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Oxidative Decarboxylation of a-Oxoacids

While Tetrahymena must have lipoic acid in its diet, we humans can make our own, and it is not considered a vitamin. Lipoic acid is present in tissues in extraordinarily small amounts. Its major function is to participate in the oxidative decarboxylation of a-oxoacids but it also plays an essential role in glycine catabolism in the human body as well as in plants.295 296 The structure is simple, and the functional group is clearly the cyclic disulfide which swings on the end of a long arm. Like biotin, which is also present in tissues in very small amounts, lipoic acid is bound in covalent amide linkage to lysine side chains in active sites of enzymes 2963... [Pg.795]

Figure 15-16 Two systems for oxidative decarboxylation of a-oxoacids and for "substrate-level" phosphorylation. The value of AG = +34.5 kj mol-1 (Table 6-6) was used for the synthesis of ATP4- from ADP3- and HP042 in computing the values of AG given. Figure 15-16 Two systems for oxidative decarboxylation of a-oxoacids and for "substrate-level" phosphorylation. The value of AG = +34.5 kj mol-1 (Table 6-6) was used for the synthesis of ATP4- from ADP3- and HP042 in computing the values of AG given.
Oxidation-reduction reactions coenzymes of 765 - 827 Oxidative decarboxylation of a-oxoacids 511, 736, 796-802... [Pg.926]

The intermediary metabolism has multienzyme complexes which, in a complex reaction, catalyze the oxidative decarboxylation of 2-oxoacids and the transfer to coenzyme A of the acyl residue produced. NAD" acts as the electron acceptor. In addition, thiamine diphosphate, lipoamide, and FAD are also involved in the reaction. The oxoacid dehydrogenases include a) the pyruvate dehydrogenase complex (PDH, pyruvate acetyl CoA), b) the 2-oxoglutarate dehydrogenase complex of the tricarboxylic acid cycle (ODH, 2-oxoglutarate succinyl CoA), and c) the branched chain dehydrogenase complex, which is involved in the catabolism of valine, leucine, and isoleucine (see p. 414). [Pg.134]

The unique function of lipoic acid is in the oxidation of the thiamin-bound active aldehyde (Fig. 15-15) in such a way that when the complex with thiamin breaks up, the acyl group formed by the oxidative decarboxylation of the oxoacid is attached to the... [Pg.796]

When present in excess methionine is toxic and must be removed. Transamination to the corresponding 2-oxoacid (Fig. 24-16, step c) occurs in both animals and plants. Oxidative decarboxylation of this oxoacid initiates a major catabolic pathway,305 which probably involves (3 oxidation of the resulting acyl-CoA. In bacteria another catabolic reaction of methionine is y-elimination of methanethiol and deamination to 2-oxobutyrate (reaction d, Fig. 24-16 Fig. 14-7).306 Conversion to homocysteine, via the transmethylation pathway, is also a major catabolic route which is especially important because of the toxicity of excess homocysteine. A hereditary deficiency of cystathionine (3-synthase is associated with greatly elevated homocysteine concentrations in blood and urine and often disastrous early cardiovascular disease.299,307 309b About 5-7% of the general population has an increased level of homocysteine and is also at increased risk of artery disease. An adequate intake of vitamin B6 and especially of folic acid, which is needed for recycling of homocysteine to methionine, is helpful. However, if methionine is in excess it must be removed via the previously discussed transsulfuration pathway (Fig. 24-16, steps h and z ).310 The products are cysteine and 2-oxobutyrate. The latter can be oxidatively decarboxylated to propionyl-CoA and further metabolized, or it can be converted into leucine (Fig. 24-17) and cysteine may be converted to glutathione.2993... [Pg.1389]

In a rare autosomal recessive condition (discovered in 1954) the urine and perspiration has a maple syrup odor/ High concentrations of the branched-chain 2-oxoacids formed by transamination of valine, leucine, and isoleucine are present, and the odor arises from decomposition products of these acids. The branched-chain amino acids as well as the related alcohols also accumulate in the blood and are found in the urine. The biochemical defect lies in the enzyme catalyzing oxidative decarboxylation of the oxoacids, as is indicated in Fig. 24-18. Insertions, deletions, and substitutions may be present in any of the subunits (Figs. 15-14,15-15). The disease which may affect one person in 200,000, is usually fatal in early childhood if untreated. Children suffer seizures, mental retardation, and coma. They may survive on a low-protein (gelatin) diet supplemented with essential amino acids, but treatment is difficult and a sudden relapse is apt to prove fatal. Some patients respond to administration of thiamin at 20 times the normal daily requirement. The branched-chain oxoacid dehydrogenase from some of these children shows a reduced affinity for the essential coenzyme thiamin diphosphate.d... [Pg.1394]

Oxidative decarboxylation of an a-oxoacid with thiamin diphosphate... [Pg.800]

Polled hereford calves in Australia develop maple syrup urine disease relatively often/ 6 One cause was established as a mutation that introduces a stop codon that causes premature termination within the leader peptide during synthesis of the thiamin diphosphate-dependent El subunit. A similar biochemical defect in a mutant of Bacillus subtilis causes difficulties for this bacterium, which requires branched-chain fatty acids in its membranes. Branched acyl-CoA derivatives are needed as starter pieces for their synthesis (Chapter 29). With the oxidative decarboxylation of the necessary oxoacids blocked, the mutant is unable to grow unless supplemented with branched-chain fatty acids. [Pg.1394]

There are two 2-oxoacid dehydrogenase multienzyme complexes in E. coli. One is specific for pyruvate, the other for 2-oxoglutarate. Each complex is about the size of a ribosome, about 300 A across. The pyruvate dehydrogenase is composed of three types of polypeptide chains El, the pyruvate decarboxylase (an a2 dimer of Mr — 2 X 100 000) E2, lipoate acetyltransferase (Mr = 80 000) and E3, lipoamide dehydrogenase (an a2 dimer of Mr = 2 X 56 000). These catalyze the oxidative decarboxylation of pyruvate via reactions 1.6, 1.7, and 1.8. (The relevant chemistry of the reactions of thiamine pyrophosphate [TPP], hydroxyethylthiamine pyrophosphate [HETPPJ, and lipoic acid [lip-S2] is discussed in detail in Chapter 2, section C3.)... [Pg.356]

In aerobic environments, eukaryotes and many eubacteria oxidatively decarboxylate the 2-oxoacids via pyruvate and 2-oxoglutarate dehydrogenase complexes [36]. For comparison with the archaebacterial oxidoreductases, the catalytic mechanism of these complexes is also shown in Fig. 4. Three enzymic activities are involved, catalysed by three distinct enzymes a 2-oxoacid decarboxylase (El), a dihydrolipoyl acyltransferase (E2) and dihydrolipoamide dehydrogenase (E3). Multiple copies of these three enzymes are found in each complex molecule, resulting in relative molecular masses in excess of 2x10 ... [Pg.7]


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A decarboxylation

A oxidative decarboxylation

Acid and the Oxidative Decarboxylation of a-Oxoacids

Decarboxylation of 3-oxoacids

Decarboxylation oxide

Decarboxylative oxidation

Of 4-oxoacid

Oxidation oxidative decarboxylation

Oxidative decarboxylation

Oxidative decarboxylation of a-oxoacid

Oxidative decarboxylation of a-oxoacid

Oxidative decarboxylation of a-oxoacid by hydrogen peroxide

Oxidative decarboxylation of a-oxoacid mechanisms

Oxidative decarboxylation of a-oxoacid with thiamin diphosphate

Oxidative oxoacids

Oxoacidic

Oxoacidity

Oxoacids

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