Pyruvate decarboxylase subunit complex

The behavior of lipoamide dehydrogenase in the intact pyruvate dehydrogenase complex has been investigated. The multienzyme complex consists of 24 pyruvate decarboxylase subunits, 24 dihydrolipoyl transacetylase subunits, and 12 lipoamide dehydrogenase subunits. The complex catalyzes a series of reactions beginning with pyruvate decarboxylation, followed by transfer of an acetyl moiety to CoA, and finally the oxidation of dihydrolipoamide by NAD. Stopped-flow smdies showed that lipoamide dehydrogenase in the complex behaves largely the same as it does when studied alone. 

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.

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. 

Within many tissues the enzymatic activities of the pyruvate and branched chain oxoacid dehydrogenases complexes are controlled in part by a phosphorylation -dephosphorylation mechanism (see Eq. 17-9). Phosphorylation of the decarboxylase subunit by an ATP-dependent kinase produces an inactive phosphoenzyme. A phosphatase reactivates the dehydrogenase to complete the regulatory cycle (see Eq. 17-9 and associated discussion). The regulation is apparently accomplished, in part, by controlling the affinity of the protein for... 

A FIGURE 3-21 Structure and function of pyruvate dehydrogenase, a large multimeric enzyme complex that converts pyruvate into acetyl CoA. (a) The complex consists of 24 copies of pyruvate decarboxylase (Ei), 24 copies of lipoamide transacetylase (E2), and 12 copies of dihydrolipoyl dehydrogenase (E3). The El and E3 subunits are bound to the outside of the core formed by the E2 subunits, (b) The reactions catalyzed by the complex include several enzyme-bound intermediates (not shown). The tight structural integration of the three enzymes increases the rate of the overall reaction and minimizes possible side reactions. 

The process is initiated by the entry of pyruvate into the mitochondrial matrix, where the enzyme pyruvate dehydrogenase will irreversibly convert pyruvate to acetyl CoA. This enzyme is a complex of a large number of subunits of independent but cooperating individual enzymic activities. (1) The first subunit (pyruvate decarboxylase) causes the decarboxylation of pyruvate and requires thiamine pyrophosphate. A twocarbon thiamine pyrophosphate adduct is formed and then transferred from this subunit to a second subunit containing oxidized lipoic acid (i.e., S-S). (2) The twocarbon adduct is transferred to form an HS and... 

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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