Dehydrogenase dihydrolipoamide

Figure 2. Mechanism of PDH. The three different subunits of the PDH complex in the mitochondrial matrix (E, pyruvate decarboxylase E2, dihydrolipoamide acyltrans-ferase Ej, dihydrolipoamide dehydrogenase) catalyze the oxidative decarboxylation of pyruvate to acetyl-CoA and CO2. E, decarboxylates pyruvate and transfers the acetyl-group to lipoamide. Lipoamide is linked to the group of a lysine residue to E2 to form a flexible chain which rotates between the active sites of E, E2, and E3. E2 then transfers the acetyl-group from lipoamide to CoASH leaving the lipoamide in the reduced form. This in turn is oxidized by E3, which is an NAD-dependent (low potential) flavoprotein, completing the catalytic cycle. PDH activity is controlled in two ways by product inhibition by NADH and acetyl-CoA formed from pyruvate (or by P-oxidation), and by inactivation by phosphorylation of Ej by a specific ATP-de-pendent protein kinase associated with the complex, or activation by dephosphorylation by a specific phosphoprotein phosphatase. The phosphatase is activated by increases in the concentration of Ca in the matrix. The combination of insulin with its cell surface receptor activates PDH by activating the phosphatase by an unknown mechanism.

The third enzyme, dihydrolipoamide dehydrogenase [E3], reoxidizes dihydrolipoamide, with NADH+H"" being formed. The electrons are first taken over by enzyme-bound FAD (3a) and then transferred via a catalytically active disulfide bond in the E3 subunit (not shown) to soluble NAD"" (3b). 

Pyruvate dehydrogenase (lipoamide) [EC 1.2.4.1], which requires thiamin pyrophosphate, catalyzes the reaction of pyruvate with lipoamide to produce 5-acetyldihydroli-poamide and carbon dioxide. It is a component of the pyruvate dehydrogenase complex (which also includes dihydrolipoamide dehydrogenase [EC 1.8.1.4] and dihy-drolipoamide acetyltransferase [EC 2.3.1.12]). Pyruvate dehydrogenase (cytochrome) [EC 1.2.2.2] catalyzes the... 

DIHYDROLIPOAMIDE DEHYDROGENASE DIHYDRONEOPTERIN ALDOLA.SE DIHYDRONEOPTERIN TRIPHOSPHATE... 

DIHYDROLIPOAMIDE DEHYDROGENASE. GLUTATHIONE REDUCTASE (EC 1.6.4.2) (GR) PUTATIVE FLAVOPROTEIN C26F1.14C. 

A very interesting application of affinity chromatography to the purification of halophilic enzymes was reported by Sundquist and Fahey (1988). These authors have purified the enzymes bis-y-glu-tamylcysteine reductase and dihydrolipoamide dehydrogenase from H. halohium using immobilized metal ion affinity chromatography in high-salt buffers. 

Lipoamide dehydrogenase, as its more proper name dihydrolipoamide dehydrogenase implies, functions physiologically in the reoxidation of... 

Brautigam CA, Chuang JL, Tomchick DR, Machius M, Chuang 25. DT. Crystal structure of human dihydrolipoamide dehydrogenase NAD-I-/NADH binding and the structural basis of disease-causing mutations. J. Mol. Biol. 2005 350 543-552. 

R. G. McCartney, J.E. Rice, S.J. Sanderson, V. Bunik, H. Lindsay, and J.G. Lindsay. 1998. Suhunit interactions in the mammalian alpha-ketoglutarate dehydrogenase complex Evidence for direct association of the alpha-ketoglutarate dehydrogenase and dihydrolipoamide dehydrogenase components J. Biol. Chem. 273 24158-24164. (PubMed)... 

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 ... 

The function of this archaebacterial dihydrolipoamide dehydrogenase in the absence of its normal multienzyme complexes is unknown [42,43]. Detailed structural studies, beginning with current experiments to clone and sequence the gene [44], may throw light on this. Meanwhile, we have surveyed a number of archaebacterial genera for the presence of the enzyme [43] and have correlated this with the presence of lipoic acid. The data available are summarised in Table 1. 

The presence of the enzyme and cofactor are co-incident, indicating that lipoic acid may indeed be the trae substrate of the archaebacterial dihydrolipoamide dehydrogenase. Interestingly, their presence can be correlated with the organisms phylogenetic positions within the archaebacteria. That is, the archaea comprise two main divisions - the methanogens, extreme halophiles. Thermoplasma and Thermococcus in one, and the remaining sulphur-dependent thermophiles in the other [66,72]. The enzyme and/or the cofactor have been detected in all the phenotypes of the former division but neither have yet been discovered in the latter. Further analyses are required to test this correlation as the data are incomplete. 

Organism Dihydrolipoamide dehydrogenase Lipoic acid References... 

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