Pyruvate, condensation with thiamin pyrophosphate

First a pyruvate molecule condenses with thiamin pyrophosphate at the thiazolium ring carbon with subsequent loss of carbon dioxide. Then a second pyruvate (or cx-ketobutyrate) condenses followed by loss of acetolactate and regeneration of the thiazolium ring. 

Diacetyl can be produced by either homolactic or heterolactic pathways of sugar metabolism (via free pyruvate) or by utilization of citric acid (see Figs. 1-1 lA and 1-1 IB). In this case, citric acid is first converted to oxaloacetic and acetic acids. The former is then decarboxylated to pyruvate which undergoes a second decarboxylation and condensation with thiamine pyrophosphate (TPP) to yield active acetaldhyde, which reacts with another pyruvate to yield a-acetolactate which undergoes oxidative decarboxylation to yield diacetyl and its equilibrium products see Fig. 1-11 A. In the case of other LAB, the precursor, a-acetolactate is not produced. Here active acetaldehyde, produced as described above, reacts with acetyl CoA to yield diacetyl see Fig. 1-1 IB. 

Diacetyl may be synthesized by either homolactic or heterolactic pathways of sugar metabolism as well as by utilization of citric acid (Fig. 2.9). Citric acid is hrst converted to acetic acid and oxaloacetate the latter is then decarboxylated to pyruvate. Although earlier reports indicated that diacetyl synthesis by lactic acid bacteria does not proceed via a-acetolactate (Gottschalk, 1986), more recent evidence suggests that this pathway is active in lactic acid bacteria (Ramos et al., 1995). Here, pyruvate undergoes a second decarboxylation and condensation with thiamine pyrophosphate (TPP) to yield active acetaldehyde. This compound then reacts with another molecule of pyruvate to yield a-acetolactate, which, in... 

Decarboxylation of an a-keto acid like pyruvate is a difficult reaction for the same reason as are the ketol condensations (see fig. 12.33) Both kinds of reactions require the participation of an intermediate in which the carbonyl carbon carries a negative charge. In all such reactions that occur in metabolism, the intermediate is stabilized by prior condensation of the carbonyl group with thiamine pyrophosphate. In figure 13.5 thiamine pyrophosphate and its hydroxyethyl derivative are written in the doubly ionized ylid form rather than the neutral form because this is the form that actually participates in the reaction even though it is present in much smaller amounts. 

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. 

The syntheses of valine, leucine, and isoleucine from pyruvate are illustrated in Figure 14.9. Valine and isoleucine are synthesized in parallel pathways with the same four enzymes. Valine synthesis begins with the condensation of pyruvate with hydroxyethyl-TPP (a decarboxylation product of a pyruvate-thiamine pyrophosphate intermediate) catalyzed by acetohydroxy acid synthase. The a-acetolactate product is then reduced to form a,/3-dihydroxyisovalerate followed by a dehydration to a-ketoisovalerate. Valine is produced in a subsequent transamination reaction. (a-Ketoisovalerate is also a precursor of leucine.) Isoleucine synthesis also involves hydroxyethyl-TPP, which condenses with a-ketobutyrate to form a-aceto-a-hydroxybutyrate. (a-Ketobutyrate is derived from L-threonine in a deamination reaction catalyzed by threonine deaminase.) a,/3-Dihydroxy-/3-methylvalerate, the reduced product of a-aceto-a-hydroxybutyrate, subsequently loses an HzO molecule, thus forming a-keto-/kmethylvalerate. Isoleucine is then produced during a transamination reaction. In the first step of leucine biosynthesis from a-ketoisovalerate, acetyl-CoA donates a two-carbon unit. Leucine is formed after isomerization, reduction, and transamination. 

Yeasts also make use of pyruvic acid to form acetoin, diacetyl and 2,3-butanediol (Figure 2.17). This process begins with the condensation of a pyruvate molecule and active acetaldehyde bound to thiamine pyrophosphate, leading to the formation of cr-acetolactic acid. The oxidative decarboxylation of a-acetolactic acid produces diacetyl. Acetoin is produced by either the non-oxidative decarboxylation of a-acetolactic acid or the reduction of diacetyl. The reduction of acetoin leads to the formation of 2,3-butanediol this last reaction is reversible. 

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