Gluconeogenesis Enzymes
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Biochemistry › Gluconeogenesis Enzymes
Which enzyme(s) bypass the pyruvate kinase reaction of glycolysis in gluconeogenesis?
pyruvate carboxylase and phosphoenolpyruvate carboxykinase (PEPCK)
phosphoenolpyruvate carboxykinase (PEPCK) only
phosphoglycerate mutase and pyruvate carboxylase
pyruvate carboxylase only
fructose bis-phosphatase only
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.
Which one of the following statements about PEP (phosphoenolpyruvate) synthesis is correct?
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 mutase.
Pyruvate is converted to PEP by the citric acid cycle.
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.