WBR0971
| Author | [[PageAuthor::Rim Halaby, M.D. [1]]] |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Biochemistry |
| Sub Category | SubCategory::General Principles |
| Prompt | [[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?]] |
| Answer A | AnswerA::Arginase |
| Answer A Explanation | [[AnswerAExp::This answer is incorrect because arginase is a cytoplasmic enzyme.]] |
| Answer B | AnswerB::Arginosuccinate synthetase |
| Answer B Explanation | [[AnswerBExp::This answer is incorrect because arginosuccinate synthetase is a cytoplasmic enzyme.]] |
| Answer C | AnswerC::Arginosuccinase |
| Answer C Explanation | [[AnswerCExp::This answer is incorrect because arginosuccinase 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.]] |
| Answer E | AnswerE::Carbamoyl phosphate synthetase II |
| Answer E Explanation | [[AnswerEExp::This answer is incorrect because carbamoyl phosphate synthetase II is a cytoplasmic enzyme.]] |
| Right Answer | RightAnswer::C |
| Explanation | [[Explanation::The urea cycle involves a set of 5 enzymes that help in the conversion of ammonia into urea which is excreted in the urine. A part of the reactions in the urea cycle occurs in the mitochondria whereas the remaining reactions occur in the cytoplasm of the hepatocytes.
The mitochondrial enzymes are: carbamoyl phosphate synthetase I and ornitithine transcarbamoylase (OTC). The cytoplasmic enzymes are: arginosuccinate synthetase, arginosuccinase and arginase. Shown below is a diagram depicting the different steps of the urea cycle.
Educational objective: The mitochondrial enzymes involved in the urea cycle are carbamoyl phosphate synthetase I and 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). First Aid for the USMLE step 1, 2013. Page 96-106. |
| Approved | Approved::No |
| Keyword | WBRKeyword::Urea cycle, WBRKeyword::Carbamoyl phosphate synthetase I |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |
