Biochemistry : Gluconeogenesis Enzymes

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #11 : Alternative Pathways

Which enzyme(s) bypass the pyruvate kinase reaction of glycolysis in gluconeogenesis?

Possible Answers:

pyruvate carboxylase only

phosphoenolpyruvate carboxykinase (PEPCK) only

pyruvate carboxylase and phosphoenolpyruvate carboxykinase (PEPCK)

fructose bis-phosphatase only

phosphoglycerate mutase and pyruvate carboxylase

Correct answer:

pyruvate carboxylase and phosphoenolpyruvate carboxykinase (PEPCK)

Explanation:

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 #12 : Alternative Pathways

Which one of the following statements about PEP (phosphoenolpyruvate) synthesis is correct?

Possible Answers:

Pyruvate can be converted to PEP by pyruvate kinase.

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.

Correct answer:

Pyruvate can be converted to PEP by a combination of reactions that use energy from two different types of nucleotide triphosphate.

Explanation:

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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