Dihydrolipoyl dehydrogenase

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. 

Klingbeil, M.M., Walker, D.J., Arnette, R., Sidawy, E., Hayton, K, Komuniecki, P.R. and Komuniecki, R. (1996) Identification of a novel dihydrolipoyl dehydrogenase-binding protein in the pyruvate dehydrogenase complex of the anaerobic parasitic nematode, Ascaris suum. Journal of Biological Chemistry 271, 5451-5457. 

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... 

PDH is a multi-enzyme complex consisting of three separate enzyme units pyruvate decarboxylase, transacetylase and dihydrolipoyl dehydrogenase. Serine residues within the decarboxylase subunit are the target for a kinase which causes inhibition of the PDH the inhibition can be rescued by a phosphatase. The PDH kinase (PDH-K) is itself activated, and the phosphatase reciprocally inhibited, by NADH and acetyl-CoA. Figure 3.12(a and b) show the role and control of PDH. 

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 pyruvate dehydrogenase complex from Escherichia coli is considerably more complex than tryptophan synthetase. It has a molecular weight of approximately 4.6 millon and contains three enzymes pyruvate dehydrogenase (Et), dihydrolipoyl transacetylase (E2), and dihydrolipoyl dehydrogenase (E3).82 The overall reaction catalyzed by the complex is... 

The PDH complex contains three enzymes—pyruvate dehydrogenase (EJ, dihydrolipoyl transacetylase (E2), and dihydrolipoyl dehydrogenase (E3)—each present in multiple copies. The number of copies of each enzyme and therefore the size of the complex varies among species. The PDH complex isolated from mammals is about 50 nm in diameter—more than five times the size of an entire ribosome and big enough to be visualized with the electron microscope (Fig. 16-5a). In the bovine enzyme, 60 identical copies of E2 form a pentagonal dodecahedron (the core) with a diameter of about 25 nm (Fig. 16-5b). (The core of the Escherichia coli enzyme contains 24 copies of E2.) E2 is the point of... 

The PDH complex is composed of multiple copies of three enzymes pyruvate dehydrogenase, Ei (with its bound cofactor TPP) dihydrolipoyl transacetylase, E2 (with its covalently bound lipoyl group) and dihydrolipoyl dehydrogenase, E3 (with its cofactors FAD and NAD). 

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

The reduced flavoprotein is reoxidized to FAD by dihydrolipoyl dehydrogenase as NAD+ is reduced. 

Dihydrolipoyl dehydrogenase (lipoamide dehydrogenase), glutathione reductase, and human thioredoxin reductase187 190 belong to a subclass... 

Further transfer of the acyl group to coenzyme A is catalyzed by the same enzyme. This displacement reaction produces reduced lipoic acid. A third enzyme, dihydrolipoyl dehydrogenase, catalyzes oxidation of this product back to the disulfide form. The electrons lost in that oxidation are transferred first to an enzyme-bound flavin (not shown in the figure) and then to NAD +. ... 

The conversion of pyruvate to acetyl-CoA. The reactions are catalyzed by the enzymes of the pyruvate dehydrogenase complex. This complex has three enzymes pyruvate decarboxylase, dihydrolipoyl transacetylase, and dihydrolipoyl dehydrogenase. In addition, five coenzymes are required thiamine pyrophosphate, lipoic acid, CoASH, FAD, and NAD+. Lipoic acid is covalently attached to... 

The enzyme complex involved in this reaction also is very similar to the pyruvate dehydrogenase complex. Indeed, the same dihydrolipoyl dehydrogenase subunit is used in both complexes. The product in both cases is the coenzyme A... 

In the reaction catalyzed by dihydrolipoyl dehydrogenase, one of three enzymes in the pyruvate dehydrogenase complex (fig. 13.5), electrons flow from oxidized lipoic acid to enzyme-bound FAD to NAD+. Compare the flow of electrons in the latter part of this scheme (FAD to NAD + ) to the flow of electrons in the electron transport scheme (Complex I). Is there a distinct difference in the flow of electrons in the two schemes If so, can you provide a possible explanation for this difference ... 

The lipoic acid is covalently attached to the transacetylase (E2). It acts as a swinging arm , interacting with pyruvate decarboxylase (E ) to accept the hydroxyethyl derivative of pyruvate, with CoA to produce acetyl-CoA, and with dihydrolipoyl dehydrogenase (E3) so that it can be reoxidized. 

Fig. 12-7 The reactions of the pyruvate dehydrogenase complex. The reactants in the overall reaction are shown in boxes. E, = pyruvate decarboxylase (TPP = thiamine pyrophosphate as prosthetic group), E2 = dihydrolipoyl trans-acetylase (oxidized lipoic acid as prosthetic group). E3 = dihydrolipoyl dehydrogenase (FAD as prosthetic group).

Complete pyruvic dehydrogenase complex containing 24 molecules of pyruvate dehydrogenase arranged on outside of dihydrolipoyl transace tylase-dihydrolipoyl dehydrogenase center... 

The multienzyme complexes eue seh-tissembling and will reeissemble to em active complex eifter resolution of the individued enzymes. The core enzyme of the complex is the dihydrolipoyl acyltransfereise (E2) the oxo-acid dehydrogenase (El) emd dihydrolipoyl dehydrogenase (E3) subunits form noncovedent bonds to this centred catedytic unit. 

Branched-Chain Oxo-acid Decarboxylase and Maple Syrup Urine Disease The third oxo-add dehydrogenase catalyzes the oxidative decarboxylation of the branched-chain oxo-acids that arise from the transamination of the branched-chain amino acids, leucine, isoleuctne, emd vtdine. It has a similEU subunit composition to pyruvate and 2-oxoglutarate dehydrogenases, and the E3 subunit (dihydrolipoyl dehydrogenase) is the stune protein as in the other two multienzyme complexes. Genetic lack of this enzyme causes maple syrup urine disease, so-called because the bremched-chain oxo-acids that are excreted in the urine have a smell reminiscent of maple syrup. 

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