Crystallographic X-Ray Diffraction of ortho-Aminophenol Oxidase SmNspF

Streptomyces bacteria, which inhabit soil, water, and plant-associated environments, are well known for producing bioactive compounds such as antibiotics and pigments. Streptomyces murayamaensis produces the o-aminophenol oxidase SmNspF, an enzyme involved in the biosynthesis of 4-hydroxy-3-nitrobenzamide. o-Aminophenol oxidases, a subclass of type-III copper proteins, are capable of oxidizing o-aminophenols either to o-quinone imines (quinone-imine-forming activity) or to nitrosophenols (nitroso-forming activity). The predominant oxidation product depends on the substitution pattern at the para position of the aminophenol substrate. SmNspF oxidizes monophenols, o-aminophenols, and o-diphenols and preferentially accepts carboxylated substrates such as 3,4-dihydroxybenzoic acid and 3-amino-4-hydroxybenzoic acid. In addition, the enzyme exhibits activity toward 2-aminoresorcinol and o-phenylenediamine, highlighting the high versatility of its binuclear copper center. Determination of the first crystal structure of an o-aminophenol oxidase revealed unique loop insertions and key second-sphere amino acid residues in the vicinity of the binuclear copper center.