Thiamine hydroxyethylthiamine

There are two 2-oxoacid dehydrogenase multienzyme complexes in E. coli. One is specific for pyruvate, the other for 2-oxoglutarate. Each complex is about the size of a ribosome, about 300 A across. The pyruvate dehydrogenase is composed of three types of polypeptide chains El, the pyruvate decarboxylase (an a2 dimer of Mr — 2 X 100 000) E2, lipoate acetyltransferase (Mr = 80 000) and E3, lipoamide dehydrogenase (an a2 dimer of Mr = 2 X 56 000). These catalyze the oxidative decarboxylation of pyruvate via reactions 1.6, 1.7, and 1.8. (The relevant chemistry of the reactions of thiamine pyrophosphate [TPP], hydroxyethylthiamine pyrophosphate [HETPPJ, and lipoic acid [lip-S2] is discussed in detail in Chapter 2, section C3.)... 

The hydroxyethylthiamine pyrophosphates are potent nucleophiles and may add to carbonyl compounds to form carbon-carbon bonds. A good illustration of carbon-carbon bond making and breaking occurs in the reactions of transketo-lase. The enzyme contains tightly bound thiamine pyrophosphate and shuttles a dihydroxyethyl group between D-xylulose 5-phosphate and D-ribose 5-phosphate to form D-sedoheptulose 7-phosphate and D-glyceraldehyde 3-phosphate (equations 2.55 and 2.56). 

Most known thiamin diphosphate-dependent reactions (Table 14-2) can be derived from the five halfreactions, a through e, shown in Fig. 14-3. Each half-reaction is an a cleavage which leads to a thiamin- bound enamine (center. Fig. 14-3) The decarboxylation of an a-oxo acid to an aldehyde is represented by step h followed by fl in reverse. The most studied enzyme catalyzing a reaction of this type is yeast pyruvate decarboxylase, an enzyme essential to alcoholic fermentation (Fig. 10-3). There are two 250-kDa isoenzyme forms, one an tetramer and one with an (aP)2 quaternary structure. The isolation of a-hydroxyethylthiamin diphosphate from reaction mixtures of this enzyme with pyruvate provided important verification of the mechanisms of Eqs. 14-14,14-15. Other decarboxylases produce aldehydes in specialized metabolic pathways indolepyruvate decarboxylase in the biosynthesis of the plant hormone indole-3-acetate and ben-zoylformate decarboxylase in the mandelate pathway of bacterial metabolism (Chapter 25). Formation of a-ketols from a-oxo acids also starts with step h of Fig. 14-3 but is followed by condensation with another carbonyl compound in step c, in reverse. An example is decarboxylation of pyruvate and condensation of the resulting active acetaldehyde with a second pyruvate molecule to give l -a-acetolactate, a reaction catalyzed by acetohydroxy acid synthase (acetolactate synthase). Acetolactate is the precursor to valine and leucine. A similar ketol condensation, which is catalyzed by the same S5mthase, is... 

To investigate thiamine metabolism in mammals, thiamine (Rf values 0.16, 0.04, and 0.03), urinary excretion of thiamine metabolites [thiochirome (Rf values 0.31, 0.28, and 0.33), thiazole (Rf values 0.85, 0.79, and 0.81), and 2-methyl-4-amino-pyrimidinecarboxylic acid (Rf values 0.42, 0.21, and 0.26)], and related compounds [pyrimidinesulfonic acid (Rf values 0.48, 0.39, and 0.46), a-hydroxyethylthiamine (Rf values 0.23, 0.09, and 0.06), A -methylnicotinamide (Rf values 0.31, 0.06, and 0.05)] were analyzed and identified by TLC on silica gel with acetonitrile-water (40 10 vol/vol) adjusted to a pH of 2.54, 4.03, and 7.85 with formic acid as solvents, respectively.Although A -methylnicotinamide and thio-chrome could not be separated in single-phase chromatography at pH 2.54, a second phase at right angle, with a pH 4.03 solvent, separated these quite clearly without affecting the resolution of the other compounds. 

Hydroxyethylthiamine (R = MeCH(OH) reacts similarly. When the sodium salt of thiamine was treated with phosgene at -20 C in aqueous ethanol, S. -carbodithiamine was formed [1462] ... 

McNelis, 1959). This subject has been reviewed recently by Metzler (1960) and will not be discussed in detail here. In brief, Breslow postulated that thiamine pyropho.sphate ionizes at the 2-position of the thiazole ring, and that the thiazolium dipolar ion (IV) reacts with pyruvate to form an intermediate (2-laetylthiamine pyrophosphate) (V) which undergoes decarboxylation to produce 2-hydroxyethylthiamine pyrophosphate (VI, VII) (Fig. 2). Species (VI) is regarded as active acetaldehyde, and can... 

HETPP acronym of hydroxyethylthiamin pyrophosphate. see Thiamin pyrophosphate. 

Ziporin and Waring (1970) used two-dimensional TLC to separate thiamine and its metabolites and precursor compounds, including hydroxyethylthiamine, pyrimidine, thiazole fragments of thiamine, carboxylic acid, and sulfonic deriva-... 

Hydroxyethylthiamine, a derivative of thiamine, is found in cells in the form of hydroxyethyl-TPP. It is converted to the corresponding thiochrome deriv-... 

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