Lipoic acid dehydrogenase

In fact, this 1 1 complex Cu+DDC serves as a "copper carrier", bringing it to the copper-susceptible intracellular system, which is the dithiol compound lipoic acid or the dithiol system lipoic acid dehydrogenase ... 

The most arsenic-sensitive of known enzymes are (a) lipoic acid dehydrogenase, and (b) the a-oxidases (e.g. pyruvate oxidase) which use dihydrolipoic acid 2,28) (thioctic acid) as a coenzyme. Lipoic acid dehydrogenase has two cysteine residues on different chains of protein which are kept adjacent by tertiary folding (Massey, Hofmann and Palmer, 1962). Arsenicals can bind these cysteine thiol-groups as in 13.1). In (b), the lipoic acid (6,8-dimercapto-octanoic acid) is easily bridged by arsenicals as in 13.1) (Peters, 1963). 

In any event, the formation of acetyl-S lipoic acid is followed by a thiol-ester interchange reaction in which the acetyl group is transferred to CoASH with the formation of acetyl-SCoA and reduced lipoic acid (32, 33). The thioltransacetylase that catalyzes this reaction has been partially purified from extracts of E. coli by Gunsalus and his co-workers (32). Finally, in the presence of a specific lipoic-acid dehydrogenase, the reduced lipoic acid is oxidized by DPN back to the ring disulfide form. The over-all reaction is the oxidation of pyruvate in the presence of CoASH to acetyl-SCoA and CO2 [reaction (8)]. 

Reaction 4 depends upon the operation of a DPN-dependent, lipoic acid dehydrogenase. A protein fraction has been obtained from E. coli which exhibits this property. Measurement of lipoic acid dehydrogenase activity directly with DPN+ and reduced lipoic acid as reductant or reoxidation of DPNH with a-lipoic acid as oxidant was not particularly successful. However, lipoic acid dehydrogenase activity could readily be determined by linking it with lactic acid dehydrogenase in the presence of pyruvate and catalytic amounts of DPN, by measuring the residual —SH groups. 

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 pyruvate dehydrogenase complex (PDC) is a noncovalent assembly of three different enzymes operating in concert to catalyze successive steps in the conversion of pyruvate to acetyl-CoA. The active sites of ail three enzymes are not far removed from one another, and the product of the first enzyme is passed directly to the second enzyme and so on, without diffusion of substrates and products through the solution. The overall reaction (see A Deeper Look Reaction Mechanism of the Pyruvate Dehydrogenase Complex ) involves a total of five coenzymes thiamine pyrophosphate, coenzyme A, lipoic acid, NAD+, and FAD. 

The mechanism of the pyruvate dehydrogenase reaction is a tour de force of mechanistic chemistry, involving as it does a total of three enzymes (a) and five different coenzymes—thiamine pyrophosphate, lipoic acid, coenzyme A, FAD, and NAD (b). 

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.)...

Arsenite and mercuric ions react with the —SH groups of lipoic acid and inhibit pyruvate dehydrogenase, as... 

Lipoic acid (the other names are a-lipoic acid or thioctic acid) (Figure 29.9) is a natural compound, which presents in most kinds of cells. Lipoic acid (LA) is contained in many food products, in particular in meat, but it is also synthesized in human organism from fatty acids. Earlier, it has been shown that in humans lipoic acid functions as a component of the pyruvate dehydrogenase complex. However, later on, attention has been drawn to the possible antioxidant activity of the reduced form of lipoic acid, dihydrolipoic acid (DHLA) (Figure 29.9). 

The PDHC catalyzes the irreversible conversion of pyruvate to acetyl-CoA (Fig. 42-3) and is dependent on thiamine and lipoic acid as cofactors (see Ch. 35). The complex has five enzymes three subserving a catalytic function and two subserving a regulatory role. The catalytic components include PDH, El dihydrolipoyl trans-acetylase, E2 and dihydrolipoyl dehydrogenase, E3. The two regulatory enzymes include PDH-specific kinase and phospho-PDH-specific phosphatase. The multienzyme complex contains nine protein subunits, including... 

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... 

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... 

PYRUVATE DEHYDROGENASE LIPOIC ACID AND DERIVATIVES Lipolysis,... 

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

Figure 7-1. Conversion of pyruvate to acetyl CoA by the pyruvate dehydrogenase complex. The three enzymes, pyruvate dehydrogenase, dihydrolipoyl transacetylase, and dihydrolipoyl dehydrogenase, exist in a complex associated with the mitochondrial matrix. Each enzyme requires at least one coenzyme that participates in the reaction. TPP, thiamine pyrophosphate Lip, lipoic acid CoA, coenzyme A.

The answer is A. This patient exhibits several signs of acute arsenic exposure, including the cholera-like gastrointestinal symptoms and probable dehydration. He may currently be in hypovolemic shock and beginning chelation therapy is the only recourse. Arsenic is a metabolic toxin because it inhibits enzymes that require lipoic acid as a coenzyme the PDH complex, the a-ketoglutarate dehydrogenase complex, and trans-ketolase of the pentose phosphate pathway. 

CS causes alkylation of sulfhydryl-containing enzymes and inhibits lactic dehydrogenase, glutamic dehydrogenase, pyruvic decarboxy-. lase, and alpha-glycerophosphate dehydrogenase.24,40 it reacts with a number of nucleophilic compounds, such as glutathione, plasma protein, and lipoic acid.24... 

Fig. 9. A schematic drawing of a possible mechanism for the reaction catalyzed by the pyruvate dehydrogenase complex. The three enzymes Elf E2, and E3 are located so that lipoic acid covalently linked to E2 can rotate between the active sites containing thiamine pyrophosphate (TPP) and pyruvate (Pyr) on Elt CoA on E2, and FAD on E3. Acetyl-CoA and GTP are allosteric effectors of E, and NAD+ is an inhibitor of the overall reaction.

Coenzymes The pyruvate dehydrogenase complex contains five coenzymes that act as carriers or oxidants for the intermediates of the reactions shown in Figure 9.3. Ei requires thiamine pyrophosphate, Ep requires lipoic acid and coenzyme A, and E3 requires FAD and NAD+. [Note Deficiencies of thiamine or niacin can cause serious central nervous system problems. This is because brain cells are unable to produce sufficient ATP (via the TCA cycle) for proper function if pyruvate dehydrogenase is inactive.]... 

The sulfhydryl form of lipoic acid is oxidized by FAD-dependent dihydrolipoyl dehydrogenase, leading to the regeneration of oxidized lipoic acid. 

Mechanism of action of the pyruvate dehydrogenase complex. TPP = thiamine pyrophosphate L = lipoic acid. 

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