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To study the biological roles of Zn, we investigated simple model systems of Zn++, coordinated with OH2 or NH3, or with O=C- in peptide. The geometrical structures and net atomic charges were calculated by the ab initio HF-SCF theory using double zeta basis sets. The ligands of O-H, N-H, and O=C- are very polar due to Zn++. Therefore, the carbon atom in peptide becomes so electrophilic that it can be easily attacked by other nucleophiles. In addition, to understand how Zn++ is coordinated with ligands in enzyme, a molecular mechanics method is applied to the system of the enzyme of carboxypeptidase A (CPA) with the substrate of glycyltyrosine. From our results, it appears that the Zn ion is coordinated not only by four ligands in enzyme and substrate but also by one water molecule.
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