This paper follows up on the work of Erb et al. and describes a biosynthetic pathway for the generation of 1‑deoxy‑D‑xylulose 5‑phosphate in Rhodospirillum rubrum via a novel methylsulfurylase. This work demonstrates the how initial elucidation of a given function often seques into discovery of linked pathways and thereby facilitates a more holistic understanding of the metabolic universe.
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.
Figure 3. 1H NMR spectra of the cupin substrates and products. (A) Mixture of MTRu 1-P and MTXu 1-P in H2O. (B) After reaction with cupin in H2O in the presence of excess DTT (spectrum obtained with solvent suppression). (C) Authentic DXP in H2O. (D) Mixture of MTRu 1-P and MTXu 1-P in D2O. (E) After reaction with cupin in D2O. The resonances are color-coded according to the structures in Figure 1.
Figure 5. Localization of the methanethiol adduct to Cys 121. (A) Doubly charged tryptic peptide corresponding to residues 112–128 with a methanethiol disulfide modification. (B) Tandem mass spectrum of the peptide ion in panel A. The complementary fragment ion pair b10+ and y12+ localize the methanethiol modification to Cys 121.
Reprinted with permission from the Journal of Biological Chemistry. © 2012 by the American Chemical Society