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{{WBRQuestion
{{WBRQuestion
|QuestionAuthor={{Rim}}
|QuestionAuthor= {{Rim}} (Reviewed by  {{YD}})
|ExamType=USMLE Step 1
|ExamType=USMLE Step 1
|MainCategory=Biochemistry
|MainCategory=Biochemistry
Line 8: Line 8:
|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=Mitonchondria are purified from the livers of pigs and samples of the mitochondrial enzymes involved in the urea cycle are isolated. Fatty acylation assays were carried out on the samples in order to study the regulatory role of fatty acylation on the kinetic activity of these enzymes.   Which of the following enzymes is most likely present in the purified mitochondrial sample?
|Prompt=In an experiment, liver mitochondrial subfractions are purified from sample of bovine livers, and mitochondrial enzymes involved in the urea cycle are isolated. Fatty acylation assays are then carried out on the samples in order to study the regulatory role of fatty acylation in the kinetic activity of these enzymes. Which of the following enzymes is most likely present in the purified mitochondrial sample?
|Explanation=Urea cycle involves a set of 5 enzymes that help in the conversion of ammonia into urea that is excreted in the urine. A part of the urea cycle occurs in the cytoplasm whereas the remaining reactions occur in the cytoplasm of hepatocytes.
|Explanation=The urea cycle involves a set of 5 hepatic enzymes that collectively convert ammonia into urea. Two of the reactions involved in urea synthesis occur within the [[mitochondria]], whereas the remaining reactions occur in the [[cytoplasm]].<br>
The [[mitochondria|mitochondrial]] enzymes are: carbamoyl phosphate synthetase I and [[OTC|ornitithine transcarbamoylase]] (OTC).
*The [[mitochondria|mitochondrial]] enzymes are carbamoyl phosphate synthetase I (requires ATP) and [[OTC|ornithine transcarbamylase]] (OTC)<br>
The cytoplasmic enzymes are: arginosuccinate synthetase, arginosuccinase and [[arginase]].
*The cytoplasmic enzymes are argininosuccinate synthetase (requires ATP), argininosuccinase, and [[arginase]]<br>
 
<br>
Shown below is a diagram depicting the different steps of the [[urea cycle]].
Shown below is an illustration of the reactions of the [[urea cycle]], all of which occur in hepatocytes in the liver<br>
 
[[File:Urea Cycle.jpg|700px]]
[[File:Urea Cycle Diagram.png|500px]]
 
Educational objective:
The [[mitochondria|mitochondrial]] enzymes involved in the [[urea cycle]] are carbamoyl phosphate synthetase I and [[OTC|ornitithine transcarbamoylase]] (OTC).
 
Reference:
M. Corv et al. Regulation of mitochondrial carbamoyl-phosphate synthetase 1 activity by active site fatty acylation. Journal of the biological chemistry (2001).
|AnswerA=Arginase
|AnswerA=Arginase
|AnswerAExp=This answer is incorrect because [[arginase]] is a [[cytoplasm|cytoplasmic]] enzyme.
|AnswerAExp=[[Arginase]] is a [[cytoplasm|cytoplasmic]] enzyme.
|AnswerB=Arginosuccinate synthetase
|AnswerB=Argininosuccinate synthetase
|AnswerBExp=This answer is incorrect because [[arginosuccinate synthetase]] is a [[cytoplasm|cytoplasmic]] enzyme.
|AnswerBExp=Argininosuccinate synthetase is a [[cytoplasm|cytoplasmic]] enzyme.
|AnswerC=Arginosuccinase
|AnswerC=Argininosuccinase
|AnswerCExp=This answer is incorrect because [[arginosuccinase]] is a [[cytoplasm|cytoplasmic]] enzyme.
|AnswerCExp=Argininosuccinase is a [[cytoplasm|cytoplasmic]] enzyme.
|AnswerD=Carbamoyl phosphate synthetase I
|AnswerD=Carbamoyl phosphate synthetase I
|AnswerDExp=Carbamoyl phosphate synthetase I is a mitochondrial enzyme involved in the [[urea cycle]].
|AnswerDExp=[[Carbamoyl phosphate synthetase I]] is a mitochondrial enzyme involved in the [[urea cycle]]. It utilizes ATP to catalyze the reaction that produces carbamoyl phosphate from carbon dioxide and ammonia.
|AnswerE=Carbamoyl phosphate synthetase II
|AnswerE=Carbamoyl phosphate synthetase II
|AnswerEExp=This answer is incorrect because carbamoyl phosphate synthetase II is a [[cytoplasm|cytoplasmic]] enzyme.
|AnswerEExp=Carbamoyl phosphate synthetase II is a [[cytoplasm|cytoplasmic]] enzyme involved in pyrimidine biosynthesis. It catalyzes the reaction that produces carbamoyl phosphate.
|RightAnswer=C
|EducationalObjectives=The [[mitochondria|mitochondrial]] enzymes involved in the [[urea cycle]] are carbamoyl phosphate synthetase I and [[OTC|ornithine transcarbamylase]] (OTC).
|Approved=No
|References=Corvi MM, Soltus CL, Berthiaume LG. Regulation of mitochondrial carbamoyl-phosphate synthetase 1 activity by active site fatty acylation. J Biol Chem. 2001;276(49):45704-12.<br>
First Aid 2014 page 108
|RightAnswer=D
|WBRKeyword=Urea cycle, Carbamoyl phosphate synthetase I, Mitochondrial enzyme, Enzyme, Mitochondria, Urea synthesis, Ammonia, Bovine liver, Experiment, Fatty acylation
|Approved=Yes
}}
}}

Latest revision as of 02:14, 28 October 2020
Author [[PageAuthor::Rim Halaby, M.D. [1] (Reviewed by Yazan Daaboul, M.D.)]]
Exam Type ExamType::USMLE Step 1
Main Category MainCategory::Biochemistry
Sub Category SubCategory::General Principles
Prompt [[Prompt::In an experiment, liver mitochondrial subfractions are purified from sample of bovine livers, and mitochondrial enzymes involved in the urea cycle are isolated. Fatty acylation assays are then carried out on the samples in order to study the regulatory role of fatty acylation in the kinetic activity of these enzymes. Which of the following enzymes is most likely present in the purified mitochondrial sample?]]
Answer A AnswerA::Arginase
Answer A Explanation [[AnswerAExp::Arginase is a cytoplasmic enzyme.]]
Answer B AnswerB::Argininosuccinate synthetase
Answer B Explanation [[AnswerBExp::Argininosuccinate synthetase is a cytoplasmic enzyme.]]
Answer C AnswerC::Argininosuccinase
Answer C Explanation [[AnswerCExp::Argininosuccinase is a cytoplasmic enzyme.]]
Answer D AnswerD::Carbamoyl phosphate synthetase I
Answer D Explanation [[AnswerDExp::Carbamoyl phosphate synthetase I is a mitochondrial enzyme involved in the urea cycle. It utilizes ATP to catalyze the reaction that produces carbamoyl phosphate from carbon dioxide and ammonia.]]
Answer E AnswerE::Carbamoyl phosphate synthetase II
Answer E Explanation [[AnswerEExp::Carbamoyl phosphate synthetase II is a cytoplasmic enzyme involved in pyrimidine biosynthesis. It catalyzes the reaction that produces carbamoyl phosphate.]]
Right Answer RightAnswer::D
Explanation [[Explanation::The urea cycle involves a set of 5 hepatic enzymes that collectively convert ammonia into urea. Two of the reactions involved in urea synthesis occur within the mitochondria, whereas the remaining reactions occur in the cytoplasm.


Shown below is an illustration of the reactions of the urea cycle, all of which occur in hepatocytes in the liver

Educational Objective: The mitochondrial enzymes involved in the urea cycle are carbamoyl phosphate synthetase I and ornithine transcarbamylase (OTC).
References: Corvi MM, Soltus CL, Berthiaume LG. Regulation of mitochondrial carbamoyl-phosphate synthetase 1 activity by active site fatty acylation. J Biol Chem. 2001;276(49):45704-12.
First Aid 2014 page 108]]

Approved Approved::Yes
Keyword WBRKeyword::Urea cycle, WBRKeyword::Carbamoyl phosphate synthetase I, WBRKeyword::Mitochondrial enzyme, WBRKeyword::Enzyme, WBRKeyword::Mitochondria, WBRKeyword::Urea synthesis, WBRKeyword::Ammonia, WBRKeyword::Bovine liver, WBRKeyword::Experiment, WBRKeyword::Fatty acylation
Linked Question Linked::
Order in Linked Questions LinkedOrder::
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