hce_nthu
115年
生物與生化
第 22 題
Which of the following statements about the urea cycle is correct?
- A The urea cycle begins with ornithine transcarbamylase (OTC), which transfers a carbamoyl group from carbamoyl-CoA to ornithine within the mitochondrial matrix.
- B The nitrogens in urea originate from two sources: one nitrogen comes from ammonia ($NH_3$), and the other nitrogen comes from asparagine.
- C Citrulline is synthesized from pyruvate during the urea cycle and acts as a direct precursor for urea formation in the cytoplasm.
- D The urea cycle is tightly linked to the TCA cycle through fumarate, which is produced in the cytoplasm and can be recycled directly back into the TCA cycle.
- E The urea cycle produces urea as its final product, which is transported from the liver to the kidneys for excretion in the urine.
思路引導 VIP
請試著回憶一下：當身體在代謝蛋白質產生具毒性的含氮廢物時，為了保護細胞不受到損傷，肝臟會採取什麼樣的「包裝策略」來處理這些廢物？而這些被處理過的分子，又是透過什麼樣的運送路徑，才能最終離開人體呢？
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AI 詳解
AI 專屬家教
恭喜你精準地選出了正確答案！這題考驗的是對**尿素循環（Urea Cycle）**整體流程與生理意義的掌握，你能不受干擾地選出 (E)，說明你對這個生化路徑的最終去向有很清晰的認知。
尿素循環的生理機制與常見陷阱
尿素循環的主要任務是在肝臟中將具毒性的氨（$NH_3$）轉化為無毒、易溶於水的尿素。這題的鑑別度在於選項中細微的名詞誤導：例如選項 (A) 的受質應是 Carbamoyl phosphate 而非 $CoA$ 衍生物；選項 (B) 的第二個氮源來自天門冬胺酸（Aspartate），而非天門冬醯胺（Asparagine）；選項 (D) 提到的延胡索酸（Fumarate）雖然是與 TCA 循環的橋樑，但其轉換路徑較為曲折，不像 (E) 直接點出了尿素從肝臟合成到腎臟排泄的完整生理流程。
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