Pyruvate dehydrogenase complex thiamin diphosphate

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.)...

The intermediary metabolism has multienzyme complexes which, in a complex reaction, catalyze the oxidative decarboxylation of 2-oxoacids and the transfer to coenzyme A of the acyl residue produced. NAD" acts as the electron acceptor. In addition, thiamine diphosphate, lipoamide, and FAD are also involved in the reaction. The oxoacid dehydrogenases include a) the pyruvate dehydrogenase complex (PDH, pyruvate acetyl CoA), b) the 2-oxoglutarate dehydrogenase complex of the tricarboxylic acid cycle (ODH, 2-oxoglutarate succinyl CoA), and c) the branched chain dehydrogenase complex, which is involved in the catabolism of valine, leucine, and isoleucine (see p. 414). 

Czerniecki J and Czygier M (2001) Cooperation of divalent ions and thiamin diphosphate in regulation of the function of pig heart pyruvate dehydrogenase complex. Joumai of Nutritionai Scence and Vitaminoiogy (Tokyo) 47,385-6. 

In brain, four major enzyme systems utilize thiamine in the form of thiamine diphosphate (TDP) as a major cofactor, i.e. a-ketoglutarate dehydrogenase complex (KGDHC), pyruvate dehydrogenase complex, branched-chain a-keto acid dehydrogenase complex (BCKDHQ and transketolase. 

Pyruvate dehydrogenase complex. Allen et al.(1964) and later de Vries et al.(1973) postulated that this complex in P. shermanii includes pyruvate dehydrogenase that uses thiamine diphosphate as coenzyme, dihydrolipoyl transacetylase containing lipoic acid, and dihydrolipoyl dehydrogenase containing NAD and FAD. The complex catalyzes the following reaction ... 

Nemeria N, Yan Y, Zhang Z et al (2001) Inhibition of the Escherichia coli pyruvate dehydrogenase complex El subunit and its tyrosine 177 variants by thiamin 2-thiazolone and thiamin 2-thiothiazolone diphosphates. Evidence for reversible tight-binding inhibition. J Biol Chem 276 45969 5978... 

Step 4 of Figure 29.12 Oxidative Decarboxylation The transformation of cr-ketoglutarate to succinyl CoA in step 4 is a multistep process just like the transformation of pyruvate to acetyl CoA that we saw in Figure 29.11. In both cases, an -keto acid loses C02 and is oxidized to a thioester in a series of steps catalyzed by a multienzynie dehydrogenase complex. As in the conversion of pyruvate to acetyl CoA, the reaction involves an initial nucleophilic addition reaction to a-ketoglutarate by thiamin diphosphate vlide, followed by decarboxylation, reaction with lipoamide, elimination of TPP vlide, and finally a transesterification of the dihydrolipoamide thioester with coenzyme A. 

Pyruvate is oxidized to acetyl-GoA by a multienzyme complex, pyruvate dehydrogenase, that is dependent on the vitamin cofactor thiamin diphosphate. 

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... 

That there is an enamine-like intermediate in all thiamin diphosphate-dependent enzymatic pathways has been suggested for many years, although the issue is still sometimes clouded by the observation that the C2a-protonated form, the C2a-hydro-xyethylThDP when added to oxidases such as pyruvate oxidase (POX) and the pyruvate dehydrogenase multienzyme complex (PDHc), will undergo oxidation. 

The studies of Peters in the 1920s and 1930s (Peters, 1963) established the coenzyme role of thiamin in the oxidative decarboxylation of pyruvate. Thiamin diphosphate is the coenzyme for three multienzyme complexes in mammalian mitochondria that are involved in the oxidative decarboxylation of oxo-acids pyruvate dehydrogenase and 2-oxoglutarate dehydrogenase in central... 

The deprotonation and addition of a base to thiazolium salts are combined to produce an acyl carbanion equivalent (an active aldehyde) [363, 364], which is known to play an essential role in catalysis of the thiamine diphosphate (ThDP) coenzyme [365, 366]. The active aldehyde in ThDP dependent enzymes has the ability to mediate an efScient electron transfer to various physiological electron acceptors, such as lipoamide in pyruvate dehydrogenase multienzyme complex [367], flavin adenine dinucleotide (FAD) in pyruvate oxidase [368] and Fc4S4 cluster in pyruvate ferredoxin oxidoreductase [369]. 

Arjunan, P, Chandrasekhar, K., Sax, M., et al. (2004) Structural determinants of enzyme binding affinity The El component of pyruvate dehydrogenase from Escherichia coli in complex with the inhibitor thiamin thiazolone diphosphate. Biochemistry 43,2405-2411. 

Deprotonation Rate of the C2-H of Thiamin Diphosphate in the Pyruvate Dehydrogenase Multienzyme Complex from Escherichia coli... 

The mechanism suggested by Kerscher and Oesterhelt is indicated in Scheme 46 for the enzyme from H. halobium (213). The initial step is identical to that of the 2-oxoacid dehydrogenase complexes and involves binding of pyruvate to thiamin diphosphate and subsequent decarboxylation yielding hydroxyethylthia-min diphosphate. This intermediate undergoes one-electron transfer to the [4Fe-4S] cluster to form the stable free radical. The cluster is then reoxidized by ferredoxin or oxygen to give the enzyme-intermediate complex. Reaction with CoA initiates the second electron transfer to the iron-sulfur cluster, acyl transfer, followed by reoxidation of the enzyme by ferredoxin or O2 to complete the cycle. Two basic questions are yet unanswered (1) What is the mechanism of the enzymic reaction between CoASH and hydroxyethyl-TPP in the absence... 

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