A "radical dance" in thiamin biosynthesis: mechanistic analysis of the bacterial hydroxymethylpyrimidine phosphate synthase.

Thiamin pyrophosphate is an important cofactor in all forms of life, where it plays a central role in the stabilization of the acyl-carbanion biosynthon1, 2. Its biosynthesis involves separate synthesis of the thiazole and the pyrimidine hererocycles, which are then linked to form the cofactor. Thiamin-thiazole biosynthesis is relatively well-understood3-7. In prokaryotes, 1-deoxy-D-xylulose-5-phosphate, cysteine and glycine or tyrosine are utilized by five proteins to construct the thiazole moiety, whereas in Saccharomyces cerevisiae, just one gene product converts NAD and glycine to thiazole, obtaining sulfur from a source yet unknown. In comparison, the mechanistic understanding of thiamin-pyrimidine (HMP) biosynthesis, in both prokaryotes and eukaryotes, is still at an early stage. In yeast, a single gene product THI5p is implicated in HMP biosynthesis from PLP and histidine, however this reaction has not yet been successfully reconstituted in vitro. In bacteria and plants HMP-P synthase (ThiC) catalyzes the conversion of aminoimidazole ribonucleotide (AIR 1), an intermediate in the purine nucleotide biosynthesis pathway, to hydroxylmethyl pyrimidine phosphate (HMP-P 2)8. In vivo and in vitro studies on the reaction catalyzed by ThiC, using labeled AIR, have revealed the involvement of a rearrangement reaction of remarkable complexity (Figure 1A)9. The ThiC catalyzed reaction has recently been reconstituted in a defined biochemical system. Spectroscopic, structural and biochemical studies established this enzyme as a unique member of the [4Fe-4S] cluster dependent radical-SAM superfamily10-11.