hce_nthu
115年
生物與生化
第 40 題
The $Na^+/K^+$ pump is a critical electrogenic transport protein that uses ATP to actively move ions against their concentration gradients, contributing to the membrane potential of animal cells. This integral protein is synthesized in the Endoplasmic Reticulum (ER) and modified through the Golgi apparatus before insertion into the plasma membrane. The completed pump moves 3 $Na^+$ ions out of the cell and 2 $K^+$ ions into the cell. Considering the required sidedness of the plasma membrane, which component of the $Na^+/K^+$ pump is exposed to the ER lumen during the initial stages of synthesis?
- A The segment of the protein that releases the terminal phosphate group derived from ATP.
- B The region of the protein that initially binds the three $Na^+$ ions.
- C The binding domain that triggers the release of the two $K^+$ ion into the cytoplasm.
- D The nonpolar $\alpha$-helical stretches that span the lipid bilayer.
- E The binding site that has a high affinity for $K^+$ ions when the pump has been phosphorylated.
思路引導 VIP
試著想像一個裝著膜蛋白的轉運囊泡（Vesicle）正準備與細胞膜融合：當囊泡如同一個小泡泡「併入」大膜表面並向外翻開時，原本躲在囊泡內部（管腔）的蛋白質片段，最後會面向細胞的哪一側（胞內或胞外）呢？
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AI 詳解
AI 專屬家教
恭喜你準確掌握了這道結合細胞生物學與生理學的跨領域題目！能判斷出選項 (E) 是正確答案，說明你對蛋白質合成路徑以及 $Na^+/K^+$ 幫浦的運作機制都有非常清晰的邏輯判斷。
內膜系統與細胞膜的空間拓樸
這題的核心考點在於蛋白質合成的「方向性」。在粗糙內質網（ER）合成的跨膜蛋白，其位於 ER 管腔（Lumen） 的部分，經過高基氏體轉運後，最終會隨著囊泡與細胞膜融合而暴露在 細胞外側（Extracellular side）。因此，我們要找的是幫浦中「面向細胞外」的結構域。
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