Oxidative decarboxylation of a-keto acids

TPP-dependent enzymes are involved in oxidative decarboxylation of a-keto acids, making them available for energy metabolism. Transketolase is involved in the formation of NADPH and pentose in the pentose phosphate pathway. This reaction is important for several other synthetic pathways. It is furthermore assumed that the above-mentioned enzymes are involved in the function of neurotransmitters and nerve conduction, though the exact mechanisms remain unclear. 

Summarizing the results obtained by controlled potential electrolysis and polarography, the reaction process for the electrolytic evolution of CO2 was estimated to be as follows the first step was one electron transfer from DMFC in NB to FMN in W as in Eq. (7). The second step was the catalytic reduction of O2 by FMNH as in Eq. (8). The final step was the oxidation of pyruvic acid by the reduction product of O2, H2O2, in W as in Eq. (9), well-known as an oxidative decarboxylation of a-keto acids [43] ... 

Two sources of acyl radicals have proved to be useful for the homolytic acylation of protonated heteroaromatic bases the oxidation of aldehydes and the oxidative decarboxylation of a-keto acids. The oxidation... 

Protonated pyridines and derivatives readily undergo acylation at C-2 or C-4 (Table 28) (76MI20503). Acyl radicals are usually generated either by hydrogen abstraction from aldehydes (Scheme 210), or by oxidative decarboxylation of a-keto acids (Scheme 211). In the former case (Scheme 210) with acridine as the substrate, reduction can take place to give a dihydroacridine. 

Acyl radicals obtained by the oxidation of aldehydes or the oxidative decarboxylation of a-keto acids react selectively at the a- or y-position of the protonated heterocyclic nitrogen. Pyridines, quinolines, pyrazines and quinoxalines all react as expected yields are typically 40 to 70%. Similarly, pyridines can be carbamoylated in acid media at C-2 (Scheme 38). 

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). 

The a-keto acid-dependent enzymes [206-208] catalyze a diverse array of reactions (Figure 26) involving functionalization of an unactivated C—H bond concomitant with the oxidative decarboxylation of a keto acid. For the hydroxylation... 

Electron transfer between pyridine nucleotides and disulfide compounds is catalyzed by several fiavoproteins and three of these are well characterized. Lipoamide dehydrogenase functions in the oxidative decarboxylation of a-keto acids catalyzing the reoxidation of reduced lipoate by NAD+ (18, 19). Glutathione reductase catalyzes electron transfer between NADPH and glutathione ZO-22). Thioredoxin reductase catalyzes the reduction of thioredoxin by NADPH (5) thioredoxin is a protein of 12,000 molecular weight containing a single cystine residue which is the electron acceptor S3). 

Oxidative decarboxylation of a-keto acids such as pyruvate and a -ketoglutarate. 

Figure 4-9. Role of lipoic acid in oxidative decarboxylation of a-keto acids.

Thiamine is required by the body as the pyrophosphate (TPP) in two general types of reaction, the oxidative decarboxylation of a keto acids catalyzed by dehydrogenase complexes and the formation of a-ketols (ketoses) as catalyzed by transketolase, and as the triphosphate (TTP) within the nervous system. TPP functions as the Mg -coordinated coenzyme for so-called active aldehyde transfers in mul-tienzyme dehydrogenase complexes that affect decarboxyia-tive conversion of a-keto (2 oxo) acids to acyl-coenzyme A (acyl-CoA) derivatives, such as pyruvate dehydrogenase and a-ketoglutarate dehydrogenase. These are often localized in the mitochondria, where efficient use in the Krebs tricarboxylic acid (citric acid) cycle follows. 

Thiamine pyrophosphate has two important coenzyme roles, both of which focus mostly on carbohydrate metabolism (Figs. 8.26 and 8.27). The active portion of the coen- rae is the thiazole ring. The first step in the oxidative decarboxylation of a-keto acids requires TPP. The two most common examples are pyruvate and a-ketoglutarate, oxidatively decarboxyatedto acetyl CoA and succinyl CoA, respectively. The same reaction is found in the metabolism of the branched-chain amino acids valine, isoleucine, leucine, and methionine. In all cases, TPP is a coenzyme in a mitochondrial multienzyme complex, consisting of TPP, lipoic acid, coenzyme A, FAD, and NAD. Note the number of vitamins required for the oxidative decarboxylation of a-keto acids thiamine (TPP), pantothenic acid (coenzyme A), riboflavin (FAD),and niacin (NAD). 

Lipoic acid participates in the coenzyme A (CoA)- and diphosphopyri-diiie nucleotide (DPN)-linkcd oxidative decarboxylation of a-keto acids [Eq. (3)]. There are alternate pathways of a-keto acid oxidation which do... 

Volatile fatty acids p resent in wine may derive from the anabolism of lipids, resulting in compounds with even number of carbon atoms, by oxidative decarboxylation of a-keto acids or by the oxidation of aldehydes. Volatile fatty acids synthesised from a-keto acids are mainly propanoic add, 2-methyl-l-propanoic acid (isobutyric acid), 2-methyl-l-butanoic acid, 3-methyl-l-butanoic acid (isovaleric acid 3-methylbutyric add) and phenylacetic add. From lipid metabolism, the following fatty acids are reported butanoic add (butyric), hexanoic acid (caproic), odanoic acid (caprylic) and decanoic add (capric) (Dubois, 1994). Although fatty adds are charaderized by unpleasant notes (Table 1), only few compounds of this family attain its perception threshold. However, their flavour is essential to the aromatic equilibrium of wines (Etievant, 1991). 

Lipoic acid is a five-membered cyclic disulphide ring with a five-carbon carboxylic add chain. When reduced it provides a constrained dithiol centre. This disulphide-dithiol cofactor is covalently bound to one of the enzymes in a multienzyme complex which catalyses oxidative decarboxylation of a-keto acids. In the course of the reaction three forms of the prosthetic group participate the cyclic disulphide, the dithiol and a thio-ester of the dithiol form. 

Study of the mechanism of the oxidation of pyruvic acid by certain bacteria led to the discovery of lipoic acid (thioctic acid) as a nutrient metabolite essential for the oxidative decarboxylation of a-keto acids. It subsequently was determined that the acetate-replacing factor " for lactic acid bacteria and protogen,a growth factor for the protozoan, Tetrahymena gelii, were also identical with lipoic acid. The occurrence of considerable quantities of lipoic acid in mammalian preparations of pyruvate and a-ketoglutarate oxidases suggests that it has the same function in animal tissues as in microorganisms. ... 

The lipoic acids are believed to have a general function in the oxidative decarboxylation of a-keto acids.The outstanding example is pyruvic acid, but it also functions in the oxidation of a-ketoglutaric and a-ketobutyric acids. The latter acid is employed as a substrate to study the characteristics of the enzyme system to avoid the complicating effect of acetoin formation which occurs with pyruvate. ... 

Thiamine pyrophosphate (TPP) is the active form of thiamine in the cells of all organisms and, as such, is important as a coenzyme in the decarboxylation of a-keto acids, as well as in the biosynthesis of certain acyloins. In view of the recent findings of Reed and De Busk (see section on pyruvate oxidation factor) TPP may form a complex with a-lipoic acid, which may be the coenzyme for the oxidative decarboxylation of a-keto acids. 

Recently Reed and DeBusk have isolated from natural materials a bound form of lipoic acid which seems to be somewhat closer to the coenzyme form. Analysis of this compound as well as its preparation from synthetic materials indicates that it is the amide of thiamine and lipoic acid (lipothiamide). This important discovery may eventually cast some light upon the mechanism of action of lipoic acid. It is significant that lipothiamide can restore pyruvate oxidation to deficient cells, and the authors conclude that lipothiamide may be part of the coenzyme for the oxidative decarboxylation of a-keto acids. 

Even more recently, Reed and DeBusk have observed that lipothiamide pyrophosphate is required for pyruvate dismutation and a-keto-glutarate oxidation by soluble enzyme preparations from an E. coli mutant. They suggest that the initial step in the oxidative decarboxylation of a-keto acids is the formation of an acyl lipothiamide pyrophosphate complex. In a subsequent step, they assume that the acyl group is transferred to CoA. These observations cast considerable light on the mechanism of the oxidative decarboxylation of a-keto acids and certainly may explain some of the difficulties already discussed in the section on TPP. However it remains to be seen whethei lipothiamide pyrophosphate carries the acyl group as a thioester as in CoA, and whether it acts exclusively as an acyl carrier and not also as a hydrogen carrier. One point which is still not clear is whether the oxidation and decarboxylation occur simultaneously or in an orderly sequence. 

Figure 3 The mechanism of oxidative decarboxylation of a-keto acids. Ej, a-keto acid dehydrogenase Ej, lipoate acyltransferase E, lipoamide dehydrogenase. R CH3, HOOC-CH2CH2" R-CH(OH)-TPP, active aldehyde LipS2, lipoate Lip (SH)2, dihydrolipoate RCOS-Lip SH, 6-acyldihydrolipoate [ ], coenzymes bound to the enzyme proteins.

The most important reaction in which thiamine pyrophosphate collaborates is the oxidative decarboxylation of a-keto acids (cf. formulas in Chapt. VI-4, VIII-10, and XI-2). CO2 is split off and the aldehyde residue is transferred by thiamine... 

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