Thiamin Biosynthesis in Saccharomyces cerevisiae

Abstract

<p>The biosynthesis of thiamin is investigated by the administration of radioactively labelled compounds to yeast. Thiamin chloride hydrochloride is isolated from the yeast cells and degraded to determine the distribution of activity within its thiazole nucleus. Label from [2-¹⁴C]-glycine, in two strains of S. cerevisiae (ATCC 24903 and 39916 H.J. Bunker), is located exclusively at C-2 of the thiazole moiety. Non-random incorporation of activity into the C₅ unit of the thiazole nucleus from [1,3-¹⁴C]glycerol, Ḏ-[1-¹⁴C]fructose, Ḏ-[U-¹⁴C]-, Ḏ-[1-¹⁴C]-, Ḏ-[2-¹⁴C]-, Ḏ-[6-¹⁴C]-, and Ḏ-[6-³H,6-¹⁴C]glucose is observed is S. cerevisiae (ATCC 24903). A new hypothesis, consistent with the positions of the label in the above experiments, is proposed for the biosynthesis of the thiazole unit. As the first step in this scheme, glycine and a 2-ketopentose are suggested to form a Schiff base which converted to the thiazole moiety in a multistep sequence. The mode of incorporation of [1,3-¹⁴C]-glycerol and the labelled hexoses indicates that the 2-ketopentose is formed from glycolytic intermediates by two pathways. Ḏ-Ribulose 5-phosphate, Ḏ-xylulose 5-phosphate, or a closely related compound is the most likely choice for the 2-ketopentose. Radioactivity from sodium [¹⁴C]formate, [3-¹⁴C ]serine, [1,3-¹⁴C]glycerol and the above hexoses is incorporated into the pyrimidine nucleus of thiamin. A new scheme is proposed to account for these results. Radioactivity from Ḻ-[Me-¹⁴C]-methionine, ḎḺ-[2-¹⁴C]tyrosine, sodium [1-¹⁴C]acetate, sodium [3-¹⁴C]-pyruvate, sodium Ḻ-[U-¹⁴C]lactate, ḎḺ-[3-¹⁴C]cysteine, Ḏ-[1-¹⁴C]ribose, sodium 2-keto[5-¹⁴C]glutarate, and [1-¹⁴C]succinic acid is not incorporated into thiamin.</p>