Glycolysis - Biochemistry of Metabolism Glycolysis...

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Glycolysis Copyright © 1998-2007 by Joyce J. Diwan. All rights reserved. Biochemistry of Metabolism
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Glycolysis takes place in the cytosol of cells. Glucose enters the Glycolysis pathway by conversion to glucose-6-phosphate . Initially there is energy input corresponding to cleavage of two ~P bonds of ATP. H O OH H OH H OH CH 2 O PO 3 2 - H OH H 1 6 5 4 3 2 glucose-6-phosphate
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1. Hexokinase catalyzes: Glucose + ATP l glucose-6-P + ADP The reaction involves nucleophilic attack of the C6 hydroxyl O of glucose on P of the terminal phosphate of ATP. ATP binds to the enzyme as a complex with Mg++ .
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Mg++ interacts with negatively charged phosphate oxygen atoms, providing charge compensation & promoting a favorable conformation of ATP at the active site of the Hexokinase enzyme. N N N N NH 2 O OH OH H H H CH 2 H O P O P O P - O O O - O - O O O - adenine ribose ATP adenosine triphosphate
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The reaction catalyzed by Hexokinase is highly spontaneous . A phosphoanhydride bond of ATP ( ~P ) is cleaved. The phosphate ester formed in glucose-6-phosphate has a lower G of hydrolysis. H O OH H OH H OH CH 2 OH H OH H H O OH H OH H OH CH 2 O PO 3 2 - H OH H 2 3 4 5 6 1 1 6 5 4 3 2 ATP ADP Mg 2+ glucose glucose-6-phosphate Hexokinase
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w the C6 hydroxyl of the bound glucose is close to the terminal phosphate of ATP, promoting catalysis. w water is excluded from the active site. This prevents the enzyme from catalyzing ATP hydrolysis, rather than transfer of phosphate to glucose. glucose Hexokinase H O OH H OH H OH CH 2 OH H OH H H O OH H OH H OH CH 2 O PO 3 2 - H OH H 2 3 4 5 6 1 1 6 5 4 3 2 ATP ADP Mg 2+ glucose glucose-6-phosphate Hexokinase Induced fit: Glucose binding to Hexokinase stabilizes a conformation in which:
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It is a common motif for an enzyme active site to be located at an interface between protein domains that are connected by a flexible hinge region. The structural flexibility allows access to the active site, while permitting precise positioning of active site residues, and in some cases exclusion of water, as substrate binding promotes a particular conformation. glucose Hexokinase
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2. Phosphoglucose Isomerase catalyzes: glucose-6-P (aldose) G fructose-6-P (ketose) The mechanism involves acid/base catalysis, with ring opening, isomerization via an enediolate intermediate , and then ring closure. A similar reaction catalyzed by Triosephosphate Isomerase will be presented in detail. H O OH H OH H OH CH 2 OPO 3 2 - H OH H 1 6 5 4 3 2 CH 2 OPO 3 2 - OH CH 2 OH H OH H H HO O 6 5 4 3 2 1 glucose-6-phosphate fructose-6-phosphate Phosphoglucose Isomerase
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3. Phosphofructokinase catalyzes: fructose-6-P + ATP l fructose-1,6-bisP + ADP This highly spontaneous reaction has a mechanism similar to that of Hexokinase. The Phosphofructokinase reaction is the
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Glycolysis - Biochemistry of Metabolism Glycolysis...

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