lec 16 b - BL/CH401 Lecture 16B Enzyme Mechanism Examples...

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Unformatted text preview: BL/CH401 Lecture 16B -- Enzyme Mechanism - Examples Part IV. A. Electron Transfers to/from NADH/NADPH We discussed dehydrogenases as having two domains one for NADH binding and one for other substrate in lecture 10. These enzymes often have the ability to make stereospecific products like L-malate and L-lactate. How is stereospecificity achieved in dehydrogenases? For lactate dehydrogenase (LDH), Arg-171 holds the carboxylate group of pyruvate (CH3-CO-COO-) while His-195 provides a proton to the carbonyl on carbon-2 to make the stereospecific product. Figure 12. Interconversion of Pyruvate/NADH and Lactate/NAD + /H + catalyzed by lactate dehydrogenase (LDH). Figure 13. The active site of LDH showing NADH and pyruvate bound by various amino acid side chains. Figure 14. Model of LDH Catalytic Mechanism showing stereospecific transfer of hydride anion from NADH to pyruvate which is held in position by Arg-171 side chain. Figure 15. More general view of the LDH Catalytic Mechanism showing the role of specific amino acid side chains. Part IV. B. Electron Transfers to/from NADH/NADPH Can an enzyme's disulfide bond be redox active? A much more complicated redox enzyme is glutathione reductase, which uses NADPH to reduce oxidized glutathione in your cells. Glutathione is a 3 AA peptide (Glu-Cys-Gly) and two molecules are linked together by a disulfide bond in the oxidized form. Figure 16. Reduction of glutathione by NADPH catalyzed by glutathione reductase. Figure 17. Structural Model of Glutathione Reductase showing the domains for binding the internal flavin molecule (FAD) and the reducing substrate, NADPH, and the interface domain for joining the two subunits. The active site where glutathione is bound is between the two subunits and it is here where the redox active disulfide of the enzyme is located. To achieve reduction of GSSG to GSH glutathione reductase must split the two electrons provided by NADPH and give them one at a time to each of the sulfurs of the GSSG. To do this, glutathione reductase contains FAD and it also uses a redox active enzyme disulfide. FAD differs from NADH/NADPH in that it can accept 2 electrons and pass them one at a time....
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This note was uploaded on 09/02/2009 for the course BIO BL 401 taught by Professor Wilbur during the Spring '07 term at Michigan Technological University.

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lec 16 b - BL/CH401 Lecture 16B Enzyme Mechanism Examples...

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