J. Biol. Chem., published online 27 November 2012; doi:10.1074/jbc.M112.424077

Indole acetic acid (IAA) is the archetypal auxin, a class of hormones that regulate numerous growth and developmental pathways in plants. A major biosynthetic source of IAA is tryptophan and its metabolite, indole-3-pyruvic acid (IPA), which is thought to be converted to IAA in an oxidative decarboxylation reaction catalyzed by a YUCCA enzyme (YUC). Although the 11 YUC genes in Arabidopsis thaliana are members of the flavin-containing monooxygenase family and have roles in localized auxin biosynthesis in various tissues, the molecular mechanisms for YUC-mediated transformations remain unresolved. Because this lack of insight was due, in part, to the poor availability of biochemical samples, Dai et al. first developed a route to purify active A. thaliana YUC6 enzyme after expression in Escherichia coli and concluded that YUC6 contains an oxidized FAD cofactor and is not an FMN-containing enzyme. Spectroscopic and stopped-flow methods showed that the FAD cofactor is reduced in the presence of NADPH and reacts with molecular oxygen to form a metastable C4a-(hydro)peroxyflavin intermediate. Further spectroscopic and HPLC data revealed that activated YUC6 converts IPA to IAA and regenerates oxidized FAD and can also use other ketoacids, such as phenylpyruvic acid, as substrates. Taken together, the studies establish that YUC6, and perhaps other YUC enzymes in plants, share mechanistic similarities to known Baeyer-Villiger monooxygenases.