View source for WBR0861

 {{WBRQuestion
|QuestionAuthor= {{SSK}} (Reviewed by Serge Korjian)
|ExamType=USMLE Step 1
|MainCategory=Biochemistry
|SubCategory=General Principles
|MainCategory=Biochemistry
|SubCategory=General Principles
|MainCategory=Biochemistry
|SubCategory=General Principles
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|SubCategory=General Principles
|MainCategory=Biochemistry
|SubCategory=General Principles
|MainCategory=Biochemistry
|SubCategory=General Principles
|MainCategory=Biochemistry
|SubCategory=General Principles
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|SubCategory=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?
|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.


[[Image:Gluconeogenesis.png|900px]]
|AnswerA=Oxaloacetate
|AnswerAExp=Oxaloacetate is the end-product produced by pyruvate carboxylase. This enzyme does not require GTP.
|AnswerB=Phosphoenolpyruvate
|AnswerBExp=Phosphoenolpyruvate is the end-product produced by PEP carboxykinase. This enzyme requires GTP to function properly.
|AnswerC=Fructose-1,6-bisphosphate
|AnswerCExp=Fructose-1,6-bisphosphate is the substrate of fructose-1,6-bisphosphatase. This enzyme does not require GTP.
|AnswerD=Fructose-6-phosphate
|AnswerDExp=Fructose-6-phosphate is the end-product of fructose-1,6-bisphosphatase. this enzyme does not require GTP.
|AnswerE=Pyruvate
|AnswerEExp=Pyruvate is the substrate of pyruvate carboxylase. This enzyme does not require GTP.
|EducationalObjectives=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.<br>
Le T, Bhushan V. First Aid for the USMLE Step 1 2013. McGraw-Hill Medical; 2013.
|RightAnswer=B
|WBRKeyword=PEP Carboxykinase, Gluconeogenesis, Pyruvate, Glycolysis, 
|Approved=Yes
}}