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{{WBRQuestion
{{WBRQuestion
|QuestionAuthor={{Rim}}
|QuestionAuthor= {{YD}} (Reviewed by Serge Korjian)
|ExamType=USMLE Step 1
|ExamType=USMLE Step 1
|MainCategory=Biochemistry
|MainCategory=Biochemistry
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|MainCategory=Biochemistry
|MainCategory=Biochemistry
|SubCategory=General Principles
|SubCategory=General Principles
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|MainCategory=Biochemistry
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|MainCategory=Biochemistry
|MainCategory=Biochemistry
|SubCategory=General Principles
|SubCategory=General Principles
|Prompt=Shown below is a diagram depicting the sorbitol metabolism. Which of the following correspond to the cofactors X and Y respectively?
|Prompt=Sorbitol concentration is measured in hepatocytes of normal rats and mutant rats with glucose intolerance. The aim of the experiment is to establish the association between the blood concentration of glucose and the concentration of sorbitol in hepatocytes. Shown below is a figure that illustrates sorbitol metabolism. Which compounds correspond to the cofactors X and Y, respectively?<br>
 
[[Image:WBR0979.png|600px]]
[[File:Sorbitol pathway question.png]]
|Explanation=NADPH is the reduced form of NADP+ whereas NAD+ is the oxidized form of NADH. Both NADPH and NAD+ are cofactors are electron carriers and donors in the redox reactions of sorbitol metabolism. Glucose is converted to sorbitol by the action of aldose reductase (requires NADPH), and sorbitol is then converted to fructose by the action of sorbitol dehydrogenase (requires NAD+). While some cells are lacking sorbitol dehydrogenase (cells of the retina, Schwann cells, and kidney cells), other cells (e.g. hepatocytes and cells of the ovaries and seminal vesicles) have both enzymes to metabolize glucose into fructose.<br>
|Explanation=NADPH is the reduced form of NADP+ and NAD+ is the oxidized form of NADH. These cofactors play a role as electon carriers and donors in redox reactions. [[Glucose]] is converted to sorbitol under the action of [[aldose reductase]] which requires NADPH as a cofactor.  Sorbitol is converted to fructose under the action of sorbitol dehydrogenase which requires NAD+. Shown below is a diagram depicting the metabolism of sorbitol.
Below is a figure that illustrates the metabolism of sorbitol<br>
 
[[Image:Sorbitol pathway illustration.png|700px]]
[[ File:Sorbitol pathway.png]]
|AnswerA=X=NADPH and Y=NAD+
 
|AnswerAExp=NADPH and NAD+ are cofactors for aldose reductase and sorbitol dehydrogenase, respectively. Both enzymes and their cofactors are involved in sorbitol metabolism.
'''Educational objective:'''
|AnswerB=X=NAD+ and Y=NADPH
Glucose is converted to sorbitol under the action of [[aldose reductase]] which requires NADPH as a cofactor.  Sorbitol is converted to fructose under the action of sorbitol dehydrogenase which requires NAD+.
|AnswerBExp=While both NAD+ and NADPH are needed for sorbitol metabolism, X corresponds to NADPH and Y corresponds to NAD+, and the order by which the enzymes and their cofactors function in the sorbitol pathway is important.
 
|AnswerC=X=ATP and Y=NAD+
'''Reference:'''
|AnswerCExp=NADPH and NAD+ are cofactors for aldose reductase and sorbitol dehydrogenase, respectively. Both enzymes and their cofactors are involved in sorbitol metabolism. ATP is not a cofactor in either reaction.
First aid for USMLE step 1, 2013. Page 104.
|AnswerD=X=NADPH and Y=ADP
|AnswerA=NADPH and NAD+
|AnswerDExp=NADPH and NAD+ are cofactors for aldose reductase and sorbitol dehydrogenase, respectively. Both enzymes and their cofactors are involved in sorbitol metabolism. ADP is not a cofactor in either reaction.
|AnswerAExp=NADPH and NAD+ are the correct cofactors corresponding to X and Y respectively.
|AnswerE=X=NAD+ and Y=ATP
|AnswerB=NAD+ and NADPH
|AnswerEExp=NADPH and NAD+ are cofactors for aldose reductase and sorbitol dehydrogenase, respectively. Both enzymes and their cofactors are involved in sorbitol metabolism. ATP is not a cofactor in either reaction.
|AnswerBExp=This answer is incorrect.  NADPH and NAD+ are the correct cofactors corresponding to X and Y respectively.
|EducationalObjectives=NADPH and NAD+ are cofactors for aldose reductase and sorbitol dehydrogenase, respectively. Both enzymes and their cofactors are involved in sorbitol metabolism.
|AnswerC=ATP and NAD+
|References=First Aid 2014 page 107
|AnswerCExp=This answer is incorrect.  NADPH and NAD+ are the correct cofactors corresponding to X and Y respectively.
|AnswerD=NADPH and ADP
|AnswerDExp=This answer is incorrect.  NADPH and NAD+ are the correct cofactors corresponding to X and Y respectively.
|AnswerE=NAD+ and ATP
|AnswerEExp=This answer is incorrect. NADPH and NAD+ are the correct cofactors corresponding to X and Y respectively.
|RightAnswer=A
|RightAnswer=A
|WBRKeyword=NADPH, NAD+, fructose, cofactor
|WBRKeyword=NADPH, NAD+, Fructose, Cofactor, Sorbitol, Sorbitol metabolism, Sorbitol pathway, Diabetes
|Approved=No
|Approved=Yes
}}
}}

Latest revision as of 02:16, 28 October 2020
Author [[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Serge Korjian)]]
Exam Type ExamType::USMLE Step 1
Main Category MainCategory::Biochemistry
Sub Category SubCategory::General Principles
Prompt [[Prompt::Sorbitol concentration is measured in hepatocytes of normal rats and mutant rats with glucose intolerance. The aim of the experiment is to establish the association between the blood concentration of glucose and the concentration of sorbitol in hepatocytes. Shown below is a figure that illustrates sorbitol metabolism. Which compounds correspond to the cofactors X and Y, respectively?

]]

Answer A AnswerA::X=NADPH and Y=NAD+
Answer A Explanation AnswerAExp::NADPH and NAD+ are cofactors for aldose reductase and sorbitol dehydrogenase, respectively. Both enzymes and their cofactors are involved in sorbitol metabolism.
Answer B AnswerB::X=NAD+ and Y=NADPH
Answer B Explanation AnswerBExp::While both NAD+ and NADPH are needed for sorbitol metabolism, X corresponds to NADPH and Y corresponds to NAD+, and the order by which the enzymes and their cofactors function in the sorbitol pathway is important.
Answer C AnswerC::X=ATP and Y=NAD+
Answer C Explanation AnswerCExp::NADPH and NAD+ are cofactors for aldose reductase and sorbitol dehydrogenase, respectively. Both enzymes and their cofactors are involved in sorbitol metabolism. ATP is not a cofactor in either reaction.
Answer D AnswerD::X=NADPH and Y=ADP
Answer D Explanation AnswerDExp::NADPH and NAD+ are cofactors for aldose reductase and sorbitol dehydrogenase, respectively. Both enzymes and their cofactors are involved in sorbitol metabolism. ADP is not a cofactor in either reaction.
Answer E AnswerE::X=NAD+ and Y=ATP
Answer E Explanation AnswerEExp::NADPH and NAD+ are cofactors for aldose reductase and sorbitol dehydrogenase, respectively. Both enzymes and their cofactors are involved in sorbitol metabolism. ATP is not a cofactor in either reaction.
Right Answer RightAnswer::A
Explanation [[Explanation::NADPH is the reduced form of NADP+ whereas NAD+ is the oxidized form of NADH. Both NADPH and NAD+ are cofactors are electron carriers and donors in the redox reactions of sorbitol metabolism. Glucose is converted to sorbitol by the action of aldose reductase (requires NADPH), and sorbitol is then converted to fructose by the action of sorbitol dehydrogenase (requires NAD+). While some cells are lacking sorbitol dehydrogenase (cells of the retina, Schwann cells, and kidney cells), other cells (e.g. hepatocytes and cells of the ovaries and seminal vesicles) have both enzymes to metabolize glucose into fructose.

Below is a figure that illustrates the metabolism of sorbitol

Educational Objective: NADPH and NAD+ are cofactors for aldose reductase and sorbitol dehydrogenase, respectively. Both enzymes and their cofactors are involved in sorbitol metabolism.
References: First Aid 2014 page 107]]

Approved Approved::Yes
Keyword WBRKeyword::NADPH, WBRKeyword::NAD+, WBRKeyword::Fructose, WBRKeyword::Cofactor, WBRKeyword::Sorbitol, WBRKeyword::Sorbitol metabolism, WBRKeyword::Sorbitol pathway, WBRKeyword::Diabetes
Linked Question Linked::
Order in Linked Questions LinkedOrder::
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