The dimer-of-trimers assembly prevents catalysis at the transferase site of prokaryotic FAD Synthase

Flavin-mononucleotide (FMN) and flavin-adenine-dinucleotide (FAD) are essential flavoprotein cofactors. A riboflavin kinase (RFK) activity catalyzes riboflavin phosphorylation to FMN, which can then be transformed into FAD by an FMN:adenylyltransferase (FMNAT) activity. Two enzymes are responsible for each one of these activities in eukaryotes, while prokaryotes have a single bifunctional enzyme, FAD synthase (FADS). FADS folds in two independent modules: the C-terminal with RFK activity and the N-terminal with FMNAT activity.

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