Pyruvate glycolytic enzyme complex

Pyruvate produced by the glycolytic pathway may be transported into the mitochondria (via an antiport with OH"), where it is converted to acetyl-CoA by the action of the enzyme complex pyruvate dehydrogenase. The pertinent enzyme activities are pyruvate dehydrogenase (PD), lipoic acid acetyltransferase, and dihydrolipoic acid dehydrogenase. In addition, several cofactors are utilized thiamine pyrophosphate (TPP), lipoic acid, NAD+, Co A, and FAD. Only Co A and NAD+ are used in stoichiometric amounts, whereas the others are required in catalytic amounts. Arsenite and Hg2+ are inhibitors of this system. The overall reaction sequence may be represented by Figure 18.5. The NADH generated may enter the oxidative phosphorylation pathway to generate three ATP molecules per NADH molecule reduced. The reaction is practically irreversible its AGq = -9.4 kcal/mol. 

Removal of CO2 from pyruvate. This reaction is carried out by the pyruvate decarboxylase (El) component of the complex. Like yeast pyruvate decarboxylase, responsible for the production of acetaldehyde, the enzyme uses a thiamine pyrophosphate cofactor and oxidizes the carboxy group of pyruvate to CO2. Unlike the glycolytic enzyme, acetaldehyde is not released from the enzyme along with CO2. Instead, the acetaldehyde is kept in the enzyme active site, where it is transferred to Coenzyme A. 

Liedvogel B, B uerle R. Fatty acid synthesis in chloroplasts from mustard Sinapis alba L.) cotyledons Formation of acetyl coenzyme A by intraplastid glycolytic enzymes and a pyruvate dehydrogenase complex. Planta 1986 169 481-489. 

Flow of carbon atoms. What is the fate of the radioactive label when each of the following compounds is added to a cell extract containing the enzymes and cofactors of the glycolytic pathway, the citric acid cycle, and the pyruvate dehydrogenase complex (The label is printed in red.)... 

Figure 6 Fatty-acid biosynthesis. Cytoplasmic acetyl-CoA (AcCoA) is the primary substrate for de novo fatty-acid synthesis. This two-carbon compound most commonly derives from the glycolytic degradation of glucose, and its formation is dependent upon several reactions in the mitochondria. The mitochondrial enzyme pyruvate carboxylase is found primarily in tissues that can synthesize fatty acids. AcCoA is converted to maionyl-CoA (MalCoA) by acetyl-CoA carboxylase. Using AcCoA as a primer, the fatty-acid synthase multienzyme complex carries out a series of reactions that elongate the growing fatty acid by two carbon atoms. In this process MalCoA condenses with AcCoA, yielding an enzyme-bound four-carbon /3-ketoacid that is reduced, dehydrated, and reduced again. The product is enzyme-bound 4 0. This process is repeated six more times, after which 16 0 is released from the complex. The reductive steps require NADPH, which is derived from enzyme reactions and pathways shown in grey. Enz refers to the fatty acid synthase multienzyme complex.

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