Lipoic acid oxidation

Answer TPP thiazolium ring adds to a carbon of pyruvate, then stabilizes the resulting car-banion by acting as an electron sink. Lipoic acid oxidizes pyruvate to level of acetate (acetyl-CoA), and activates acetate as a thioester. CoA-SH activates acetate as thioester. FAD oxidizes lipoic acid. NAD+ oxidizes FAD. (See Fig. 16-6.)... 

The thioredoxins appear to have a highly specific relationship with the enzyme carrying out ttieir reduction. Yeast thioredoxin for example is not reduced by the thioredoxin reductase from E. coli. In contrast reduced thioredoxins may donate electrons to a variety of acceptors. Reduced thioredoxin is a good general disulphide reductant. In combination with its reductase a disulphide reductase system is formed which is capable of reducing lipoic acid, oxidized glutathione and other similar structures. In these cases the thioredoxin-disulphide redox system does not appear to require additional enzymatic components. 

Fig. 60. The respiratory chain of higher plants. Ubiquinone appears to serve as an electron reservoir. = probable site of ATP formation. SD = succinate dehydrogenase. It used to be assumed that, with the exception of the reaction catalyzed by SD, the hydrogen acceptor in dehydrogenation reactions was NAD+ and that the hydrogen then entered the respiratory chain in the form of NADH+H+. In reality the situation is more complicated since the lipoic acid oxidizing flavoproteid of the pyruvate dehydrogenase and the a-ketoglutarate dehydrogenase complexes—in both cases the same flavoproteid is involved—can establish direct contact with the flavoproteins of the respiratory chain just like succinate dehydrogenase. associated with encircled flavoproteins means that ATP can be formed as a result of transitions between the various flavoproteins, except those involving SD.

Although a variety of oxidizing agents are available for this transformation it occurs so readily that thiols are slowly converted to disulfides by the oxygen m the air Dithiols give cyclic disulfides by intramolecular sulfur-sulfur bond formation An example of a cyclic disulfide is the coenzyme a lipoic acid The last step m the laboratory synthesis of a lipoic acid IS an iron(III) catalyzed oxidation of the dithiol shown... 

Lipoic acid exists as a mixture of two structures a closed-ring disulfide form and an open-chain reduced form (Figure 18.33). Oxidation-reduction cycles interconvert these two species. As is the case for biotin, lipoic acid does not often occur free in nature, but rather is covalently attached in amide linkage with lysine residues on enzymes. The enzyme that catalyzes the formation of the lipoamide nk.2Lg c requires ATP and produces lipoamide-enzyme conjugates, AMP, and pyrophosphate as products of the reaction. 

Lipoic acid is an acyl group carrier. It is found in pyruvate dehydrogenase zard a-ketoglutarate dehydrogenase, two multienzyme complexes involved in carbohydrate metabolism (Figure 18.34). Lipoie acid functions to couple acyl-group transfer and electron transfer during oxidation and decarboxylation of a-keto adds. 

The special properties of lipoic acid arise from the ring strain experienced by oxidized lipoic acid. The closed ring form is approximately 20 kj higher in energy than the open-chain form, and this results in a strong negative reduction potential of about —0.30 V. The oxidized form readily oxidizes cyanides to isothiocyanates and sulfhydryl groups to mixed disulfides. 

FIGURE 18.33 The oxidized and reduced forms of lipoic acid and the structure of the lipoic acid-lysine conjugate. 

Hydroxyetliyl group is transferred to lipoic acid and oxidized to form acetyl dihydro lipoamide... 

The reaction of hydroxyethyl-TPP with the oxidized form of lipoic acid yields the energy-rich thiol ester of reduced lipoic acid and results in oxidation of the hydroxyl-carbon of the two-carbon substrate unit (c). This is followed by nucleophilic attack by coenzyme A on the carbonyl-carbon (a characteristic feature of CoA chemistry). The result is transfer of the acetyl group from lipoic acid to CoA. The subsequent oxidation of lipoic acid is catalyzed by the FAD-dependent dihydrolipoyl dehydrogenase and NAD is reduced. 

Figure 17-5. Oxidative decarboxylation of pyruvate by the pyruvate dehydrogenase complex. Lipoic acid is joined by an amide link to a lysine residue of the transacetylase component of the enzyme complex. It forms a long flexible arm, allowing the lipoic acid prosthetic group to rotate sequentially between the active sites of each of the enzymes of the complex. (NAD nicotinamide adenine dinucleotide FAD, flavin adenine dinucleotide TDP, thiamin diphosphate.)...

As in the case of other cardiovascular diseases, the possibility of antioxidant treatment of diabetes mellitus has been studied in both animal models and diabetic patients. The treatment of streptozotocin-induced diabetic rats with a-lipoic acid reduced superoxide production by aorta and superoxide and peroxynitrite formation by arterioles providing circulation to the region of the sciatic nerve, suppressed lipid peroxidation in serum, and improved lens glutathione level [131]. In contrast, hydroxyethyl starch desferrioxamine had no effect on the markers of oxidative stress in diabetic rats. Lipoic acid also suppressed hyperglycemia and mitochondrial superoxide generation in hearts of glucose-treated rats [132],... 

Keenoy et al. [141] treated type 1 diabetic patients with Daflon 500, a mixture of flavonoids diosmin (90%) and hesperdin (10%). It was found that flavonoid therapy resulted in a decrease in the levels of the HbAic hemoglobin and the in vitro oxidability of non-HDL lipoproteins. Lipoic acid was found to improve microcirculation in patients with diabetic... 

The clinical significance of thiamine and its necessity for pyruvic acid oxidation has been discussed. Recent reports concerning the coenzyme function of thiamine in pentose (H13), tryptophan (D2), and lipoic acid metabolism (R6) have increased our knowledge of thiamine in metabolism and lend added interest to the role of thiamine in clinical problems. This method has also been used to assay thiamine in liver and brain. 

ATP and magnesium were required for the activation of acetate. Acetylations were inhibited by mercuric chloride suggesting an SH group was involved in the reaction either on the enzyme or, like lipoic acid, as a cofactor. Experiments from Lipmann s laboratory then demonstrated that a relatively heat-stable coenzyme was needed—a coenzyme for acetylation—coenzyme A (1945). The thiol-dependence appeared to be associated with the coenzyme. There was also a strong correlation between active coenzyme preparations and the presence in them of pantothenic acid—a widely distributed molecule which was a growth factor for some microorganisms and which, by 1942-1943, had been shown to be required for the oxidation of pyruvate. 

Direct evidence of the reaction of PAN with sulfhydryl compounds has since been obtained (PAN at 115 ppm for 1-10 min). - In the reaction with glutathione, the major products are oxidized glutathione (disulfide) and 5-acetylglutathione. Other sulfhydryl compounds (e.g., coenzyme A, lipoic acid, and cysteine) yield only oxidation products, with no evidence of 5-acetylation. However, acetylation reactions have been observed with alcohols and amines. Sulfur compounds other than thiols can undergo oxidation by PAN methionine is converted to methionine sulfoxide, and oxidized lipoic acid (disulfide) is converted to sulfoxide. 

Now this reaction is effectively a repeat of the pyruvate acetyl-CoA oxidative decarboxylation we saw at the beginning of the Krebs cycle. It similarly requires thiamine diphosphate, lipoic acid, coenzyme A and NAD+. A further feature in common with that reaction is that 2-oxoglutarate dehydrogenase is also an enzyme complex comprised of three separate enzyme activities. 2-Oxoglutarate is thus transformed into succinyl-CoA, with the loss of... 

In oxidative decarboxyiation of pyruvate to acetyi-CoA, the enzyme-bound disulfide-containing coenzyme lipoic acid is also involved. The electron-rich enamine intermediate, instead of accepting a proton, is used to attack a sulfur in the lipoic acid moiety. This leads to fission of the S-S bond, and thereby effectively reduces the lipoic acid fragment. Regeneration of the TPP ylid via the reverse aldol-type... 

In lipoic acid (6), an intramolecular disulfide bond functions as a redox-active structure. As a result of reduction, it is converted into the corresponding dithiol. As a prosthetic group, lipoic acid is usually covalently bound to a lysine residue (R) of the enzyme, and it is then referred to as lipoamide. Lipoamide is mainly involved in oxidative decarboxylation of 2-0X0 acids (see p. 134). The peptide coenzyme glutathione is a similar disulfide/ dithiol system (not shown see p. 284). 

An acyl-transfer and redox coenzyme containing two sulfhydryl groups that form a dithiolane ring in the oxidized (disulfide) form. The redox potential at pH 7 is -0.29 volts. Lipoic acid is attached to the e-amino group of lysyl residues of transacetylases (subunit of a-ketoacid dehydrogenase complexes), thereby permitting acyl... 

Dihydrolipoyl dehydrogenase transfers electrons from lipoic acid to NAD to form NADH and regenerate the oxidized form of lipoic acid. 

An interesting synthetic approach to thietanes is the selective desulfurization of cyclic disulfides.The treatment of dithiolanes with a diethyl-aminophosphine results in a ring contraction to thietanes, (Eq. 19). This has been demonstrated with a-lipoic acid, a coenzyme with a dithiolane structure involved in the biological oxidation of pyruvic acid. The reaction is proposed to be initiated by the electrophilic attack of the phosphorus on the ring sulfur atom, resulting in the formation of an acyclic internal phosphonium salt, which by subsequent elimination of a phosphine sulfide, closes to the four-membered ring. °... 

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