WBR0971

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. The mitochondrial enzymes are carbamoyl phosphate synthetase I (requires ATP) and ornithine transcarbamylase (OTC) The cytoplasmic enzymes are argininosuccinate synthetase (requires ATP), argininosuccinase, and arginase 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::