Fragmentation pyruvate decarboxylation

Pyruvate decarboxylase is an enzyme that requires thiamine pyrophosphate. Pyruvate decarboxylate catalyzes the decarboxylation of pyruvate and transfers the resulting two-carbon fragment to a proton, resulting in the formation of acetaldehyde. 

The tricarboxylic acid cycle was therefore validated, having been tested not only in pigeon-breast muscle but also with brain, testis, liver, and kidney. The nature of the carbohydrate fragment entering the cycle was still uncertain. The possibility that pyruvate and oxaloacetate condensed to give a 7C derivative which would be decarboxy-lated to citrate, was dismissed partly because the postulated compound was oxidized at a very low rate. Further, work on the oxidation of fatty acids (see Chapter 7) had already established that a 2C fragment like acetate was produced by fatty acid oxidation, en route for carbon dioxide and water. It therefore seemed likely that a similar 2C compound might arise by decarboxylation of pyruvate, and thus condense with oxaloacetate. For some considerable time articles and textbooks referred to this unknown 2C compound as active acetate. ... 

Since the decarboxylation of 161 to 162 proceeds in a poor yield, it was suggested that formation of 162 in benzene occurs directly from 2-benzopyrylium salts 62 through primary nucleophilic attack by amine in position 3. In this case, an enamine fragment of pyruvic acid appears in the ring-opened intermediate 163, which undergoes easy decarboxylation (82TL459). The vinylic carbanion 164, formed by the loss of carbon dioxide, captures a proton by intra- or intermolecular process, then hetero-cyclization takes place. 

A fourth fate of pyruvate is its oxidative decarboxylation to acetyl CoA. This irreversible reaction inside mitochondria is a decisive reaction in metabolism it commits the carbon atoms of carbohydrates and amino acids to oxidation by the citric acid cycle or to the synthesis of lipids. The pyruvate dehydrogenase complex, which catalyzes this irreversible funneling, is stringently regulated by multiple allosteric interactions and covalent modifications. Pyruvate is rapidly converted into acetyl CoA only if ATP is needed or if two-carbon fragments are required for the synthesis of lipids. 

The transketolase (TK EC 2.2.1.1) catalyzes the reversible transfer of a hydroxy-acetyl fragment from a ketose to an aldehyde [42]. A notable feature for applications in asymmetric synthesis is that it only accepts the o-enantiomer of 2-hydroxyaldehydes with effective kinetic resolution [117, 118] and adds the nucleophile stereospecifically to the re-face of the acceptor. In effect, this allows to control the stereochemistry of two adjacent stereogenic centers in the generation of (3S,4R)-configurated ketoses by starting from racemic aldehydes thus this provides products stereochemically equivalent to those obtained by FruA catalysis. The natural donor component can be replaced by hydroxy-pyruvate from which the reactive intermediate is formed by a spontaneous decarboxylation, which for preparative purposes renders the overall addition to aldehydic substrates essentially irreversible [42]. 

The major reaction is oxidative dehydrogenation at the secondary hydroxyl site of lactic acid, but the product pyruvic acid in its free-acid form is unstable to decompose. Thus the substrate was supplied as ethyl ester to protect the carboxyl moiety. Esterification is also of benefit to vapor-phase flow operation in making acids more volatile. Hydrolysis of ethyl lactate gives free pyruvic acid with further decarboxylation to actaldehyde. Ethanol, which is another fragment of ester hydrolysis, eould be either oxidized to acetaldehyde or dehydrated to ethylene at higher temperature above 350°G. The reaction network is summerized in Scheme 1. 

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 decarboxylation of pymvic acid to yield carbon dioxide and a two-carbon fragment is catalyzed by a class of enzymes that require thiamin pyrophosphate (TPP) and a divalent metal ion. We will restrict our attention to the simplest member of the class, pyruvate decarboxylase 132, 133), which catalyzes reaction 18). 

Acetolactate synthase is another TPP-requiring enzyme. It also catalyzes the decarboxylation of pyruvate but transfers the resulting two-carbon fragment to another molecule of pyruvate, forming acetolactate. This is the first step in the biosynthesis of the amino acids valine and leucine. Propose a mechanism for acetolactate synthase. 

A functional difference between CAM and C4-photosynthesis is the immediate source of P-enolpyruvate for carboxylation. In C -photosynthesis, it is largely assumed that PEP is regenerated from the 3-C fragment after malate (or oxalace-tate) utilization. Two possibilities seem apparent (1) pyruvate is converted to P-enolpyruvate by pyruvate, Pi dikinase (Hatch and Slack, 1970) or (2) oxalacetate rather than malate is decarboxylated to form P-enolpyruvate by PEP carboxyki-nase (Black et al, 1973). 

Lipmann s discovery in 1939 that acetyl phosphate results from the oxidative decarboxylation of pyruvate foretold the fundamental role of TPP in initiating reactions leading to the production of an activated 2-car-bon fragment. 

The functional group of TPP involved in the attack on a-keto acids still, therefore, remains in doubt. The nature of the primary product formed upon decarboxylation of the keto acid also remains to be unequivocally elucidated. Rather good evidence exists, however, which indicates that pyruvate is attacked by TPP enzymes to yield CO2 and a 2-carbon frag-ment-TPP complex in which the C2 fragment is at the aldehyde level of oxidation. This primary product will here be called the acetaldehyde-... 

A single copper containing protein with a requirement for ascorbic acid—p-hydroxyphenylpyruvate oxidase—catalyses the simultaneous hydroxylation of the phenyl ring, migration of the aliphatic side chain to an adjacent position on the ring, and oxidative decarboxylation of the pyruvate fragment . The enzyme... 

In thiamine deficiency, therefore, the formation from pyruvate of acetyl-coenzyme A, which is normally followed by the oxidation of the 2-C acetyl fragment to CO2 and H2O via the citric acid cycle is blocked, and the pyruvate formed by glycolysis consequently accumulates. In addition in its role in the oxidative decarboxylation of pyruvate, thiamine pyrophosphate is also needed as a co-enzyme in the similar decarboxylation of the closely related a-oxoglutarate, one of the intermediaries in the citric acid cycle. 

---