Download MendeleyStructure and function of a flavin-dependent S-monooxygenase from garlic (Allium sativum).
Valentino, H., Campbell, A.C., Schuermann, J.P., Sultana, N., Nam, H.G., LeBlanc, S., Tanner, J.J., Sobrado, P.(2020) J Biol Chem 295: 11042-11055
- PubMed: 32527723
- DOI: https://doi.org/10.1074/jbc.RA120.014484
- Primary Citation of Related Structures:
6WPU - PubMed Abstract:
Allicin is a component of the characteristic smell and flavor of garlic ( Allium sativum ). A flavin-containing monooxygenase (FMO) produced by A. sativum (AsFMO) was previously proposed to oxidize S -allyl-l-cysteine (SAC) to alliin, an allicin precursor. Here, we present a kinetic and structural characterization of AsFMO that suggests a possible contradiction to this proposal. Results of steady-state kinetic analyses revealed that AsFMO exhibited negligible activity with SAC; however, the enzyme was highly active with l-cysteine, N -acetyl-l-cysteine, and allyl mercaptan. We found that allyl mercaptan with NADPH was the preferred substrate-cofactor combination. Rapid-reaction kinetic analyses showed that NADPH binds tightly ( K D of ∼2 μm) to AsFMO and that the hydride transfer occurs with pro- R stereospecificity. We detected the formation of a long-wavelength band when AsFMO was reduced by NADPH, probably representing the formation of a charge-transfer complex. In the absence of substrate, the reduced enzyme, in complex with NADP + , reacted with oxygen and formed an intermediate with a spectrum characteristic of C4a-hydroperoxyflavin, which decays several orders of magnitude more slowly than the k cat The presence of substrate enhanced C4a-hydroperoxyflavin formation and, upon hydroxylation, oxidation occurred with a rate constant similar to the k cat The structure of AsFMO complexed with FAD at 2.08-Å resolution features two domains for binding of FAD and NADPH, representative of class B flavin monooxygenases. These biochemical and structural results are consistent with AsFMO being an S-monooxygenase involved in allicin biosynthesis through direct formation of sulfenic acid and not SAC oxidation.
Organizational Affiliation:
Department of Biochemistry, Virginia Tech, Blacksburg, Virginia, USA.
