Characterization of an Isoeugenol Monooxygenase (Iem) from Pseudomonas nitroreducens Jin1 That Transforms Isoeugenol to Vanillin

Abstract

The isoeugenol monooxygenase (iem) gene from Pseudomonas nitroreducens Jin1, responsible for the conversion of isoeugenol to vanillin, was cloned and overexpressed in Escherichia coli. The purified Tern had a predicted molecular mass of 54 kDa. The V-max, K-M, and k(cat) values for it, using isoeugenol as substrate, were 4.2 mu mol vanillin min(-1) mg(-1) of protein, 120 mu m, and 3.8 s(-1), respectively. Maximum substrate turnover for Iem occurred at 30 degrees C and pH 9.0. An (18)Oxygen-labeling experiment revealed that oxidative cleavage of isoeugenol by Tern was catalyzed via a monooxygenation reaction, and that incorporation of the oxygen atom from O-2 into vanillin was preferable to incorporation from water. While the catalytic activity of Iem, as prepared, did not require the addition of any organic or metal cofactor, ICP-MS analysis showed 0.7 mol of iron per mol of Iem. Moreover site-directed mutagenesis of Iem of four conserved histidine residues individually, His(167), His(218), His(282) and His(471), which appear to be involved in ligand bonding with Fe2+, resulted in a loss of activity. Enzyme activity was not appreciably influenced by preincubation of Tern with high concentrations of chelators, suggesting that iron is tightly bound. Iem has an iron-mediated mechanism that is widely spread among the three domains of life.