Pyruvate enol intermediate

Figure 2. Mechanism for exchange in methyl pyruvate and for its hydrogenation to methyl lactate via an enol intermediate. R = COOCH3...

The 500-residue subunits of pyruvate kinase consist of four domains,891 the largest of which contains an 8-stranded barrel similar to that present in triose phosphate isomerase (Fig. 2-28). Although these two enzymes catalyze different types of reactions, a common feature is an enolic intermediate. One could imagine that pyruvate kinase protonates its substrate phosphoenolpyruvate (PEP) synchronously with the phospho group transfer (Eq. 12-42). However, the enzyme catalyzes the rapid conversion of the enolic form of pyruvate to the oxo form (Eq. 12-43) adding the proton sterospecifically to the si face. This and other evidence favors the enol as a true intermediate... 

The glycolysis of glucose proceeds through several steps involving enol intermediates to afford pyruvate, which can be converted into acetyl coenzyme A (acetyl-CoA) to participate in the Krebs cycle. Kinetic and crystal structure studies point to the key role played in enzyme catalysis by the stabilization of such intermediates on binding of enolate to the metal ion(s) of the enzymes. 

In most enzyme systems, enolate intermediates are stabilized by metal ion complexation. Although few good numbers are available, it appears that metal ion complexation of the oxygen of the keto and enol forms can increase the acidity of an adjacent carbon-hydrogen bond by four to six orders of magnitude. For example, complexation with lowers the ipK at C-3 of oxaloacetate from 13 to 9 (76), and a similar shift is seen with pyruvate (75). Enolates of a-keto acids can be effectively stabilized by metal complexation. For example, in the cases of pyruvic acid and oxaloacetic acid, both the keto oxygen and one of the carboxyl oxygens coordinate to the metal (Scheme 11). 

In the first stage of peptidoglycan synthesis, UDP-N-acetylmuramic acid is synthesized, and then a pentapeptide chain becomes attached to the carboxyl group of N-acetylmuramic acid (see Scheme 2). Strom-inger found that the reaction of enolpyruvate phosphate with UDP-2-acetamido-2-deoxy-D-glucose results in the formation of a compound that appeared to be a UDP-2-acetamido-2-deoxy-D-glucose-pyruvate enol ether, and he predicted that its reduction would aflFord UDP-N-acetylmuramic acid. The occurrence of these reactions has now been confirmed, and the intermediate product has been characterized in greater detail. ... 

Initial Cu(oxac)k formation is accompanied by the appearance of a transient in the u.v. region of the spectrum. Since the keto-enol equilibrium for Cu(oxac) lies closer to the enol form than for free oxac, this transient disappears at comparable rates by enolization and by decarboxylation to a steady-state concentration. The third phase of the reaction is decarboxylation through this steady-state species. Between these two phases is an intermediate process which is manifest only in pH-stat measurements and involves no decarboxylation. This is assigned to protonation of the copper pyruvate-enolate product Cu(pyr)ei ... 

Pyruvate has been shown to trap an intermediate in the Zn +-catalysed decarboxylation of oxaloacetic acid. Identification of this species as parapyruvate provides evidence that both the zinc-catalysed dimerization of pyruvate and the decarboxylation of oxaloacetate proceed with formation of the pyruvate enolate. Since the rate of... 

The thiazolium ion then behaves as an electron sink or electrophile and decarboxylation follows. The enolic intermediate, on the other hand, acts as a nucleophile which can be protonated. This intermediate has been isolated. Finally, acetaldelyde is formed and the coenzyme (ylid form) is regenerated at the same time. The liberation of acetaldelyde is the rate-limiting step in the pyruvate decarboxylase mechanism. 

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. 

The process of converting an enol to a ketone. Pyruvate kinase catalyzes a ketonization reaction in the conversion of the enolpyruvate intermediate to pyruvate. See... 

UDP-GlcNAC enolpyruvyltransferase (MurZ) catalyzes the reaction between the phosphate of the enol pyruvate and the UDP-GlcNAC to form the corresponding enolpyruvate. This reaction is the first stage of the biosynthesis of the peptidoglycan of the bacterial wall. Phosphates of mono- and difluoroenolpyruvates are substrates of MurZ (Figure 7.30). The tetrahedral intermediates formed after incubation... 

Since fluoro-carbonyl compounds are such useful and versatile synthetic intermediates, much effort has been devoted to their preparation [124], but only in a few instances has elemental fluorine been used directly. One of the earliest successful direct fluorinations of a simple carbonyl compound was the fluorina-tion of pyruvic acid derivatives which have a high enol content (R = Aryl, Acyl) (Fig. 47) [125] in the solvent being used (mixtures of CF2C1CFC12 and acetonitrile). However, in derivatives where the enol content was low (R = Alkyl), complicated mixtures of products were obtained. 

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... 

These enzymes do not catalyze any proton exchange at C-3 of pyruvate or at C-2 of an acyl-CoA unless the biotin is first carboxylated. This suggested that removal of the proton to the biotin oxygen and carboxylation might be synchronous. However, 13C and 2H kinetic isotope effects and studies of 3H exchange66 support the existence of a discrete enolate anion intermediate as shown in Eq. 14-11.165/67 This mechanism is also consistent with the observation that propionyl-CoA... 

Similar enol ethers probably serve as intermediates in another common modification-reaction of monosaccharide units especially characteristic of exocellular polysaccharides, namely, the formation of cyclic acetals of pyruvic acid. 

In both schemes, the upper pathway represents the formation of an unstable intermediate that is then trapped by reaction with ATP. By contrast, the lower pathways represent activation of a carbonyl group by ATP, followed by loss of a proton (to form enol pyruvate eq. 9) or reaction with ammonia (to yield an intermediate for the formation of CTP eq. 10). ATP has long been known to drive metabolic processes if it phosphorylates carbonyl groups, it will prove to serve a catalytic role as well. 

This key intermediate has given its name to Nature s general route to aromatic compounds and many other related six-membeied ring compounds the shikimic acid pathway. This pathway contains some of the most interesting reactions (from a chemist s point of view) in biology. It starts with an aldol reaction between phosphoenol pyruvate as the nucleophilic enol component and the C4 sugar erythrose 4-phosphate as the electrophilic aldehyde. 

Pyruvate kinase-catalyzed removal of phosphate from PEP (128) yields the unstable intermediate enol form of pyruvate (129), presumably stabilized by metal binding (Figure 15B) . The intermediate 129 undergoes fast acid-catalyzed conversion to the keto form of pyruvate (130) (/ h+ = 1-7 x 10 s ) and can be further enhanced by... 

Covey and Leussing have subsequently carried out detailed studies of the zinc(IT)-catalyzed decarboxylation of oxaloacetate. Two processes are observed using stopped flow techniques an initial absorbance increase being complete in about 30 s and a subsequent absorbance decrease being complete after 15 to 30 min. The first process is due to metal ion-promoted keto-enol tautomerism and the second to catalyzed decarboxylation. The subsequent protonation of the zinc pyruvate intermediate is rapid and unobservable. The rate constant for the decarboxylation... 

On the other hand, the enolate of pyruvate is often an intermediate in these reactions, and if the equilibrium is considered from the point of view of the enolate [Eq. (5)],... 

Interestingly, although mechanistic details vary, it is likely that a metal-chelated enolate of pyruvate is an intermediate in all these reactions and that all transition states for the carboxylation/decarboxylation step are similar. 

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