Tham khảo tài liệu 'biomimetics learning from nature part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 178 Biomimetics Learning from Nature apparently completely different model systems one with an azamacrocyclic and the other with a pyrazolyl type ligand is not a problem. Apparently the deprotonation energies of the zinc-bound water are very similar. Thus we assume that our experiment using the TpZnSH complex is suited to corroborate and illustrate the calculated mechanism. Although a true catalysis is not observed the substrate COS can be transformed into CO2 and H2S in the presence of TpPh MeZnOH just by altering the pH of the solution. According to our calculations the protonation free energies of the zinc-bound hydroxide and hydrosulfide differ by ca. 84 kJ mol. This is in good agreement with our experimental observation that a fast desulfuration occurs only at pH values at which a zinc-bound water is not deprotonated. Nevertheless a well-balanced pH at the active site of natural CA could allow both a predominantly deprotonated zinc-bound water ligand and small but sufficient protonation of the zinc-bound hydrosulfide. As we already pointed out above we hold the view that a small amount of protonated hydrosulfide ligand at the zinc ion is sufficient for complete desulfuration of CA due to the fact that the dissociation of H2S is practically irreversible. In our opinion the calculated mechanism is thus very likely to occur the way it is depicted in Figure 5. The role of other amino acid residues in the catalytic mechanism has been addressed in studies by Bottoni Bottoni et al. 2004 and Liedl Tautermann et al. 2003 . They have demonstrated that some of the residues especially Glu106 and Thr199 are directly involved in some steps of the CO2 fixation. It has also been commented upon that a histidine residue in the enzyme cavity near the active site so-called proton shuttle influences the pKa of the zinc-bound water. The residue which is located in a distance of approx. 7 À from the zinc centre can be present in both protonated or deprotonated state. For both cases
