Acetaldehyde, from pyruvate

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

The homolytic acylation of protonated heteroaromatic bases is, as with alkylation, characterized by high selectivity. Only the positions a and y to the heterocyclic nitrogen are attacked. Attack in the position or in the benzene ring of polynuclear heteroaromatics has never been observed, even after careful GLC analysis of the reaction products. Quinoline is attacked only in positions 2 and 4 the ratio 4-acyl- to 2-acylquinoline was 1.3 with the acetyl radical from acetaldehyde, 1.7 with the acetyl radical from pyruvic acid, and 2.8 with the benzoyl radical from benzaldehyde. 

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. 

Ketols can also be formed enzymatically by cleavage of an aldehyde (step a, Fig. 14-3) followed by condensation with a second aldehyde (step c, in reverse). An enzyme utilizing these steps is transketolase (Eq. 17-15),132b which is essential in the pentose phosphate pathways of metabolism and in photosynthesis. a-Diketones can be cleaved (step d) to a carboxylic acid plus active aldehyde, which can react either via a or c in reverse. These and other combinations of steps are often observed as side reactions of such enzymes as pyruvate decarboxylase. A related thiamin-dependent reaction is that of pyruvate and acetyl-CoA to give the a-diketone, diacetyl, CH3COCOCH3.133 The reaction can be viewed as a displacement of the CoA anion from acetyl-CoA by attack of thiamin-bound active acetaldehyde derived from pyruvate (reverse of step d, Fig. 14-3 with release of CoA). 

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... 

Controversy remains in the determination of substrate tolerance for KdgA/KhgA aldolases from different sources. Early assay studies with KhgA prepared [127-129] from rat liver concluded that the catalyst had an unusually wide ranging tolerance for nucleophilic components, including a number of 3-substituted pyruvate derivatives as well as pyruvaldehyde, acetaldehyde, and pyruvic esters [135], Later, other workers using enzymes from rat or bovine liver and from E. coli reported their inability to reproduce these results but noted a rather limiting specificity [136]. 

The incorporation of [2-14C]pyruvate and [l-14C]acetate into sugars 17 and 18 was investigated.27 Oxidation of the methyl glycosides of sugar 17 with periodate yielded acetaldehyde from the 1-hydroxyethyl branch. The acetaldehyde (2,4-dinitrophenyl)hydrazone was further oxidized by Kuhn-Roth oxidation to acetic acid, which was degraded by the Schmidt reaction to methylamine and carbon dioxide. Periodate oxidation of the methyl glycosides of sugar 18 produced acetic acid from the C-acetyl branch. The acetic acid was isolated, and purified as 1-acetamidonaphthalene. 

Diacetyl, and its reduction products, acetoin and 2,3-butanediol, are also derived from acetaldehyde (Fig 8D.7), providing additional NADH oxidation steps. Diacetyl, which is formed by the decarboxylation of a-acetolactate, is regulated by valine and threonine availability (Dufour 1989). When assimilable nitrogen is low, valine synthesis is activated. This leads to the formation of a-acetolactate, which can be then transformed into diacetyl via spontaneous oxidative decarboxylation. Because valine uptake is suppressed by threonine, sufficient nitrogen availability represses the formation of diacetyl. Moreover, the final concentration of diacetyl is determined by its possible stepwise reduction to acetoin and 2,3-butanediol, both steps being dependent on NADH availability. Branched-chain aldehydes are formed via the Ehrlich pathway (Fig 8D.7) from precursors formed by combination of acetaldehyde with pyruvic acid and a-ketobutyrate (Fig 8D.7). 

Ethanol is formed from pyruvate in yeast and several other microorganisms. The first step is the decarboxylation of pyruvate. This reaction is catalyzed by pyruvate decarboxylase, which requires the coenzyme thiamine pyrophosphate. This coenzyme, derived from the vitamin thiamine (Bj), also participates in reactions catalyzed by other enzymes (Section 17.1.1). The second step is the reduction of acetaldehyde to ethanol by NADH, in a reaction catalyzed by... 

From these studies of the concomitant organic products it is clear that the observed G(NHS) values represent the combined yield of a number of different modes of degradation of the peptide chain. And, before proceeding to detailed considerations of elementary processes, it is useful here to formulate working hypotheses as to the stoichiometry of these reactions. First of all we note that the maximal yields of lactic acid and of the carbonyl products, acetaldehyde and pyruvic acid from acetylalanine are obtained only after mild hydrolysis of the irradiated... 

Removal of CO2 from pyruvate. This reaction is carried out by the pyruvate decarboxylase (El) component of the complex. Like yeast pyruvate decarboxylase, responsible for the production of acetaldehyde, the enzyme uses a thiamine pyrophosphate cofactor and oxidizes the carboxy group of pyruvate to CO2. Unlike the glycolytic enzyme, acetaldehyde is not released from the enzyme along with CO2. Instead, the acetaldehyde is kept in the enzyme active site, where it is transferred to Coenzyme A. 

Karl, T, Curtis, A.I, Rosenstiel, T.N., Monson, R.K., Fall, R. (2002) Transient releases of acetaldehyde from tree leaves—products of a pyruvate overflow mechanism Plant, Cell Environment, 25,1121-1131. 

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