Rhodospirillum rubrum produces 5-methylthioadenosine (MTA) from S-adenosylmethionine in polyamine biosynthesis; however, R. rubrum lacks the classical methionine salvage pathway. Instead, MTA is converted to 5-methylthio-d-ribose 1-phosphate (MTR 1-P) and adenine; MTR 1-P is isomerized to 1-methylthio-d-xylulose 5-phosphate (MTXu 5-P) and reductively dethiomethylated to 1-deoxy-d-xylulose 5-phosphate (DXP), an intermediate in the nonmevalonate isoprenoid pathway [Erb, T. J., et al. (2012) Nat. Chem. Biol., in press]. Dethiomethylation, a novel route to DXP, is catalyzed by MTXu 5-P methylsulfurylase. An active site Cys displaces the enolate of DXP from MTXu 5-P, generating a methyl disulfide intermediate.
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