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[[Image:Gluconeogenesis.png|500px]]
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Revision as of 01:49, 7 November 2013

 
Author [[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type ExamType::USMLE Step 1
Main Category MainCategory::Biochemistry
Sub Category SubCategory::General Principles
Prompt [[Prompt::A researcher is studying the mechanisms of maintenance of glucose levels in rats. After a series of experiments in which he uses rat liver to measure enzyme activities, he notes that a the activity of a specific cytosolic enzyme involved in gluconeogenesis varies greatly with the concentration of cytosolic GTP. Which of the following is the end product produced by this enzyme?]]
Answer A AnswerA::Oxaloacetate
Answer A Explanation AnswerAExp::Oxaloacetate is the end-product produced by pyruvate carboxylase. This enzyme does not require GTP.
Answer B AnswerB::Phosphoenolpyruvate
Answer B Explanation AnswerBExp::Phosphoenolpyruvate is the end-product produced by PEP carboxykinase. This enzyme requires GTP to function properly.
Answer C AnswerC::Fructose-1,6-bisphosphate
Answer C Explanation AnswerCExp::Fructose-1,6-bisphosphate is the substrate of fructose-1,6-bisphosphatase. This enzyme does not require GTP.
Answer D AnswerD::Fructose-6-phosphate
Answer D Explanation AnswerDExp::Fructose-6-phosphate is the end-product of fructose-1,6-bisphosphatase. this enzyme does not require GTP.
Answer E AnswerE::Pyruvate
Answer E Explanation AnswerEExp::Pyruvate is the substrate of pyruvate carboxylase. This enzyme does not require GTP.
Right Answer RightAnswer::B
Explanation [[Explanation::Gluconeogenesis occurs primarily in the liver and involves a number of irreversible enzymes to bypass some of the unidirectional enzymes in the glycolytic pathway. A summary of the four irreversible enzymes is shown below. PEP carboxykinase responsible for converting oxaloacetate into phosphoenolpyruvate (PEP) while bypassing pyruvate. PEP carboxykinase is a cytosolic enzyme that has a binding site that binds GTP and is crucial for proper enzyme functioning.



Educational objective: PEP carboxykinase is an important enzyme involved in gluconeogenesis that converts oxaloacetate to PEP. It requires GTP to function properly.


References:

Hanson RW, Garber AJ. Phosphoenolpyruvate carboxykinase. I. Its role in gluconeogenesis. Am J Clin Nutr. 1972;25(10):1010-21.
Le T, Bhushan V. First Aid for the USMLE Step 1 2013. McGraw-Hill Medical; 2013.
Educational Objective:
References: ]]

Approved Approved::No
Keyword WBRKeyword::PEP Carboxykinase, WBRKeyword::gluconeogenesis
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