanism through such a bridge. Mutations o f various residues around the active site provide further insight into the corresponding experimental results and, in fact, suggest an impor tant role for the solvent water molecules in the CA II catalytic mecha nism. (C) 1998 Wiley Liss, Inc.