Structure and function of a flavin-dependent S-monooxygenase from garlic (Allium sativum)
- Virginia Polytechnic Inst. and State Univ. (Virginia Tech), Blacksburg, VA (United States)
- Univ. of Missouri, Columbia, MO (United States)
- Cornell Univ., Ithaca, NY (United States)
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. In this paper, 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.
- Research Organization:
- Argonne National Laboratory (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
- Sponsoring Organization:
- National Institute of General Medical Sciences (NIGMS); National Institutes of Health (NIH); USDOE Office of Science (SC)
- Grant/Contract Number:
- P30 GM124165; AC02-06CH11357
- OSTI ID:
- 1647382
- Journal Information:
- Journal of Biological Chemistry, Vol. 295, Issue 32; ISSN 0021-9258
- Publisher:
- American Society for Biochemistry and Molecular BiologyCopyright Statement
- Country of Publication:
- United States
- Language:
- ENGLISH
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Related Subjects
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
flavin-dependent monooxygenases
sulfenic acid
S-oxygenation
allicin
class B monooxygenases
allyl mercaptan
hydroxylation
garlic (Allium sativum)
natural product biosynthesis
structure-function
flavin
FAD
natural product
plant biochemistry
oxidation-reduction (redox)
C4a-hydroperoxyflavin