Ch. 11 outline - Ch. 11-The Citric Acid Cycle -The Citric...

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-The Citric Acid cycle is a metabolic pathway that converts carbon atoms to carbon dioxide and in doing so, conserves metabolic free energy that ultimately drives the synthesis of ATP. -The 8 reactions of the citric acid cycle take place in the cytosol of prokaryotes and in the mitochondria of eukaryotes. -Carbon atoms enter the citric acid cycle in the form of acetyl groups derived from amino acids, fatty acids, and carbohydrates The Pyruvate Dehydrogenase Complex Coverts Pyruvate to Acetyl-CoA -Pyruvate produced by glycolysis in the cytosol must first be transported into the mitochondria -Together, they catalyze the oxidative decarboxlation of pyruvate and the transfer of the acetyl unti to coenzyme A. 1. In the first step, which is catalyzed by E1, pyruvate is decarboxylated. This reaction requires the cofactor thiamine pyrophosphate. TPP attacks the carbonyl carbon of pyruvate, and the departure of carbon dioxide leaves a hydroxyethyl group attached to TPP. This carbanion is stabilized by the positively charged thiazolium ring group of TPP 2. The hydroxyethyl group is then transferred to E2 of the pyruvate dehydrogenase complex. The hydroxyethyl acceptor is a lipoamide prosthetic group. The transfer reaction regenerates the TPP cofactor of E1 and oxidizes the hydroxyethyl group to an acetyl group. 3. Next, E2 transfers the acetyl group to coenzyme A, producing acetyl-CoA and leaving a reduced lipoamide group. 4. The final two steps of the reaction restore the pyruvate dehydrogenase complex to its original state. E3 reoxidizes the lipoamide group of E2 by reducing the cofactor flavin adenine dinucleotide. 5. Finally, NAD+ reoxidizes the FADH2 prosthetic group, and NADH diffuses away. -Multienzyme complex such as the pyruvate dehydrogenase complex can carry out a multistep reaction sequence efficiently because the product of one reaction can quickly become the substrate for the next reaction without diffusing away or reacting with another substance. -Flux through the pyruvate dehydrogenase complex is regulated by product inhibition: Both
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This note was uploaded on 12/31/2010 for the course CH 369 taught by Professor Kbrowning during the Spring '07 term at University of Texas at Austin.

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Ch. 11 outline - Ch. 11-The Citric Acid Cycle -The Citric...

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