WBR0861

Author	[[PageAuthor::Serge Korjian M.D. (Reviewed by Serge Korjian)]]
Exam Type	USMLE Step 1
Main Category	Biochemistry
Sub Category	General Principles
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	Oxaloacetate
Answer A Explanation	Oxaloacetate is the end-product produced by pyruvate carboxylase. This enzyme does not require GTP.
Answer B	Phosphoenolpyruvate
Answer B Explanation	Phosphoenolpyruvate is the end-product produced by PEP carboxykinase. This enzyme requires GTP to function properly.
Answer C	Fructose-1,6-bisphosphate
Answer C Explanation	Fructose-1,6-bisphosphate is the substrate of fructose-1,6-bisphosphatase. This enzyme does not require GTP.
Answer D	Fructose-6-phosphate
Answer D Explanation	Fructose-6-phosphate is the end-product of fructose-1,6-bisphosphatase. this enzyme does not require GTP.
Answer E	Pyruvate
Answer E Explanation	Pyruvate is the substrate of pyruvate carboxylase. This enzyme does not require GTP.
Right Answer	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.]]
Approved	Yes
Keyword	PEP Carboxykinase, Gluconeogenesis, Pyruvate, Glycolysis
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