ORF7 from Amycolatopsis orientalis catalyzes decarboxylation of Nδ-methylarginine and amine oxidation of arginine: Biosynthetic implications

•ORF7 is a flavin monooxygenase involved in ECO-0501 biosynthesis.•ORF7 catalyzes the physiological oxidative decarboxylation of Nδ-methylarginine.•Arginine exclusively and efficiently undergoes ORF7-catalyzed amine oxidation.

A key step in the biosynthesis of the polyene polyketide ECO-0501 by Amycolatopsis orientalis ATCC 43491 is thought to involve oxidative decarboxylation of arginine or Nδ-methylarginine to the corresponding primary amide. This reaction is the centerpiece of a recently identified biosynthetic cassette that generates 4-guanidinobutyryl thioesters to serve as starter units for polyketide synthesis. We examined the reaction of ORF7, the predicted ECO-0501 biosynthetic decarboxylase, with arginine, and saw no evidence of decarboxylation. Instead, we observed exclusive amine oxidation to generate 2-oxoarginine, with a kcat/KM,Arg of 5.6 × 106 M− 1 s− 1, typical of values measured for physiological amino acid decarboxylases. In contrast, when ORF7 was incubated with Nδ-methylarginine, we observed exclusive decarboxylation to generate 4-(N1-methylguanidino)butyramide. These differing reactive pathways provide insight into the biosyntheses of guanidinobutyryl-derived polyketides and demonstrate the biosynthetic versatility of arginine-processing decarboxylases. In addition, it suggests that ORF7 may be an incisive model system for dissecting the determinants of flavoprotein-catalyzed oxidase and monooxygenase modes of reactivity.