The Lycopene Cyclase CrtY from Pantoea ananatis (Formerly Erwinia uredovora) Catalyzes an FAD_red-dependent Non-redox Reaction

Abstract

The cyclization of lycopene generates provitamin A carotenoids such as β-carotene and paves the way toward the formation ofcyclic xanthophylls playing distinct roles in photosynthesis and as precursors for regulatory molecules in plants and animals.The biochemistry of lycopene cyclization has been enigmatic, as the previously proposed acid-base catalysis conflicted withthe possibility of redox catalysis as predicted by the presence of a dinucleotide binding site. We show that reduced FAD isthe essential lycopene cyclase (CrtY) cofactor. Using flavin analogs, mass spectrometry, and mutagenesis, evidence was obtainedbased on which a catalytic mechanism relying on cryptic (net) electron transfer can be refuted. The role of reduced FAD isproposed to reside in the stabilization of a transition state carrying a (partial) positive charge or of a positively chargedintermediate via a charge transfer interaction, acid-base catalysis serving as the underlying catalytic principle. Lycopenecyclase, thus, ranks among the novel class of non-redox flavoproteins, such as isopentenyl diphosphate:dimethylallyl diphosphateisomerase type 2 (IDI-2) that requires the reduced form of the cofactor.