Biosynthesis of Thiamin

There are differences in the pathways of thiamin biosynthesis between prokaryotes and eukaryotes, and also between organisms that are aerobes and facultative anaerobes. Some organisms are completely autotrophic for thiamin, whereas others require the presence of either preformed pyrimidine or thiazole in the culture medium, and indeed some require both.

The pyrimidine moiety is not synthesized by the usual pathway for pyrimidine synthesis, but arises from amino-imidazole ribonucleotide, an intermediate in purine synthesis. Bacterial mutants with a single point mutation in the purine biosynthetic pathway show requirements for both thiamin and preformed purines. High concentrations of adenosine inhibit the synthesis of the hydroxymethylpyrimidine moiety of thiamin. The amino-imidazole ribonucleotide undergoes ring cleavage and the insertion of a two-carbon unit to yield the six-membered pyrimidine ring and a hydroxymethyl group, which becomes the methylene bridge of thiamin.

The thiazole ring is synthesized from a pentulose or deoxypentulose 5-phosphate and either glycine or tryptophan, depending on the organism. Incorporation of sulfur leads to formation of hydroxymethyl thiazole, which is then phosphorylated. The sulfur comes from cysteine and is incorporated by formation of a thiocarboxylate at the carboxyl terminal of the enzyme, unlike biotin synthesis (Section 11.1.1), where an iron-sulfur cluster at the active site of the enzyme is the donor.

The final stepin the synthesis of thiamin involves condensation between the hydroxymethylpyrimidine diphosphate andhydroxyethylthiazole monophosphate to yield thiamin monophosphate, which can then be dephosphorylated to free thiamin or phosphorylated further to the diphosphate and triphosphate (Hohmann and Meacock, 1998; Begley et al., 1999).

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