Biochemistry : Gluconeogenesis Enzymes
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Example Questions
Example Question #1 : Gluconeogenesis
Which enzyme(s) bypass the pyruvate kinase reaction of glycolysis in gluconeogenesis?
phosphoglycerate mutase and pyruvate carboxylase
pyruvate carboxylase and phosphoenolpyruvate carboxykinase (PEPCK)
pyruvate carboxylase only
phosphoenolpyruvate carboxykinase (PEPCK) only
fructose bis-phosphatase only
pyruvate carboxylase and phosphoenolpyruvate carboxykinase (PEPCK)
In the first step of gluconeogenesis, pyruvate carboxylase (with ATP and bicarbonate) converts pyruvate to oxaloacetate. Then phosphoenolpyruvate carboxykinase (PEPCK) (with GTP) releases carbon dioxide to give phosphoenolpyruvate.
Example Question #11 : Alternative Pathways
Which one of the following statements about PEP (phosphoenolpyruvate) synthesis is correct?
Pyruvate is converted to PEP by the citric acid cycle.
Pyruvate can be converted to PEP by a mutase.
Pyruvate can be converted to PEP by a combination of reactions that use energy from two different types of nucleotide triphosphate.
Pyruvate can be converted to PEP by pyruvate kinase.
Pyruvate can be converted to PEP by a combination of reactions that use energy from two different types of nucleotide triphosphate.
GTP and ATP are used to drive the reactions that make pyruvate into PEP. Specifically, ATP catalyzes PEP carboxylase and GTP catalyzes PEP carboxykinase. A mutase moves phosphate groups already on a molecule, so it cannot be used to introduce one. The citric acid cycle can make pyruvate into any of the citric acid intermediates, but PEP isn't one of them. Pyruvate kinase is the enzyme that makes pyruvate into PEP, which is only favorable in the forward direction.
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