Chapter 17 Biochemistry6e

Chapter 17 Biochemistry6e - Chapter 17 Biochemistry The...

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Chapter 17 Biochemistry The Citric Acid Cycle - 6 th Edition by Berg, Tymoczko and Stryer The aerobic processing of glucose is the source of the most ATP generated from the metabolism of glucose. The process begins with the complete oxidation of the glucose metabolism derivatives to CO 2 . This oxidation of pyruvate begins with the synthesis of acetyl-CoA and then its metabolism to CO 2 occurs in the Krebs, or Tricarboxylic Acid- or Citric Acid Cycle. The TCA Cycle is the final common pathway for the oxidation of fuel molecules, with most entering as acetyl-CoA but some entering as the other intermediates of the pathway. In eukaryotes, the Krebs Cycle takes place in the mitochondrion, which contrasts with glycolysis, which occurs in the cytoplasm. Krebs Cycle Overview The Krebs Cycle is the central metabolic hub of the cell. It allows for the aerobic oxidation of any molecule that can be converted either into acetyl-CoA or into a dicarboxylic acid. The Krebs Cycle not only provides for energy conservation, it also allows for biosynthetic precursors to be made which are then used to make storage molecules (fats) or to make amino acids, nucleic acids, cholesterol and porphyrin. With respect to the conservation of energy, the Krebs Cycle’s role is to harvest high energy electrons from carbon fuels and to provide those electrons to the electron transport system so that the efficient synthesis of ATP can occur. The Citric Acid Cycle uses a series of oxidations which result in the synthesis of 3 CO 2 , 3 NADH, 1 FADH 2 and 1 GTP (Show Figure 17.2) . During the process, an acetyl-CoA condenses with an oxaloacetic acid to form citrate. Citrate is re-arranged into an isomer, which is then oxidatively decarboxylated. The resulting molecule is itself oxidatively decarboxylated into the 4 C molecule succinyl -CoA which is then converted to succinate while yielding a GTP. Finally, oxaloacetic acid is regenerated via a series of hydration and oxidation steps (Show Figure 17.15) . 1
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Two C atoms enter as acetyl-CoA while 2 C atoms leave as CO 2 . Three hydride ions leave as NADH, a pair of H atoms (and the associated electrons) exits as FADH 2 . The key function of the Krebs Cycle is thus to harvest high-energy electrons from carbon molecules being used as food while neither using molecular oxygen directly nor generating a substantial amount of high-energy phosphate bonds. The key function, energetically, of the Krebs Cycle is to remove the high-energy electrons of acetyl-CoA and to donate them to NADH and FADH 2 . During the process of oxidative phosphorylation, these high- energy electrons are removed from NADH and FADH 2 , made to flow through the electron-transport chain whereby the energy released results in the establishment of a proton gradient across the inner mitochondrial membrane. The gradient is then used to make ATP and the electrons are delivered to O 2 so that the NAD + and FAD may be regenerated (Show Figure 17.3) . These two processes, the Krebs Cycle
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This note was uploaded on 04/26/2008 for the course CHEM 361 taught by Professor Defreitas during the Spring '08 term at Loyola Chicago.

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Chapter 17 Biochemistry6e - Chapter 17 Biochemistry The...

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