Pyruvic decarboxylation system

That 2-acetylthiamine pyrophosphate is an intermediate in the oxidative decarboxylation of pyruvate may also be. inferred from (a) the demonstration by Goedde et al. (1961) that the hydroxyethyl group of 2-hydroxyethyl-thiamine pyrophosphate can be converted to acetyl CoA in the presence of a pyruvate oxidation system from yeast mitochondria and (b) the demonstration by Krampitz et al. (1961) that the hydroxyethyl group of 2-hydrox-yethylthiamine pyrophosphate is oxidized to acetate in the presence of fcrricyanide and the acetoin-forming system from A. aerogenes. 

FIGURE 12.2 Oxidative decarboxylation of pyruvate, (a) Overall reaction, (b) Mechanism of action of the pyruvate dehydrogenase system... 

The authors chose pyruvic acid as their model compound this C3 molecule plays a central role in the metabolism of living cells. It was recently synthesized for the first time under hydrothermal conditions (Cody et al., 2000). Hazen and Deamer carried out their experiments at pressures and temperatures similar to those in hydrothermal systems (but not chosen to simulate such systems). The non-enzymatic reactions, which took place in relatively concentrated aqueous solutions, were intended to identify the subsequent self-selection and self-organisation potential of prebiotic molecular species. A considerable series of complex organic molecules was tentatively identified, such as methoxy- or methyl-substituted methyl benzoates or 2, 3, 4-trimethyl-2-cyclopenten-l-one, to name only a few. In particular, polymerisation products of pyruvic acid, and products of consecutive reactions such as decarboxylation and cycloaddition, were observed the expected tar fraction was not found, but water-soluble components were found as well as a chloroform-soluble fraction. The latter showed similarities to chloroform-soluble compounds from the Murchison carbonaceous chondrite (Hazen and Deamer, 2007). 

Marcus theory, first developed for electron transfer reactions, then extended to atom transfer, is now being applied to catalytic systems. Successful applications to catalysis by labile metal ions include such reactions as decarboxylation of oxaloacetate, ketonization of enolpyru-vate, and pyruvate dimerization (444). 

The combined dehydrogenation and decarboxylation of pyruvate to the acetyl group of acetyl-CoA (Fig. 16-2) requires the sequential action of three different enzymes and five different coenzymes or prosthetic groups—thiamine pyrophosphate (TPP), flavin adenine dinucleotide (FAD), coenzyme A (CoA, sometimes denoted CoA-SH, to emphasize the role of the —SH group), nicotinamide adenine dinucleotide (NAD), and lipoate. Four different vitamins required in human nutrition are vital components of this system thiamine (in TPP), riboflavin (in FAD), niacin (in NAD), and pantothenate (in CoA). We have already described the roles of FAD and NAD as electron carriers (Chapter 13), and we have encountered TPP as the coenzyme of pyruvate decarboxylase (see Fig. 14-13). 

The oxidative decarboxylation of pyruvate and a-ketoglutaraie I which plays a key role in energy metabolism of most cells, is parn ularly important in tissues of the nervous system. In thiaminel deficiency, the activity of these two dehydrogenase reactions <1 decreased, resulting in a decreased production of ATP and, fxsi I... 

Oxidative decarboxylations of a-keto acids are mediated by either enzymes having more than one cofactor or complex multienzyme systems utilizing a number of cofactors. For example, pyruvate oxidase uses TPP and FAD as coenzymes, the function of the latter being to oxidize the intermediate (41). Conversion of pyruvate to acetyl-CoA requires a multienzyme complex with the involvement of no less than five coenzymes, TPP, CoA, dihydrolipoate, FAD and NAD+ (74ACR40). 

There are significant differences in the reactivity of synthetic intermediate analogues for these reactions and the corresponding intermediates in the enzymic system. Lienhard and coworkers42,43 reported that the rate of decarboxylation of 2-(l-carboxy-l-hydroxyethyl)-3,4-dimethylthiazolium chloride is very fast relative to pyruvate (whose reaction is too slow to observe) but slower than the enzymic decarboxylation of pyruvate decarboxylase (PDC) by a factor of 105. Similar observations of a catalytic gap were seen for the rate of decarboxylation of lactylthiamin compared to PDC and the... 

Some simple dithiolethiones, including the parent (3b) and 5-aryl derivatives, have been isolated from Brassica species, and some Streptomyces strains produce four antibiotics which possess pyrrolo[3,2-c][l,2]dithiole skeletons (174a-d). Some of these have been synthesized (B-66MI43100,77JOC2891). Lipoic acid (29) is a growth factor and an essential component of enzyme systems involved in oxidative decarboxylation of pyruvic and related acids (B-61MI43101). 

No 3-carboxy-substituted TBCs, derived from L-tryptophan by the Pic-tet-Spengler route, have yet been isolated from mammalian tissues. The same is also true for the dicarboxylic acid 23a derived from the condensation of L-tryptophan with pyruvic acid (36). The 1-carboxy-substituted TBCs 37 and 38, on the other hand, occur in mammalian systems (70,71) and are metabolically decarboxylated (65,S5). Whether a direct enzymatic decarboxylation of racemic material, occurring with the (S) and (R) enantiomers at a different rate, could account for the formation of unequal amounts of the enantiomers of TBC has not been investigated so far. The pyruvic acid route to optically active TBC (Fig. 12) leading from TBC 38a to TBC 29a via DBC 34 is at tifie moment the preferred pathway (85,86,89), although the enzymes involved in the asymmetric reduction leading to TBC 29a and the hydroxylated metabolites TBCs 30a and 33a have been neither isolated nor characterized. 

Further proof that COj is assimilated by means of the enzyme oxalacetate /3-carboxylase was obtained by Evans and coworkers, who succeeded in preparing a cell-free preparation of this enzyme from liver. The enz3rme was able to catalyze the decarboxylation of oxalacetate to pyruvate. These investigators were able to demonstrate an uptake of C Oj. Utter and Wood have demonstrated conclusively, however, that in the presence of isotopic CO2, pyruvate can be converted to oxalacetate containing isotopic carbon and that the process is, of course, reversible. Addition of adenosine triphosphate to this liver enzyme system increased the rate of incorporation of C Os. Wood, Vennesland and Evans S have also shown that during the fixation of C02, isotopic carbon is incorporated solely and in equal concentrations into the carboxyl groups of pyruvate, lactate, malate and fumarate. 

Thiamine is required by the body as the pyrophosphate (TPP) in two general types of reaction, the oxidative decarboxylation of a keto acids catalyzed by dehydrogenase complexes and the formation of a-ketols (ketoses) as catalyzed by transketolase, and as the triphosphate (TTP) within the nervous system. TPP functions as the Mg -coordinated coenzyme for so-called active aldehyde transfers in mul-tienzyme dehydrogenase complexes that affect decarboxyia-tive conversion of a-keto (2 oxo) acids to acyl-coenzyme A (acyl-CoA) derivatives, such as pyruvate dehydrogenase and a-ketoglutarate dehydrogenase. These are often localized in the mitochondria, where efficient use in the Krebs tricarboxylic acid (citric acid) cycle follows. 

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