Carboxylate Binding Modes in Zinc Proteins: a Theoretical Study

The relative energies of different coordination modes (bidentate, monodentate, syn, and anti) of a carboxylate group bound to a zinc ion have been studied by the density functional method B3LYP with large basis sets on realistic models of the active site of several zinc proteins. In positively charged four-coordinate complexes, the mono- and bidentate coordination modes have almost the same energy (within 10 kJ/mol). However, if there are negatively charged ligands other than the carboxylate group, the monodentate binding mode is favored. In general, the energy difference between monodentate and bidentate coordination is small, 4-24 kJ/mol, and it is determined more by hydrogen-bond interactions with other ligands or second-sphere groups than by the zinc-carboxylate interaction. Similarly, the activation energy for the conversion between the two coordination modes is small, approximately 6 kJ/mol, indicating a very flat Zn-O potential surface. The energy difference between syn and anti binding modes of the monodentate carboxylate group is larger, 70-100 kJ/mol, but this figure again strongly depends on interactions with second-sphere molecules. Our results also indicate that the pK(a) of the zinc-bound water ligand in carboxypeptidase and thermolysin is 8-9.