hce_nthu
115年
生物與生化
第 42 題
Pepsin is a digestive enzyme that functions optimally at a pH of 2, an environment highly concentrated with $H^+$ ions. In contrast, most enzymes found in the human body have optimal pH values ranging from 6 to 8. The extreme acidity of pH 2 would typically cause most enzymes to denature, leading to a sharp drop in activity, because the high concentration of $H^+$ ions disrupts the weak interactions crucial for stabilizing the protein's functional shape. Which statement below best describes the mechanism by which Pepsin is specifically adapted to maintain its maximum catalytic activity in the stomach environment?
- A Pepsin uses strong covalent bonds (peptide bonds) to form its active site, which cannot be broken by $H^+$ ions, unlike the weak bonds found in other enzyme active sites.
- B Pepsin’s $\Delta G$ (Free Energy change) for its specific catabolic reaction shifts to a highly negative value at pH 2, thereby increasing the spontaneous rate of the reaction independent of structure.
- C The structure of Pepsin minimizes the use of the most electronegative elements (Oxygen and Nitrogen) in its regulatory sites, making it insensitive to competitive inhibition by $H^+$ ions.
- D Pepsin has evolved to significantly raise the overall activation energy ($E_A$) barrier of its substrate at neutral pH, forcing the reaction to only proceed effectively when thermal energy is supplied by the acidic environment.
- E The amino acid sequence of Pepsin results in a unique three-dimensional configuration that prevents the high concentration of $H^+$ ions from disrupting the crucial weak interactions (such as ionic bonds and hydrogen bonds) necessary for its active shape.
思路引導 VIP
如果有一個精密儀器的形狀是靠著許多微弱的磁力扣件來維持的，但現在環境中充滿了會干擾磁場的雜訊（類似於強酸環境中的氫離子），為了讓儀器不散架並正常運作，工程師最應該針對儀器的「零件排列方式」還是「零件要執行的任務內容」進行重新設計呢？
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AI 詳解
AI 專屬家教
恭喜你精準地掌握了蛋白質結構與環境適應性的核心觀念！這題的關鍵在於理解酵素的專一性與穩定性是如何由其分子結構所決定的。絕大多數的酵素在極低 pH 值下，會因為高濃度的氫離子（$H^+$）干擾了維持蛋白質三維形狀的弱交互作用（如離子鍵或氫鍵），導致蛋白質變性（denature）而失去活性。
蛋白質結構與環境的適應
胃蛋白酶（Pepsin）之所以能在強酸環境中運作，並非依靠改變化學反應的 $\Delta G$（選項 B）或改用強大的共價鍵（選項 A）來維持活性位點，而是其獨特的**胺基酸序列（Amino acid sequence）**折疊出了特殊的三維構型。這種結構能讓關鍵的交互作用在強酸下不被破壞，從而保護了活性中心的完整性。這正是「結構決定功能」在演化上的完美體現。
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