ENZYMES OF THIAMIN METABOLISM

Cornell University, Ithaca NY

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Abstract

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Thiamin pyrophosphate, the active cofactor form of vitamin B1 found within the cell, is an essential vitamin for humans. Although humans cannot biosynthesize this compound, most plants and bacteria are capable of producing thiamin. The pathways for thiamin biosynthesis vary between bacteria, which utilize a pathway with slight variations, and plants and fungi, which rely upon a different pathway. It is also possible for thiamin and degraded forms of thiamin to be salvaged. In fungal and plant systems, THI1 and THI4 respectively, are responsible for the formation of the thiazole precursor adenosine diphospho-5-([unreadable]-ethyl)-4-methylthiazole-2-carboxylic acid. THI6, a bifunctional enzyme, catalyzes the phosphorylation of the thiazole moiety and couples the product of this reaction to the pyrimidine moiety to yield thiamin monophosphate. In yeast, THI20 is a second bifunctional enzyme that both hydrolyzes thiamin and phosphorylates the pyrimidine moiety. In bacteria, the thiazole and pyrimidine moieties are formed separately and then joined together relying on many enzymes. ThiE, thiamin phosphate synthase, is responsible for this coupling reaction. 4-Amino-5-hydroxymethyl-2-methylpyrimidine phosphate (HMP-P) synthase, or ThiC, is an iron-sulfur cluster protein responsible for the dramatic rearrangement of 5-aminoimidazole ribonucleotide (AIR) to form HMP-P. An important step in the biosynthesis of thiamin is the incorporation of an atom of sulfur into the growing thiazole moiety. In thiazole biosynthesis in bacteria, this is achieved by the activation of a sulfur carrier protein, ThiS, and the formation of a thioester at the carboxy terminus of ThiS in cooperation with a second protein, ThiF. Once the thioester has been formed on ThiS, the sulfur atom is then incorporated into the thiazole moiety via ThiG and ThiO/ThiH. Recently, a new sulfate assimilation pathway from Wolinella succinogenes has been identified, containing four enzymes which have primary structure similarity to the sulfur transfer enzymes of thiamin biosynthesis. These enzymes in W. succinogenes include a ThiS-like protein, a ThiF-like protein, a QBSD-like protein, and a putative O-acetylhomoserine sulfydrylase (OAHS), which might be involved in the transfer of sulfide to the activated WsThiS-like protein.