Role of the amino acid residues in the catalysis of catechol 2,3-dioxygenase from Pseudomonas putida SU10 as probed by chemical modification and random mutagenesis

The catechol 2,3-dioxygenase (C23O) encoded by the Pseudomonas putida xylE gene was over-produced in Escherichia coli and purified to homogeneity. The activity of the C23O required the reduced form of the Fe(II) ion since the enzyme was highly susceptible to inactivation with hydrogen peroxide but reactivated with the addition of ferrous sulfate in conjunction with ascorbic acid. The C23O activity was abolished by treatment with the chemical reagents, diethylpyrocarbonate (DEPC), tetranitromethane (TNM), and 1-cyclohexyl-3-(2-morpholinoethyl) carbodiimidemetho-rho-toluenesulfonate (CMC), which are modifying reagents of histidine, tyrosine and glutamic acid, respectively. These results suggest that histidine, tyrosine and glutamic acid residues may be good active sites for the enzyme activity. These amino acid residues are conserved residues among several extradiol dioxygenases and have the chemical potential to serve as ligands for Fe(II) coordination. Analysis of random point mutants in the C23O gene derived by PCR technique revealed that the mutated positions of two mutants, T179S and S211R, were located near the conserved His165 and His217 residues, respectively. This finding indicates that these two positions, along with the conserved histidine residues, are specially effective regions for the enzyme function.