Menaquinone
Biosynthesis: Formation of Aminofutalosine
Requires a Unique Radical SAM Enzyme
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Abstract
Menaquinone
(MK, vitamin K<sub>2</sub>) is a lipid-soluble molecule
that participates in the bacterial electron transport chain. In mammalian
cells, MK functions as an essential vitamin for the activation of
various proteins involved in blood clotting and bone metabolism. Recently,
a new pathway for the biosynthesis of this cofactor was discovered
in <i>Streptomyces coelicolor</i> A3(2) in which chorismate
is converted to aminofutalosine in a reaction catalyzed by MqnA and
an unidentified enzyme. Here, we reconstitute the biosynthesis of
aminofutalosine and demonstrate that the missing enzyme (aminofutalosine
synthase, MqnE) is a radical SAM enzyme that catalyzes the addition
of the adenosyl radical to the double bond of 3-[(1-carboxyvinyl)oxy]benzoic
acid. This is a new reaction type in the radical SAM superfamily