Gluconeogenesis - Molecular Biochemistry I Gluconeogenesis;...

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Gluconeogenesis; Copyright © 1999-2007 by Joyce J. Diwan. All rights reserved. Molecular Biochemistry I
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Gluconeogenesis occurs mainly in liver . Gluconeogenesis occurs to a more limited extent in kidney & small intestine under some conditions. Synthesis of glucose from pyruvate utilizes many of the same enzymes as Glycolysis . Three Glycolysis reactions have such a large negative G that they are essentially irreversible . Hexokinase (or Glucokinase) Phosphofructokinase Pyruvate Kinase. These steps must be bypassed in Gluconeogenesis. Two of the bypass reactions involve simple hydrolysis reactions.
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Hexokinase or Glucokinase (Glycolysis) catalyzes: glucose + ATP glucose-6-phosphate + ADP Glucose-6-Phosphatase (Gluconeogenesis) catalyzes: glucose-6-phosphate + H 2 O glucose + P i 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 H 2 O 1 6 5 4 3 2 + P i glucose-6-phosphate glucose Glucose-6-phosphatase
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Glucose-6-phosphatase enzyme is embedded in the endoplasmic reticulum (ER) membrane in liver cells. The catalytic site is found to be exposed to the ER lumen. Another subunit may function as a translocase, providing access of substrate to the active site. 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 H 2 O 1 6 5 4 3 2 + P i glucose-6-phosphate glucose Glucose-6-phosphatase
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Phosphofructokinase (Glycolysis) catalyzes: fructose-6-P + ATP fructose-1,6-bisP + ADP Fructose-1,6-bisphosphatase (Gluconeogenesis) catalyzes: fructose-1,6-bisP + H 2 O fructose-6-P + P i fructose-6-phosphate fructose-1,6-bisphosphate Phosphofructokinase CH 2 OPO 3 2 - OH CH 2 OH H OH H H HO O 6 5 4 3 2 1 CH 2 OPO 3 2 - OH CH 2 O PO 3 2 - H OH H H HO O 6 5 4 3 2 1 ATP ADP P i H 2 O Fructose-1,6-biosphosphatase
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Bypass of Pyruvate Kinase: Pyruvate Kinase (last step of Glycolysis) catalyzes: phosphoenolpyruvate + ADP pyruvate + ATP For bypass of the Pyruvate Kinase reaction, cleavage of 2 ~P bonds is required. G for cleavage of one ~P bond of ATP is insufficient to drive synthesis of phosphoenolpyruvate (PEP). PEP has a higher negative G of phosphate hydrolysis than ATP.
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Bypass of Pyruvate Kinase (2 enzymes): Pyruvate Carboxylase (Gluconeogenesis) catalyzes: pyruvate + HCO 3 - + ATP oxaloacetate + ADP + P i PEP Carboxykinase (Gluconeogenesis) catalyzes: oxaloacetate + GTP PEP + GDP + CO 2 C C CH 2 O O - O PO 3 2 - C C CH 3 O O - O ATP ADP + P i C CH 2 C C O O O - O - O HCO 3 - GTP GDP CO 2 pyruvate oxaloacetate PEP Pyruvate Carboxylase PEP Carboxykinase
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Contributing to spontaneity of the 2-step process: Free energy of one ~P bond of ATP is conserved in the carboxylation reaction. Spontaneous decarboxylation
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This note was uploaded on 09/25/2010 for the course BCH 4024 taught by Professor Allison during the Spring '08 term at University of Florida.

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Gluconeogenesis - Molecular Biochemistry I Gluconeogenesis;...

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