WBR0972: Difference between revisions

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|MainCategory=Biochemistry
|MainCategory=Biochemistry
|SubCategory=General Principles
|SubCategory=General Principles
|Prompt=A newborn baby who was normal at birth is noted to be lethargic, hypothermic and feeding poorly at his first days of life. The nurse witnesses an episode of seizure following which blood laboratory tests are ordered. An elevated level of ammonia is detected.  Subsequent studies reveals deficiency in N acetylglutamate synthase. N-acetylglutamate is recognized as the regulator of the rate-limiting step of the urea cycle, and its absence leads to abnormalities in urea production and elevated ammonia levels.
|Prompt=A 35-year-old woman complains that her newborn is feeding poorly. The newborn was born one day ago by an uneventful vaginal delivery. His mother received all her prenatal vitamins during pregnancy and followed up with her obstetrician regularly. On physical examination, the newborn is hypothermic and appears lethargic. Further investigation demonstrates an elevated concentration of serum ammonia and deficiency in N-acetylglutamate synthase activity. The physician explains to the newborn's mother that his condition is caused by abnormal functioning of an enzyme of the urea cycle in hepatocytes. Which of the following reactions of the urea cycle is directly involved in this patient's condition?<br>
Shown below is a diagram depicting the different steps of the urea cycle occurring in the mitochondria and cytoplasm of hepatocytes. Which of the following reactions is the rate-limiting step of the urea cycle?
[[File:WBR0972figure.png|600px]]
 
[[File:WBR0972.png|600px]]
|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>
|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 (requires ATP) and [[OTC|ornithine transcarbamylase]] (OTC)<br>
*The [[mitochondria|mitochondrial]] enzymes are carbamoyl phosphate synthetase I (requires ATP) and [[OTC|ornithine transcarbamylase]] (OTC)<br>
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|RightAnswer=A
|RightAnswer=A
|WBRKeyword=Urea cycle, Carbamoyl phosphate synthetase I, N acetylglutamate, Urea synthesis, Ammonia, Rate limiting step, Reaction, Enzyme
|WBRKeyword=Urea cycle, Carbamoyl phosphate synthetase I, N acetylglutamate, Urea synthesis, Ammonia, Rate limiting step, Reaction, Enzyme
|Approved=No
|Approved=Yes
}}
}}

Revision as of 16:54, 10 March 2015

 
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::A 35-year-old woman complains that her newborn is feeding poorly. The newborn was born one day ago by an uneventful vaginal delivery. His mother received all her prenatal vitamins during pregnancy and followed up with her obstetrician regularly. On physical examination, the newborn is hypothermic and appears lethargic. Further investigation demonstrates an elevated concentration of serum ammonia and deficiency in N-acetylglutamate synthase activity. The physician explains to the newborn's mother that his condition is caused by abnormal functioning of an enzyme of the urea cycle in hepatocytes. Which of the following reactions of the urea cycle is directly involved in this patient's condition?

]]

Answer A AnswerA::A
Answer A Explanation AnswerAExp::A corresponds to the enzyme carbamoyl phosphate synthetase I, which is the rate-limitng step of the urea cycle. Carbamoyl phosphate synthetase I activation requires ATP and the cofactor N-acetylglutamate.
Answer B AnswerB::B
Answer B Explanation [[AnswerBExp::B corresponds to ornithine transcarbamylase, which catalyzes the reaction that produces citrulline.]]
Answer C AnswerC::C
Answer C Explanation AnswerCExp::C corresponds to argininosuccinate synthetase, which is an ATP-requiring enzyme that catalyzes the reaction that produces argininosuccinate.
Answer D AnswerD::D
Answer D Explanation AnswerDExp::D corresponds to arginosuccinase, which catalyzes the reaction that produces arginine and fumarate.
Answer E AnswerE::E
Answer E Explanation [[AnswerEExp::E corresponds to arginase, which catalyzes the reaction that produces ornithine.]]
Right Answer RightAnswer::A
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

The rate limiting step in the urea cycle is the reaction catalyzed by mitochondrial enzyme carbamoyl phosphate synthetase I that produces carbamoyl phosphate from carbon dioxide and ammonia. is the conversion of ammonia and carbon dioxide into carbamoyl phosphate. The enzyme of this rate limiting step requires ATP and the cofactor N-acetylglutamate. N-acetylglutamate synthetase deficiency in this newborn results in a reduced activation of carbamoyl phosphate synthetase I enzyme and consequently, a reduced concentration of carbamoyl phosphate.

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

Educational Objective: The rate limiting step of the urea cycle is the reaction catalyzed by the mitochondrial enzyme carbamoyl phosphate synthetase I that produces carbamoyl phosphate from carbon dioxide and ammonia. This enzyme requires ATP and the cofactor N-acetylglutamate for activation.
References: First Aid 2014 page 108]]

Approved Approved::Yes
Keyword WBRKeyword::Urea cycle, WBRKeyword::Carbamoyl phosphate synthetase I, WBRKeyword::N acetylglutamate, WBRKeyword::Urea synthesis, WBRKeyword::Ammonia, WBRKeyword::Rate limiting step, WBRKeyword::Reaction, WBRKeyword::Enzyme
Linked Question Linked::
Order in Linked Questions LinkedOrder::