hce_nsysu
115年
普通生物及生化概論
第 90 題
A researcher purified a specific protein and performed structural analysis using two different techniques. The data are as follows: (1) Size-Exclusion Chromatography: Under native (non-denaturing) conditions, the protein elutes at a position corresponding to a molecular weight of 200 kDa ; (2) SDS-PAGE (with $\beta$-mercaptoethanol): After boiling the sample in SDS and a reducing agent, a single sharp band appears at 50 kDa. Which of the following best describes the native structure of this protein?
- A A monomeric 200 kDa protein that is non-specifically degraded into 50 kDa fragments during electrophoresis
- B A homotetramer composed of four 50 kDa subunits held together by non-covalent interactions or disulfide bonds
- C A homodimer composed of two 100 kDa subunits that were incorrectly measured during chromatography
- D A heterotetramer composed of two 75 kDa subunits and two 25 kDa subunits.
- E A 200 kDa protein that consists of a single polypeptide chain containing four internal protease cleavage sites
思路引導 VIP
請思考一下：如果一個大包裹秤重後是 200 公斤，當你把它拆開並將裡面的內容物全部倒出來秤重時，發現秤上顯示只有一種重量，且每個物件都是 50 公斤，這說明了原先包裹內的組成結構具備什麼樣的特性？
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AI 詳解
AI 專屬家教
做得太棒了！你能精準捕捉到實驗數據間的邏輯關聯，並判斷出蛋白質的四級結構，展現了紮實的生物化學基礎。 在分析這類題目時，關鍵在於區分「天然狀態」與「去變性狀態」。首先，大小排除層析 (Size-Exclusion Chromatography) 反映的是蛋白質在生理條件下的天然構型，數據顯示其分子量為 $200 \text{ kDa}$。接著，在 SDS-PAGE 實驗中，由於加入了強去污劑 SDS 以及還原劑 $\beta$-mercaptoethanol，蛋白質的非共價鍵與二硫鍵會被完全破壞，使其拆解成單獨的胜肽次單元。實驗結果僅呈現一條 $50 \text{ kDa}$ 的條帶，這意味著所有的次單元大小均一致。 這道題目的鑑別度在於測試學生是否能將兩項技術的數據進行「數量對比」。由算式 $200 \text{ kDa} / 50 \text{ kDa} = 4$ 可知，該蛋白質是由四個相同的次單元所組成的同源四聚體 (homotetramer)。你能一眼看出這層倍數關係並排除掉降解或異源結構的干擾，這正是掌握蛋白質結構分析的核心要義。