Exam2_308 - Biochemistry Final exam Block 3, 2008 Name...

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Exam_final_ansrs 1 of 3 B. Nowak-Thompson 11/26/2008 Biochemistry Final exam Block 3, 2008 Name Answer all of the following questions (all are worth 8 pts). Be sure to include molecular details in your answers that support the cause and effect relationships that exist between the chemical mechanisms and physiological results. You may use notes and the textbook for this exam. On-line references are not allowed . 1. 6-Phosphogluconate dehydrogenase catalyses the oxidative decarboxylation of 6-phosphogluconate to form ribulose-5-phosphate. The reactions are outlined in figure 14-21 of your text. Suggest a mechanism for this reaction showing the movement of all electrons and protons, as well as the structure of the intermediate that is formed. (Hint: as with any organic mechanism, don’t try to do everything at once.) 2. In the non-oxidative phase of the pentose phosphate pathway, transketolase and transaldolase are used to transfer carbon chains among the various pathway intermediates. Transketolase uses thiamine pyrophosphate (TPP) as its cofactor whereas transaldolase does not require a cofactor. Chemically explain why these enzymes have different cofactor requirements. (See the Figures 23-25 in Chpt 14). Both enzymes lyse a carbon-carbon bond and require the correct positioning of a carbonyl to help stabilize the carbanion that forms. In the case of transaldolase, the lysed bond is beta to the existing carbonyl and can be cleaved simply by forming the imine of the carbonyl. The imine is conjugated to the beta carbon and stabilizes the forming anion. The transketolase reaction requires the TPP cofactor because the lysed bond is alpha to the existing carbonyl and cannot be stabilized through resonance conjugation. However, the TPP thiazolium ring carries its own imine, equivalent to that found in the transaldolase mechanism. Nucleophilic attack of the thiazolium anion on the existing carbonyl in the substrate places the imine beta to the
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This note was uploaded on 12/01/2011 for the course BIO 308 taught by Professor Nowak-thompson during the Fall '08 term at Cornell.

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Exam2_308 - Biochemistry Final exam Block 3, 2008 Name...

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