WBR0861

Author	[[PageAuthor::Serge Korjian M.D. (Reviewed by Serge Korjian)]]
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.]]
Approved	Approved::Yes
Keyword	WBRKeyword::PEP Carboxykinase, WBRKeyword::Gluconeogenesis, WBRKeyword::Pyruvate, WBRKeyword::Glycolysis
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::