Dihydrolipoate-Lipoate

The protein contains a selenocysteine residue. Steroid and lipid hydroperoxides, but not the product of reaction of EC 1.13.11.12 on phospholipids, can act as acceptor, but more slowly than H2O2 (cf. EC 1.11.1.12) 

Trolox (5(X) pg/kg i.p.) protected lung tissues of Swiss albino female mice from sulphur mustard (1 LC50 = 42.3 lag/m for 1 h duration) intoxication (Kumar et al. 2001). 

Epigallocatechin gallate suppressed the initiation rate and prolonged the lag phase duration of per-oxyl radical-induced oxidation in a phospholipid liposome model to a greater extent (P 0.01) compared to both Trolox and a-tocopherol (Hu and Kitts 2001). 

The dihydrolipoic acid/lipoic acid redox couple has been found to exert a synergistic action in the antioxidant recycling mechanisms of natural membranes and low-density lipoproteins in vitro and in the protection against oxidative injury in vivo (Packer 1991). 

Combined treatment with D,L-a-lipoic acid and meso-2,3-dimercaptosuccinic acid reversed the decreases in the activities of renal y-glutamyl transferase (EC 2.3.2.2) and N-acetyl P-D-glucosaminidase (EC 3.2.1.17) evident in rats after the administration of lead citrate in drinking water for 5 weeks (Sivaprasad et al. 2002). 

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Fuchs, J. and Milbradt, R., Antioxidant inhibition of skin inflammation induced by reactive oxidants evaluation of the redox couple dihydrolipoate/lipoate, Skin Pharmacol., 1, 278, 1994. 

The P-protein consists of two identical 100-kDa subunits, each containing a molecule of PLP. This protein is a glycine decarboxylase which, however, replaces the lost CO2 by an electrophilic sulfur of lipoate rather than by a proton. Serine hydroxymethyltransferase can also catalyze this step of the sequence. The lipoate is bound to a second protein, the H-protein. A third protein, the T-protein, carries boxmd tetrahydrofolate which displaces the aminomethyl group from the dihydrolipoate and converts it to N, N -methylene... 

In the first step of the mechanism for the FAD-catalyzed oxidation of dihydrolipoate to lipoate, the thiolate ion attacks the C-4a position of the flavin ring. This reaction is general-acid catalyzed—as the thiolate ion attacks the ring, a proton is donated to the N-5 nitrogen. A second nucleophilic attack by a thiolate ion, this time on the sulfur that is covalently attached to the coenzyme, generates the oxidized product and FADH2. Section 25.4 discusses where this FAD-catalyzed reaction fits into metabolism. 

The first enzyme in the system catalyzes the reaction of TPP with pyruvate to form the same resonance-stabilized carbanion formed by pyruvate decarboxylase and by the enzyme in Problems 8 and 9. The second enzyme of the system (E2) requires lipoate, a coenzyme that becomes attached to its enzyme by forming an amide with the amino group of a lysine side chain. The disulfide linkage of lipoate is cleaved when it undergoes nucleophilic attack by the carbanion. In the next step, the TPP carbanion is eliminated from the tetrahedral intermediate. Coenzyme A (CoASH) reacts with the thioester in a transthioesterification reaction (one thioester is converted into another), substituting coenzyme A for dihydrolipoate. At this point, the final reaction product (acetyl-CoA) has been formed. However, before another catalytic cycle can occur, dihydrolipoate must be oxidized back to lipoate. This is done by the third enzyme (E3), an FAD-requiring enzyme (Section 25.3). Oxidation... 

The biotin and lipoate synthases catalyze similar reactions, the insertion of sulfur into unactivated C—bonds to generate essential cofactors (Figures 8C and 8D). The substrate in the case of biotin synthase is dethiobiotin, with a single sulfur inserted into two C— FI bonds to generate the tetrahydrothiophene ring of biotin. In the case of lipoate synthase, two atoms of sulfur are inserted, one each into the C—bonds at positions 6 and 8 of octanoic acid to produce dihydrolipoate, which is typically isolated in the oxidized form shown in Figure 8D. (The actual... 

Figure 11.2 Reaction sequences catalyzed by 2-oxoacid dehydrogenase complex Pyruvate dehydrogenase complex (PDC) and a-ketoglutarate dehydrogenase complex (aKGDC) catalyze the oxidative decarboxylation of pyruvate (R = CH3) and a-ketoglutarate (R = CH2CH2COOH) to Acetyl-CoA and succinyl CoA respectively. Three component enzymes 2-oxoacid (pyruvate/a-ketoglutarate) decarboxylase, lipoate acetyltransferase/succinyltransferase, dihydrolipoate dehydrogenase as well as five cofactors, namely (1) thiamine pyrophosphate (TPP) and its acylated form, (2) lipoamide (LipS2), reduced form and acylated form, (3) flavin adenine dinucleotide (FAD) and its reduced form, (4) nicotinamide adenine dinucleotide (NAD ) and its reduced form, and (5) coenzyme A (CoASH) and its acylated product are involved.

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 mechanism proposed for the FAD-catalyzed oxidation of dihydrolipoate to lipoate is shown here. 

The mechanism proposed for the FAD-catalyzed oxidation of succinate to fiimarate is similar to the mechanism you have just seen for the FAD-catalyzed oxidation of dihydrolipoate to lipoate. 

Before another catalytic cycle can occur, dihydrolipoate must be oxidized back to lipoate. This is done by the third enzyme (E3), an FAD-requiring enzyme. We saw the mechanism for this reaction in Section 24.2. When dihydrolipoate is oxidized by FAD, the coenzyme is reduced to FADH2. 

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