Oxidation of Phenolate Siderophores by the Multicopper Oxidase Encoded by theEscherichia coli yacKGene

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Abstract
A gene (yacK) encoding a putative multicopper oxidase (MCO) was cloned fromEscherichia coli, and the expressed enzyme was demonstrated to exhibit phenoloxidase and ferroxidase activities. The purified protein contained six copper atoms per polypeptide chain and displayed optical and electron paramagnetic resonance (EPR) spectra consistent with the presence of type 1, type 2, and type 3 copper centers. The strong opticalA610(Ε610= 10,890 M−1cm−1) and copper stoichiometry were taken as evidence that, similar to ceruloplasmin, the enzyme likely contains multiple type 1 copper centers. The addition of copper led to immediate and reversible changes in the optical and EPR spectra of the protein, as well as decreased thermal stability of the enzyme. Copper addition also stimulated both the phenoloxidase and ferroxidase activities of the enzyme, but the other metals tested had no effect. In the presence of added copper, the enzyme displayed significant activity against two of the phenolate siderophores utilized byE. colifor iron uptake, 2,3-dihydroxybenzoate and enterobactin, as well as 3-hydroxyanthranilate, an iron siderophore utilized bySaccharomyces cerevisiae. Oxidation of enterobactin produced a colored precipitate suggestive of the polymerization reactions that characterize microbial melanization processes. As oxidation should render the phenolate siderophores incapable of binding iron,yacKMCO activity could influence levels of free iron in the periplasm in response to copper concentration. This mechanism may explain, in part, howyacKMCO moderates the sensitivity ofE. colito copper.