A-Keto acids intermediate

Decarboxylation. Decarboxylation of the pyruvate/thiamine addition product occurs in much the same way that decarboxylation of a -keto acid intermediate occurs in the acetoacetic ester synthesis (Section 22.8). The C=N double bond of the pyruvate addition product acts like the C=0 double bond of a 0-keto acid to accept electrons as COj leaves. 

Oxidative deavage of 1,2-diols to carboxylic acids by HP was achieved using catalytic amounts of tungstate and phosphate ions, under acidic conditions [34c,37vj. The reaction was conducted at 90 °C and pH 2 in aqueous solution, with a slight excess of HP with respect to the stoichiometric amount required. A 94% yield of AA was obtained from trans-1,2-cyclohexandiol, and a slightly lower 92% yield from the cis isomer. The reaction proceeded via an initial C—H bond fission of the secondary carbinol to form the related a-ketol, followed by oxidative cleavage of the latter to yield a keto acid intermediate. 

Isoleucine and valine. The first four reactions in the degradation of isoleucine and valine are identical. Initially, both amino acids undergo transamination reactions to form a-keto-/T methyl valerate and a-ketoiso valerate, respectively. This is followed by the formation of CoA derivatives, and oxidative decarboxylation, oxidation, and dehydration reactions. The product of the isoleucine pathway is then hydrated, dehydrogenated, and cleaved to form acetyl-CoA and propionyl-CoA. In the valine degradative pathway the a-keto acid intermediate is converted into propionyl-CoA after a double bond is hydrated and CoA is removed by hydrolysis. After the formation of an aldehyde by the oxidation of the hydroxyl group, propionyl-CoA is produced as a new thioester is formed during an oxidative decarboxylation. 

Such a transition state might be significantly stabilized by the neighboring carbonyl function, and it would explain the requirement for an a-keto acid intermediate in the uridine diphosphoglucuronate decarboxylase reaction. 

The hypothesis that branched amino acids are metabolized to branched fatty acids via a-keto acid intermediates is further supported by incubations with deuterated 2-oxo-3-methylpentanoate and 2-oxo-4-methylpentanoate/ the transamination products of lie and Leu respectively. GC/MS of acyl groups resulting from incubation with d2 -2-oxo-4-methylpentanoate (deuterated at C-3) revealed deuterium incorporation into glucose esters containing d2-3-methylbutyrate and d2-9-methyldecanoate. Trace amounts of d2-7-methyloctanoate were also observed, supporting the hypothesis that branched chain precursors are elongated by the addition of acetate. 

The initial aims of this study were to test the full-length glycine-containing substrate with BaeJ KSl and examine the tolerance towards other amino acids. The glycine, alanine and valine derived a-keto acid intermediates 27-29 were... 

Work 197) has suggested that the divergence of lysine from the usual patterns of amino acid biosynthesis is due to the ease of ring closure of the six-carbon c-amino-a-keto acid intermediate. To avoid ring closure, bacteria have developed the synthesis of a C, compound and then de-carboxylate this to lysine. 

The NAD- and NADP-dependent dehydrogenases catalyze at least six different types of reactions simple hydride transfer, deamination of an amino acid to form an a-keto acid, oxidation of /3-hydroxy acids followed by decarboxylation of the /3-keto acid intermediate, oxidation of aldehydes, reduction of isolated double bonds, and the oxidation of carbon-nitrogen bonds (as with dihydrofolate reductase). 

Azlactones like 6 are mainly used as intermediates in the synthesis of a-amino acids and a-keto acids. The Erlenmeyer-Pldchl reaction takes place under milder conditions than the Perkin reaction. 

Yet a third method for the synthesis of a-amino acids is by reductive amination of an a-keto acid with ammonia and a reducing agent. Alanine, for instance, is prepared by treatment of pyruvic acid with ammonia in the presence of NaBH As described in Section 24.6, the reaction proceeds through formation of an intermediate imine that is then reduced. 

Figure 30-11. Intermediates in i-hydroxyproline catabolism. (a-KA, a-keto acid a-AA, a-amino acid.) Numerals identify sites of metabolic defects in hyperhydroxyprolinemia and type II hyperprolinemia.

Transketolase (TKase) [EC 2.2.1.1] essentially catalyzes the transfer of C-2 unit from D-xylulose-5-phosphate to ribose-5-phosphate to give D-sedoheptulose-7-phosphate, via a thiazolium intermediate as shown in Fig. 16. An important discovery was that hydroxypyruvate works as the donor substrate and the reaction proceeds irreversibly via a loss of carbon dioxide (Fig. 17). In this chapter, we put emphasis on the synthesis with hydroxypyruvate, as it is the typical TPP-mediated decarboxylation reaction of a-keto acid. ... 

Alternative methods to access the same Breslow intermediate using NHCs utilising acyl silanes [3] or a-keto-acids have been developed [4], although these processes have not been utilised in asymmetric transformations to date. 

Recently, bacterial NRPS modules with the organization of A-KR-PCP have been discovered in the valino-mycin and cereulide synthetases. The A domains of these modules selectively activate a-keto acids. After the resulting adenylate is transferred to the PCP domain, the a-ketoacyl- -PCP intermediate is reduced to a PCP-bound, a-hydroxythioester by the KR domain. These domains use NAD(P)H as a cofactor and are inserted into A domains between two conserved core motifs analogous to MT domains. Their substrate specificity differs from that of polyketide synthase KR domains, which reduce /3-ketoacyl substrates. Similar fungal NRPSs, such as beauvericin synthetase, utilize A domains that selectively activate a-hydroxy acids. These molecules are thought to be obtained using an in trans KR domain, which directly reduces the necessary, soluble a-keto acid. 

More drastic hydrolysis conditions of unsaturated oxazolones 448 leads to further hydrolysis of the intermediate 2-acylamino-2-alkenoic acid 449 and produces the corresponding a-keto acids 450. For example, phenylpyruvic acid " and other aryl(heteroaryl)pyruvic acids of biological interest have been obtained in this manner (Scheme 7.148). 

The a-keto acids are extremely versatile intermediates. For example, reduction of an arylpyruvic acid 450 yields the corresponding p-aryllactic acid 451, condensation of 450 with amines followed by reduction affords amino acid derivatives 454, ° and condensation of 450 with hydroxylamine yields a-oximi-noacids 452 as shown in Scheme 7.149. ... 

Another regulatory mechanism involves the nocturnal inhibitor 2-carboxyarabinitol 1-phosphate, a naturally occurring transition-state analog (see Box 6-3) with a structure similar to that of the /3-keto acid intermediate of the rubisco reaction (Fig. 20-7 see also Fig. 20-20). This compound, synthesized in the dark in some plants, is a potent inhibitor of carbamoylated ru-bisco. It is either broken down when light returns or is expelled by rubisco activase, activating the rubisco. 

Oxidative decarboxylations of a-keto acids are mediated by either enzymes having more than one cofactor or complex multienzyme systems utilizing a number of cofactors. For example, pyruvate oxidase uses TPP and FAD as coenzymes, the function of the latter being to oxidize the intermediate (41). Conversion of pyruvate to acetyl-CoA requires a multienzyme complex with the involvement of no less than five coenzymes, TPP, CoA, dihydrolipoate, FAD and NAD+ (74ACR40). 

When the substrate is substituted at the /3 carbon with a potential leaving group, such as —OH, —SH, —0P033-(see fig. 10.3d), the corresponding a-carbanion intermediate (see fig. 10.4d) can eliminate the group. This is an essential step in a,/3 eliminations. Upon hydrolysis, the elimination intermediate produces pyridoxal-5 -phosphate and the substrate-derived enamine, which spontaneously hydrolyzes to ammonia and an a-keto acid. 

The full series of intermediates in a transamination is shown in figure 10.5a. After protonation at the aldimine carbon of pyridoxal-5 -phosphate (step 3), hydrolysis (step 4) forms an a-keto acid and pyridoxamine-5 -phosphate. The reverse of this sequence with a second a-keto acid (steps 5 through 8) completes the transamination reaction. 

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. 

Terminal alkynes under these conditions undergo oxidative cleavage to the carboxylic acid, presumably because the intermediate keto aldehyde would yield the unstable a-keto acid. 

The simplest member of the series of aliphatic a-keto acids is pyruvic acid. It is conveniently prepared by the distillation of tartaric acid with a dehydrating agent such as postassium hydrogen sulphate (Expt 5.173). The reaction probably involves dehydration to the tautomeric oxaloacetic acid (13) intermediate, which then decarboxylates by virtue of its constitution as a / -keto acid. 

Asymmetric transamination.2 This planar chiral pyridoxamine analog in the presence of Zn(C104), (l/Zn(C104)2 = 1.0.5) converts a-keto acids into (R)-amino tieids in 60 96%ee. Use of (R)-l in place of (S)-l produces (S)-amino acids with the wime elliciency. Chemical yields range from 50 75%. The preferred solvent is tnel li.mol. The pyridoxal-type analog is recovered in 75-85%yield. The transamination is considered to involve kinetically controlled stereoselective protonation of an octahedral Ztr 1 chelate intermediate. 

First steps to elucidate the reaction mechanism of PDC were achieved by the investigation of model reactions using ThDP or thiamine [36,37], Besides the identification of C2-ThDP as the catalytic center of the cofactor [36], the mechanism of the ThDP-catalyzed decarboxylation of a-keto acids as well as the formation of acyloins was explained by the formation of a common reaction intermediate, active acetaldehyde . This active species was first identified as HEThDP 7 (Scheme 3) [38,39]. Later studies revealed the a-carbanion/enamine 6 as the most likely candidate for the active acetaldehyde [40 47] (for a comprehensive review see [48]). The relevance of different functional groups in the ThDP-molecule for the enzymatic catalysis was elucidated by site-directed substitutions of the cofactor ThDP by chemical means (for a review see... 

Despite the limited information about the coordination environment of the metal center, a mechanism for the a-keto acid-dependent enzymes has been proposed (Figure 27) [222] in which the a-keto acid binds to the iron center and primes it for 02 binding. Attack of the bound 02 on the coordinated a-keto acid at the C-2 position results in decarboxylation of the a-keto acid to give an Fe(II)-peroxy derivative that can react with substrate either directly or via a high-valent iron-oxo intermediate to give the oxygenated substrate, a carboxylic acid, and the starting Fe(II) enzyme. 

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