Interaction between Polyoxin and Active Center of Chitin Synthetase

Abstract

Various derivatives of polyoxin C, other polyoxins and several uridine analogues have been known as competitive inhibitors of chitin-UDP acetylglucosaminyitransferase (EC 2.4.1.16, chitin synthetase). Their inhibitory activities were more or less dependent on pH. The variation of inhibitor constants Ki or Michaelis-Menten parameters Km and V with pH was investigated and the data obtained were plotted according to the method proposed by Dixon et al. From the results of the pKi-pH plots for the above competitive inhibitors, it was concluded that the ionized amino group at C−2″ position acted a very important role for the binding of polyoxins to chitin synthetase. The carbonyl oxygen atoms at C−1″ and of the carbamoyloxy group probably participated in the hydrogen bond formation with the enzyme. And pH scarcely influenced on the interaction between the carboxyl group at C−5″ and the enzyme. The results of the Dixon plots for variations of Km and V with pH suggested that an unionized imidazole group (pK_a = 6.3) and an ionized amino group (pK_a = 7.7) of chitin synthetase were concerned in the enzyme reaction.