2.4 Enzymes: Past Paper Practice | 2.4 酶：真题精练

📚 2.4 Enzymes: Past Paper Practice | 2.4 酶：真题精练

Enzymes are biological catalysts that speed up metabolic reactions by lowering activation energy. In A-level Biology, Topic 2.4 covers enzyme structure, mechanism, factors affecting activity, inhibition, and practical applications. This article consolidates key concepts through typical past paper questions, providing bilingual explanations to strengthen your exam technique.

酶是生物催化剂，通过降低活化能加速代谢反应。A-Level 生物课程 2.4 节涵盖酶的结构、作用机制、影响活性的因素、抑制作用及实际应用。本文通过典型真题精练，辅以双语解析，帮助你巩固核心概念，提升应试技巧。

1. Enzyme Action and Activation Energy | 酶的作用与活化能

Enzymes lower activation energy by providing an alternative reaction pathway. They form enzyme-substrate complexes at the active site. The activation energy is the minimum energy required for a reaction to occur.

酶通过提供替代反应途径来降低活化能。它们在活性位点形成酶-底物复合物。活化能是反应发生所需的最小能量。

Typical Exam Question: Explain how enzymes catalyse metabolic reactions. (3 marks)

典型真题：解释酶如何催化代谢反应。(3分)

Model Answer: Enzymes lower the activation energy of a reaction; they bind to substrate(s) at the active site forming an enzyme-substrate complex; this puts strain on bonds or brings substrates closer together, facilitating bond breaking or formation; the products are then released, and the enzyme remains unchanged.

参考答案：酶降低反应的活化能；它们在活性位点与底物结合，形成酶-底物复合物；这使化学键受到张力或使底物更接近，促进键断裂或形成；随后产物释放，酶本身不变。

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2. Lock-and-Key vs Induced-Fit Model | 锁钥模型与诱导契合模型

The lock-and-key model suggests the active site is exactly complementary to the substrate. The induced-fit model proposes that the active site changes shape slightly to wrap around the substrate, straining bonds.

锁钥模型认为活性位点与底物严格互补。诱导契合模型提出活性位点略微改变形状以包裹底物，使化学键产生应变。

Exam Question: Contrast the lock-and-key and induced-fit models of enzyme action. (4 marks)

真题：比较锁钥模型和诱导契合模型的作用方式。(4分)

Answer: Lock-and-key: active site has a rigid, fixed shape complementary to substrate; substrate fits without conformational change. Induced-fit: active site is flexible; upon substrate binding, the active site moulds around the substrate; this distorts bonds in the substrate, lowering activation energy. The induced-fit model explains the broad specificity of some enzymes better.

答案：锁钥模型：活性位点具有刚性的、与底物互补的固定形状；底物无需构象变化即可结合。诱导契合模型：活性位点是灵活的；底物结合时，活性位点发生形变包裹底物；这使底物内的键发生扭曲，降低活化能。诱导契合模型能更好地解释某些酶的广泛特异性。

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3. Factors Affecting Enzyme Activity: Temperature | 温度影响酶活性

At low temperatures, molecules have less kinetic energy, so fewer successful collisions occur. As temperature rises to the optimum, reaction rate increases. Beyond the optimum, the enzyme denatures: hydrogen bonds and hydrophobic interactions break, altering the tertiary structure.

低温下，分子动能较低，成功碰撞较少。温度升至最适温度时，反应速率增加。超过最适温度后，酶变性：氢键和疏水相互作用被破坏，三级结构改变。

Past Paper: A student investigated the effect of temperature on the activity of catalase. Explain the shape of the temperature-activity graph. Include reference to Q₁₀. (4 marks)

真题：一名学生研究了温度对过氧化氢酶活性的影响。解释温度-活性图的形状，并提及Q₁₀。(4分)

Answer: Activity increases with temperature due to greater kinetic energy and more frequent collisions. The Q₁₀ (temperature coefficient) for enzyme-controlled reactions is typically around 2, meaning rate doubles for every 10 °C rise up to optimum. Above the optimum, the rate drops sharply because the enzyme loses its specific shape (denaturation); the active site is no longer complementary to the substrate.

答案：活性随温度升高而增加，因为动能增大和碰撞更频繁。酶促反应的Q₁₀（温度系数）通常约为2，即每升高10°C速率翻倍，直到最适温度。超过最适温度后，速率急剧下降，因为酶丧失特异性形状（变性）；活性位点不再与底物互补。

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4. Factors Affecting Enzyme Activity: pH | pH 影响酶活性

pH affects the ionisation of amino acid side chains in the active site. Each enzyme has an optimum pH. Deviation from this pH changes the charge distribution, disrupting ionic bonds and hydrogen bonds, leading to denaturation.

pH 影响活性位点氨基酸侧链的电离状态。每种酶都有最适 pH。偏离此 pH 会改变电荷分布，破坏离子键和氢键，导致变性。

Question: Pepsin works in the stomach at pH 2, while trypsin works in the small intestine at pH 8. Explain why these enzymes have different optimum pH values. (3 marks)

问题：胃蛋白酶在胃中 pH 2 条件下工作，而胰蛋白酶在小肠 pH 8 条件下工作。解释为什么这些酶有不同最适 pH。(3分)

Answer: The amino acid composition at the active site determines the charge pattern for substrate binding and catalysis. Pepsin has evolved to function in acidic environments, with ionisable groups that maintain correct shape at low pH. Trypsin has ionisable groups that are correctly protonated at alkaline pH. A change in pH alters the ionic charges, causing the tertiary structure to unfold.

答案：活性位点的氨基酸组成决定了底物结合和催化所需的电荷模式。胃蛋白酶已进化到在酸性环境中工作，其可电离基团在低 pH 下保持正确形状。胰蛋白酶的可电离基团在碱性 pH 下能正确质子化。pH 改变会改变离子电荷，导致三级结构展开。

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5. Substrate Concentration and Enzyme Kinetics | 底物浓度与酶动力学

At low substrate concentration, the rate increases linearly with [S] because more active sites are occupied. At high [S], the enzyme becomes saturated; all active sites are occupied, and the maximum rate (Vₘₐₓ) is reached. The Michaelis-Menten constant, Kₘ, represents the substrate concentration at which the rate is ½ Vₘₐₓ.

在低底物浓度下，反应速率随[S]线性增加，因为更多活性位点被占据。在高[S]下，酶被饱和；所有活性位点都被占据，达到最大速率(Vₘₐₓ)。米氏常数Kₘ表示反应速率为½ Vₘₐₓ时的底物浓度。

Past Paper: Sketch a graph of initial rate against substrate concentration for an enzyme-catalysed reaction. Label Vₘₐₓ and Kₘ. Explain the significance of Kₘ. (4 marks)

真题：绘制酶促反应初始速率相对底物浓度的曲线图。标出Vₘₐₓ和Kₘ。解释Kₘ的意义。(4分)

Answer: The graph is a hyperbola increasing to a plateau. Vₘₐₓ is the asymptote. Kₘ is the substrate concentration at ½ Vₘₐₓ. It indicates the enzyme’s affinity for the substrate: a low Kₘ means high affinity because a low substrate concentration is needed to reach half-maximal velocity.

答案：图形为双曲线，逐渐达到平台。Vₘₐₓ是渐近线。Kₘ是½ Vₘₐₓ时的底物浓度。它指示酶对底物的亲和力：Kₘ低意味着亲和力高，因为只需较低的底物浓度即可达到半最大速度。

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6. Competitive Inhibition | 竞争性抑制

A competitive inhibitor resembles the substrate and binds reversibly to the active site. It does not affect Vₘₐₓ because high substrate concentration can outcompete the inhibitor. However, Kₘ increases (more substrate needed to reach ½ Vₘₐₓ).

竞争性抑制剂与底物结构相似，可逆地与活性位点结合。它不影响Vₘ

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