Acetaldehyde formation from pyruvate

It may be stated at this point that the presence of a /3-hydroxy-butyrate fat in certain organisms is a matter of general biochemical importance. Usually /3-hydroxybutyric acid and the acetone bodies are derived from n-butyric acid directly. The unambiguous formation of jS-hydroxybutyric acid anhydrides from carbohydrates opens up new vistas its formation from acetaldehyde, and from pyruvic acid, through aldol intermediates can be understood without difficulty. Kirrmann s reaction, to which little attention has been paid, is at the same time an example of an oxygen shift, leading from hydroxyaldehydes to fatty acids. 

Nineteen different compounds (or compound classes) that are known to be rapidly assimilated by bacterioplankton have been identified as DOM photoproducts (Table I). Five of these photoproducts are formed with source DOM from both freshwater and marine environments acetaldehyde, formaldehyde, glyoxylate, pyruvate, and amino acids. Nine others have been reported only from freshwater systems (acetate, butyrate, citrate, formate, levulinate, malonate, oxalate, succinate, and dissolved carbohydrates), whereas five have been reported only from marine systems (acetone, butanal,... 

Fig. 7. Branch point between fermentation and respiration. At low pyruvate flux, the low of the Pdh complex for pyruvate results in oxidative decarboxylation to form acetyl CoA and NADH. The acetyl CoA can then can go into energy generation (via respiration) or fatty acid synthesis. At high glycolytic flux, pyruvate accumulates, and the higher of Pdc favors acetaldehyde formation and ethanol production. Accumulation of acetate can interfere with mitochondrial function. Pyk Pyruvate kinase Pdh pyruvate dehydrogenase Pdc pyruvate decarboxylase Aid (Dha) aldehyde dehydrogenase Adh alcohol dehydrogenase Acs acetyl CoA synthetase. (Taken from Postma et al. [169])...

Fig. 25. Formation of acetaldehyde from pyruvic acid by pyruvate decarboxylase...

According to Breslow, the active aldehyde intermediate in the decarboxylation of pyruvate could be an a-hydroxyethyl derivative of thiamine pyrophosphate, the substituent being attached in position 2 to the thiazole ring . His starting point was the observation that thiazolium salts easily lose a proton at C-2. Thus a stable and reactive zwitterion results that could be capable of forming an acyl carbanion derivative. The near-by amino-pyrimidine ring would have an inductive effect upon electron withdrawal at C-2. Breslow pictures the formation of acetoin from pyruvate and acetaldehyde as follows ... 

Indeed both -lactylthiamine pyrophosphate (XX) and a-hydroxyethyl-thiamine pyrophosphate (XXI) have been isolated and identified as products after incubation of pyruvate with a purified carboxylase preparation " . When [2- - C]pyruvate is used, the radioactivity is found in the thiazole part of the molecule after sulfite cleavage of XXL Acetaldehyde is formed from pyruvic acid by yeast carboxylase by enzymic cleavage of intermediate XXI, Uberating thiamine pyTophosphate . XXI has also been identified as intermediate in the formation of acetyl-coenzyme A from pyruvic acid by p3u uvic oxidase . The transketolase reaction has been shown to proceed via a gly-colaldehyde-enzyme intermediate here one may expect to find dihydroxy-ethylthiamine pyrophosphate as active glycol-aldehyde . Such experiments strongly support Breslow s concept of the reaction mechanism. 

Three mechanisms, at least, exist for the formation of AMC enzymatically. These are (1) from 2 moles of acetaldehyde, (2) from 1 mole of pyruvate and 1 mole of acetaldehyde, and (3) from 2 moles of pyruvate. 

The highly purified wheat germ carboxylase of Singer carries out the simple decarboxylation of pyruvate to acetaldehyde and CO2. It can also synthesize AMC from pyruvate and acetaldehyde or form 2 moles of acetaldehyde, at a slower rate. The fact that this enzyme can form AMC from acetaldehyde alone indicates that this TPP-enzyme complex can activate acetaldehyde directly. This strongly suggests the formation of an acetaldehyde-TPP intermediate directly from acetaldehyde, which has the same capacity as that derived from the decarboxylation of pyruvate, to condense with a free acetaldehyde molecule to form AMC. 

The formation of acetaldehyde in the reaction shown in Eq. (8.15) involves oxidation of the CH3CHOH radical, which is formed by decarboxylation of the lactate anion. By contrast, the formation of pyruvic acid in the reaction shown in Eq. (8.16) involves hydrogen abstraction from CH3CH(0H)C02H by the excited state of the uranyl ion to give the CH3C(0H)C02H radical. Subsequent oxidation of this radical leads to the formation of pyruvic acid. These pathways are supported by EPR measurements at 77... 

In 1961 Jimi proposed the two-site theory of the mechanism of acyloin formation by pyruvate decarboxylase [15]. This theory was later confirmed by others [18,28]. According to the model, at the first site pyruvate is decarboxylated to an aldehyde-diphosphatamine complex (HETPP) called active acetaldehyde. The active acetaldehyde moiety is then irreversibly transferred to the second site, where reversible dissociation to free aldehyde takes place. The model is based on the observation that pyruvate decarboxylase not only forms free acetaldehyde as the major end-product of decarboxylation of an a-keto acid but also catalyzes formation of C-C bonds via an acyloin reaction in which free aldehyde competes with a proton for bond formation with the a carbanion of EDETPP. Thus the addition of a C2 unit equivalent to acetaldehyde by means of HETPP to a carbonyl group results in an (i )-hydroxy ketone [29]. For instance, the production of acetoin (methylacetyl carbinol) results when acetaldehyde is allowed to accumulate or is added to the reaction mixture [28]. This phenomenon was confirmed using pyruvate decarboxylase from different sources (wheat germ, yeast, and bacteria) [15,28,30]. 

Vojtisek and Netrval [43] studying phenylacetyl carbinol formation from sucrose, acetaldehyde, and benzaldehyde by S. carlsbergensis (variant Budvar ) detected the highest initial rate of biotransformation and the highest phenylacetyl caibinol production in the cells with Ihe lowest pyruvate decarboxylase activity. They suggested that the total amount of phenylacetyl carbinol produced would depend primarily on the actual intracellular concentration of pyruvate, i.e., biotransformation stops due to exhaustion of intracellular pyruvate, prior to inactivation of pyruvate decarboxylase. Additions of pyruvate did not influence the rate of phenylacetyl carbinol production but increased significantly the overall production of this compound. 

This scheme suggests the possibility of a new C—C bond formation. The nucleophiUc attack of enol intermediate (= carbanion intermediate) on another aldehyde affords ultimately a hydroxy ketone as shown in Eq. (19). The simplest product, acetoin (R = CH,) (10), has been found for the first time in fermenting yeast [45,46]. It can be formed by the reaction between pyruvate and acetaldehyde, which itself is originated from pyruvate by the action of the same enzyme. Later, in brewers yeast [47-51], wheat germ [52], and mammalian tissue [51], it was proved that PDase is responsible for the formation of acetoin. 

Formation of a-ketols from a-oxo acids also starts with step b 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 R-a-acetolactate, a reaction catalyzed by acetohydroxy acid synthase (acetolactate synthase).128 Acetolactate is the precursor to valine and leucine. A similar ketol condensation, which is catalyzed by the same synthase, is... 

Alkaloids from tryptophan. The alkaloid harmine, which is found in several families of plants, can be formed from tryptophan and acetaldehyde (or pyruvate) in the same manner as is indicated for the formation of papaverine in Fig. 25-10. Some other characteristic plant metabolites such as psilocybine, an hallucinogenic material from the mushroom... 

Functionally and mechanistically reminiscent of the pyruvate lyases, the 2-deoxy-D-ribose 5-phosphate (121) aldolase (RibA EC 4.1.2.4) [363] is involved in the deoxynucleotide metabolism where it catalyzes the addition of acetaldehyde (122) to D-glyceraldehyde 3-phosphate (12) via the transient formation of a lysine Schiff base intermediate (class I). Hence, it is a unique aldolase in that it uses two aldehydic substrates both as the aldol donor and acceptor components. RibA enzymes from several microbial and animal sources have been purified [363-365], and those from Lactobacillus plantarum and E. coli could be induced to crystallization [365-367]. In addition, the E. coli RibA has been cloned [368] and overexpressed. It has a usefully high specific activity [369] of 58 Umg-1 and high affinity for acetaldehyde as the natural aldol donor component (Km = 1.7 mM) [370]. The equilibrium constant for the formation of 121 of 2 x 10M does not strongly favor synthesis. Interestingly, the enzyme s relaxed acceptor specificity allows for substitution of both cosubstrates propional-dehyde 111, acetone 123, or fluoroacetone 124 can replace 122 as the donor [370,371], and a number of aldehydes up to a chain length of 4 non-hydrogen atoms are tolerated as the acceptor moiety (Table 6). 

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