Pyruvate condensation reactions

Theoretically, a fall in concentration of oxaloacetate, particularly within the mitochondria, could impair the ability of the citric acid cycle to metabolize acetyl-CoA and divert fatty acid oxidation toward ketogenesis. Such a fall may occur because of an increase in the [NADH]/[NAD+] ratio caused by increased P-oxida-tion affecting the equilibrium between oxaloacetate and malate and decreasing the concentration of oxaloacetate. However, pyruvate carboxylase, which catalyzes the conversion of pyruvate to oxaloacetate, is activated by acetyl-CoA. Consequently, when there are significant amounts of acetyl-CoA, there should be sufficient oxaloacetate to initiate the condensing reaction of the citric acid cycle. 

As for the reaction path from pyruvic acid to citraconic anhydride, it is considered that a condensation reaction first takes place by a reaction between an oxygen atom of carbonyl group and two hydrogn atoms of methyl group in another molecule, followed by oxidative decarboxylation to form citraconic acid. The produced citraconic acid is dehydrated under the reaction conditions used. The proposed reaction path is shown in Figure 7. 

Phosphoenolpyruvate, a key metabolic intermediate. A compound of central importance in metabolism is the phosphate ester of the enol form of pyruvate, commonly known simply as phosphoenolpyruvate (PEP).249 It is formed in the glycolysis pathway by dehydration of 2-phosphoglycerate (Eq. 13-15) or by decarboxylation of oxaloacetate. Serving as a preformed enol from which a reactive enolate anion can be released for condensation reactions,250 251 PEP... 

Further developments are shown in Figure 4. On the basis that glucosamine reacted with pyruvic acid in the presence of alkali to yield pyrrole-2-carboxylic acid, in 1% yield, Gottschalk (21) proposed that sialic acid was formed by an aldol condensation reaction between N-ace-tylglucosamine and pyruvic acid. Kuhn and Brossmer (15) and Zilliken and Glick (22) showed that the reverse reaction also took place under alkaline conditions. Cornforth, Firth, and Gottschalk (23) synthesized crystalline N-acetylneuraminic acid (NANA) from N-acetylglucosamine and oxaloacetic acid (pH 11, 20°C). Under conditions less subject to misinterpretation, Heimer and Meyer (24) found that Vibrio cholerae enzymes cleaved NANA into an N-acetylhexosamine and pyruvic acid. 

Precursors. Precursors for this reaction are compounds exhibiting keto-enol tau-tomerism. These compounds are usually secondary metabolites derived from the glycolysis cycle of yeast metabolism during fermentation. Pyruvic acid is one of the main precursor compounds involved in this type of reaction. During yeast fermentation it is decarboxylated to acetaldehyde and then reduced to ethanol. Acetone, ace-toin (3-hydroxybutan-2-one), oxalacetic acid, acetoacetic acid and diacetyl, among others, are also secondary metabolites likely to participate in this kind of condensation reaction with anthocyanins. 

Mechanism of reaction. The adduct of malvidin-3-glucoside with pyruvic acid, also known as vitisin A(Fig. 9A.3h), was firstly detected in fortified red wines (Bakker et al. 1997) and in a grape marc (Fulcrand et al. 1998) and further isolated and characterized by NMR (Bakker et al. 1997 Fulcrand et al. 1998). According to Fulcrand et al. (1998), the reaction between pyruvic acid and grape anthocyanins occurs through a series of steps similar to those previously described for the hydroxyphenyl-pyranoanthocyanins (Sect. 9A.2.4.1 Fig. 9A.3f). Later studies performed by NMR (Mateus et al. 2001b) and mass spectrometry (Asenstorfer et al. 2001 Hayasaka and Asenstorfer 2002) have confirmed the structure proposed by Fulcrand et al. (1998). This mechanism is extended to the condensation reaction between anthocyanins and other enolizable precursors found in wine (Benabdeljalil et al. 2000). 

Pyruvate decarboxylase catalyzes the nonoxidative decarboxylation of pyruvate to acetaldehyde and carbon dioxide. When an aldehyde is present with pyruvate, the enzyme promotes an acyloin condensation reaction. The mechanistic reason for this fortuitous reaction is well understood and involves the aldehyde outcompeting a proton for bond formation with a reactive thiamine pyrophosphate-bound intermediate (90,91). When acetaldehyde is present, the product formed is acetoin. Benzalde-hyde results in the production of phenylacetylcarbinol (Fig. 26). Both of these condensations are enantioselective, forming the R enantiomer preferentially in both cases. 

Studies on thiamine (vitamin Bi) catalyzed formation of acyloins from aliphatic aldehydes and on thiamine or thiamine diphosphate catalyzed decarboxylation of pyruvate have established the mechanism for the catalytic activity of 1,3-thiazolium salts in carbonyl condensation reactions. In the presence of bases, quaternary thiazolium salts are transformed into the ylide structure (2), the ylide being able to exert a cat ytic effect resembling that of the cyanide ion in the benzoin condensation (Scheme 2). Like cyanide, the zwitterion (2), formed by the reaction of thiazolium salts with base, is nucleophilic and reacts at the carbonyl group of aldehy s. The resultant intermediate can undergo base-catalyzed proton... 

Figure 10 The condensation reaction between pyruvate and (2R,3S)-2,3-dihydroxy-4-oxo-A/,A/-dipropylbutyramide catalyzed by NAL. The large changes in the facial selectivity, in either direction, as a result of mutations identified during directed evolution are listed in the table (inset).

Importantly, salt-induced peptide formation could provide an abiotic route for the formation of peptides directly from amino acids in concentrated NaCl solutions containing copper ions. Montmorillonite and similar minerals apparently promote the condensation reaction that could have taken place in evaporating tidal pools -Darwin s warm little ponds - where the required salty brine solutions were easily available. Obviously, this is a likely and hence a credible prebiotic scenario. There might a pearl hidden beneath muddy waters. Besides, it is fascinating to assume that the primitive enolase enzyme known to be a highly conserved ancient enzyme could have evolved in an RNA-peptide world. Enolase catalyzes the for enantio-selective carbon-carbon bond addition of water to phosphoenol pyruvate to yield D-2-phospho-glycerate. 

Since the enzyme reaction is reversible, conditions for the synthesis of Neu5Ac and natural or synthetic derivatives in high yield as well as of Kdn with the aid of bacterial lyase were elaborated (see section 6.1). The recombinant and overexpressed sialate-pyruvate lyase from E. coli is now in wide use as a speeific chiral catalyst which mediates highly enantioselective aldol condensation reactions leading to a variety of sialic acids. 

Terpenoids do not necessarily contain exact multiples of five carbons and allowance has to be made for the loss or addition of one or more fragments and possible molecular rearrangements during biosynthesis. In reality the terpenoids are biosynthesized from acetate units derived from the primary metabolism of fatty acids, carbohydrates and some amino acids (see Fig. 2.10). Acetate has been shown to be the sole primary precursor of the terpenoid cholesterol. The major route for terpenoid biosynthesis, the mevalonate pathway, is summarized in Fig. 2.16. Acetyl-CoA is involved in the generation of the C6 mevalonate unit, a process that involves reduction by NADPH. Subsequent decarboxylation during phosphorylation (i.e. addition of phosphate) in the presence of ATP yields the fundamental isoprenoid unit, isopentenyl pyrophosphate (IPP), from which the terpenoids are synthesized by enzymatic condensation reactions. Recently, an alternative pathway has been discovered for the formation of IPP in various eubacteria and plants, which involves the condensation of glyceraldehyde 3-phosphate and pyruvate to form the intermediate 1-deoxy-D-xylulose 5-phosphate (Fig. 2.16 e.g. Eisenreich et al. 1998). We consider some of the more common examples of the main classes of terpenoids below. 

The Sia synthase, or aldolase, is responsible for the reversible aldol reaction that occurs between six-carbon mannosamines and pyruvate (Figure 22). In the forward direction, this condensation reaction creates a new carbon-carbon bond to produce nine-carbon Sias. Aldolases capable of Sia synthesis are found in a variety of bacteria. The E. coli enzyme has proven to be extremely adaptable in chemoenzymatic reactions. Its substrate scope has been studied extensively and is described in a large body of literature published in the late 1980s and early 1990s. Taken together, these reports indicate that the enzyme exhibits a strong preference for pyruvate as... 

The five-carbon backbone of PNP is derived from DXP, which is produced from G3P and pyruvate in a condensation reaction catalyzed by DXPS. The recombinant enzyme from E. coli has been characterized and shown to have a /feat of 270 s and a of 240 pM for G3P and 96 pM for pyruvate. DXPS uses thiamin... 

The thioxo compounds 109 and 111 are readily desulfurized by shaking with Raney nickel. This reaction provides a link for the establishment of orientations of compounds obtained by condensation reactions of a-keto esters (RCOC02R ) and the corresponding a-keto aldehydes (RCOCHO) with 2,3-diaminopyridines. Thus the 0x0 compounds obtained from the former reactions may be converted into the products from the latter reactions by successive treatment with phosphorus pentasulhde and Raney nickel. For example, the compound 114, obtained as the only product from the condensation of 2,3-diamino-5-bromopyridine with ethyl pyruvate in strongly acid solution (see Section II2B), has been converted to the thioxo compound 115, which on desulfurization yields compound 116, identical with that obtained from the condensation of the pyridine with pyruvaldehyde in neutral solution (see Section II2A). 

Deoxy-D-/w<2 0-2-octulosonate aldolase (EC 4.1.2.23), also named 2-keto-3-deoxyoctonate (KDO) aldolase catalyzes reversible condensation of pyruvate with D-arabinose to form KDO. Preliminary investigations of the substrate specificity indicated a high specificity for KDO in the direction of the cleavage. KDO aldolase has been tested in the condensation reaction with several unnatural substrates including D-ribose, D-xylose, D-lyxose, L-arabinose, D-arabinose 5-phosphate and Af-acetylmannosamine, giving poor yield in all cases [64],... 

The first step is a decarboxylation of pyruvate and its conversion to acetyl-CoA, with the concommitant production of NADH. The next step is a condensation reaction between acetyl-CoA and oxaloacetate to produce the six-carbon compound, citrate. Citrate is isomerized to isocitrate. 

Acetyl-CoA. There is a key metabolite of energy metabolism. It is produced in mitochondria by decarboxylation of pyruvate, beta oxidation of fatty acids, or hydrolysis of acetoacetate. In condensation reaction with oxaloacetate, acetyl-CoA yields citrate, which is the first intermediate in the tricarboxylic acid chain. Acetyl-CoA is also used for the synthesis of acetylcholine and the acetylation of several low molecular weight compounds and proteins. In liver and adipose tissue, acetyl-CoA is used for the synthesis of the fatty acids chain. 

In biosynthesis, pyruvic acid, a representative 1,2-dicarbonyl compound, is used as a key C2 and C3 donor unit. The use of related 1,2-dicarbonyl compounds, such as a-keto esters and ct-keto anilides, as nucleophiles in catalytic asymmetric synthesis, however, is rather limited due to their high reactivity as electrophiles. Chemoselective activation of 1,2-dicarbonyl compounds as nucleophiles is required to avoid undesired self-condensation reactions of 1,2-dicarbonyl compounds. Applications of 1,2-dicarbonyl compounds as donors in asymmetric Michael reactions remained unsolved until a recent report by Sodeoka et oL Indeed, these authors have described the first example of a diastereo- and... 

In examining the overall conversion of pyruvate into a fatty acid (Schemes 1.1 and 1.2) it is interesting to note the exploitation of particular chemical properties of sulphur (i), as an easily reduced disulphide (1.6) (ii), as an easily oxidized dithiol and (iii) in reactive thioesters which aid the Claisen-type condensation reactions. Also of crucial importance for the condensation is the use of a malonic acid derivative (1.14) as a source of a stable anion. (Further discussion of fatty acid biosynthesis in relation to polyketide formation is taken up in Chapter 3.)... 

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