Thiocarboxylation of molybdopterin synthase provides evidence for the mechanism of dithiolene formation in metal-binding pterins

Molybdopterin (MPT) is a pyranopterin with a unique dithiolene group coordi nating molybdenum (Mo) or tungsten (W) in all Mo- and W-enzymes except nitr ogenase. In Escherichia coli, MPT is formed by incorporation of two sulfur atoms into precursor Z, which is catalyzed by MPT synthase. The recently so lved crystal structure of MPT synthase (Rudolph, M. J., Wuebbens, M. M., Ra jagopalan, K. V., and Schindelin, H. (2000) Nat. Struct. Biol. 8, 42-46) sh ows the heterotetrameric nature of the enzyme that is composed of two small (MoaD) and two large subunits (MoaE). According to sequence and structural similarities among MoaD, ubiquitin, and ThiS, a thiocarboxylation of the C terminus of MoaD is proposed that would serve as the source of sulfur that is transferred to precursor Z. Here, we describe the in vitro generation o f carboxylated and thiocarboxylated MoaD. Both forms of MoaD are monomeric and are able to form a heterotetrameric complex after coincubation in equim olar ratios with MoaE. Only the thiocarboxylated MPT synthase complex was f ound to be able to convert precursor Z in vitro to MPT. Slight but signific ant differences between the carboxylated and the thiocarboxylated MPT synth ase can be seen using size exclusion chromatography. A two-step reaction of MPT synthesis is proposed where the dithiolene is generated by two thiocar boxylates derived from a single tetrameric MPT synthase.