Essential amino acid residues in diphtheria toxin catalytic domain were determined by quantum chemistry calculation and the enzymatic activities of mutant in the active-site were tested in order to provide high effective cytotoxin for target anticancer drugs. According to the research on relationships between diphtheria toxin structure and function, associated with the results of quantum chemistry calculation, Tyr-149 in diphtheria toxin catalytic domain was replaced by Phe, and the activity of enzyme and capacity of binding substrate were tested. Tyr-149 is located in strong positive electricity centre and has the capacity to receive electrons, so it is an active amino acid residue. Compared with the wild diphtheria toxin, the enzymic activity of Phe-mutant was elevated, and its NAD binding capacity showed slight change. Y149 is an important amino acid residue lying in active centre of diphtheria toxin catalytic domain and substitution on it could influence the bioactivity of protein.

Key words: diphtheria toxin, quantum chemistry, enzymic kinetics