📚 Enzymes | 酶
Enzymes are biological catalysts that speed up chemical reactions in living organisms. They are essential for metabolism, digestion, respiration, and many other processes. This revision guide covers the key concepts of enzymes for the Edexcel IGCSE Biology syllabus.
酶是生物催化剂，能够加快生物体内的化学反应。它们对代谢、消化、呼吸以及许多其他过程至关重要。本复习指南涵盖 Edexcel IGCSE 生物学大纲中关于酶的核心概念。
1. What Are Enzymes? | 什么是酶？
Enzymes are proteins made of long chains of amino acids folded into specific three-dimensional shapes. Each enzyme is unique to the reaction it catalyses.
酶是由氨基酸长链折叠成特定三维形状的蛋白质。每种酶对于它所催化的反应都是独特的。
- Enzymes are not used up in reactions – they can be reused.
- 酶在反应中不会被消耗——可以重复使用。
- Enzymes lower the activation energy needed for a reaction.
- 酶降低反应所需的活化能。
- Enzymes are affected by temperature, pH, concentration, and inhibitors.
- 酶受温度、pH、浓度和抑制剂的影响。
2. Active Sites and Specificity | 活性位点与专一性
An enzyme contains a small region called the active site. The substrate – the molecule that the enzyme acts upon – binds to this site temporarily.
酶上有一个称为活性位点的小区域。底物——即酶作用的分子——会暂时结合到这个位点上。
The shape of the active site is complementary to the shape of the substrate. This is why enzymes are specific: only one type of substrate fits a particular enzyme.
活性位点的形状与底物的形状互补。这就是酶具有专一性的原因：只有一种类型的底物能契合特定的酶。
3. Lock and Key vs. Induced Fit | 锁钥模型与诱导契合
The lock and key model suggests that the active site is a rigid shape (the lock) that only the correct substrate (the key) can fit into.
锁钥模型认为活性位点是一个刚性的形状（锁），只有正确的底物（钥匙）才能插入其中。
The induced fit model is more modern: the active site is slightly flexible and changes shape to wrap around the substrate, making the binding stronger.
诱导契合模型更为先进：活性位点略微柔性，会改变形状来包裹底物，使结合更加牢固。
Both models explain enzyme specificity, but the induced fit model better explains how enzymes stabilize the transition state.
两种模型都解释了酶的专一性，但诱导契合模型更好地解释了酶如何稳定过渡态。
4. How Enzymes Speed Up Reactions | 酶如何加速反应
Enzymes provide an alternative reaction pathway with lower activation energy. This means more reactant particles have enough energy to react, so the rate increases.
酶提供了活化能更低的替代反应途径。这意味着更多的反应物粒子具有足够的能量进行反应，因此速率加快。
The enzyme binds to the substrate, bringing reactive groups close together and stressing bonds so they break more easily.
酶与底物结合，使反应基团靠近并拉伸化学键，使其更容易断裂。
Substrate + Enzyme → Enzyme-Substrate Complex → Product + Enzyme
底物 + 酶 → 酶-底物复合物 → 产物 + 酶
5. Factors Affecting Enzyme Activity: Temperature | 影响酶活性的因素：温度
As temperature increases, particles move faster and collide more often. The rate of reaction increases up to the optimum temperature.
随着温度升高，粒子运动加快，碰撞更加频繁。反应速率在达到最适温度之前不断升高。
However, above the optimum temperature, the enzyme begins to denature. The heat breaks hydrogen bonds and other interactions that hold the protein’s shape together.
然而，超过最适温度后，酶开始变性。热量会破坏维持蛋白质形状的氢键和其他相互作用。
| Temperature | Effect on rate |
| Low (0–10°C) | Slow; molecules move slowly |
| Optimum (~40°C in humans) | Maximum rate |
| Above optimum (60°C+) | Denaturation; shape permanently lost |
| 温度 | 对速率的影响 |
| 低温（0–10°C） | 缓慢；分子运动慢 |
| 最适温度（人体内约40°C） | 速率最大 |
| 超过最适温度（60°C以上） | 变性；形状永久丧失 |
6. Factors Affecting Enzyme Activity: pH | 影响酶活性的因素：pH
Each enzyme has an optimum pH. Any change in pH affects the charges on amino acid residues and disrupts bonding within the enzyme.
每种酶都有一个最适 pH。pH 的任何变化都会影响氨基酸残基的电荷，并破坏酶内部的键合。
Extreme pH values cause denaturation. For example, pepsin in the stomach works best at pH 2, while trypsin in the intestine works best at pH 8.
极端的 pH 会导致变性。例如，胃中的胃蛋白酶最适 pH 为 2，而肠道中的胰蛋白酶最适 pH 为 8。
In an exam, you should be able to explain the shape of the rate–pH curve: a bell-shaped curve with the maximum at the optimum pH.
在考试中，你应该能够解释速率–pH 曲线的形状：一条钟形曲线，最高点对应最适 pH。
7. Enzyme Concentration and Substrate Concentration | 酶浓度和底物浓度
When substrate concentration is fixed, increasing enzyme concentration increases the rate of reaction, as long as more substrate is available.
当底物浓度固定时，增加酶浓度会提高反应速率，只要有更多的底物可用。
When enzyme concentration is fixed, increasing substrate concentration increases the rate until the active sites are all occupied. Further increases have no effect because the enzyme is saturated.
当酶浓度固定时，增加底物浓度会提高速率，直到所有活性位点都被占据。再增加底物浓度则没有影响，因为酶已经饱和。
Similar to substrate concentration, increasing enzyme concentration at high substrate levels continues to increase the rate, since more active sites are available.
与底物浓度类似，在高底物水平下增加酶浓度会继续提高速率，因为有更多的活性位点可用。
8. Inhibitors | 抑制剂
Inhibitors are molecules that reduce enzyme activity. Competitive inhibitors have a similar shape to the substrate and compete for the active site.
抑制剂是降低酶活性的分子。竞争性抑制剂具有与底物相似的形状，与底物竞争活性位点。
Non-competitive inhibitors bind elsewhere on the enzyme (allosteric site), changing the shape of the active site so the substrate can no longer bind.
非竞争性抑制剂结合在酶的其他位置（变构位点），改变活性位点的形状，使底物无法再结合。
Competitive inhibition can be overcome by adding more substrate, but non-competitive inhibition cannot.
竞争性抑制可以通过增加底物来克服，而非竞争性抑制则不能。
9. Uses of Enzymes in Industry and Medicine | 酶在工业和医药中的应用
Enzymes are widely used in biotechnology. For example, amylase breaks down starch into sugars, glucose isomerase converts glucose into fructose, and proteases break down proteins.
酶在生物技术中广泛应用。例如，淀粉酶将淀粉分解为糖类，葡萄糖异构酶将葡萄糖转化为果糖，蛋白酶分解蛋白质。
- Biological washing powders contain proteases and lipases to digest stains.
- 生物洗衣粉含有蛋白酶和脂肪酶以分解污渍。
- Lactase is used to produce lactose-free milk.
- 乳糖酶用于生产无乳糖牛奶。
- Immobilised enzymes are used in industry to make the process more efficient and reusable.
- 固定化酶在工业中用于提高效率并能够重复使用。
10. Core Practical: Investigating Enzyme Activity | 核心实验：研究酶活性
A common experiment investigates the effect of pH or temperature on the rate of amylase breaking down starch.
常见实验是研究 pH 或温度对淀粉酶分解淀粉速率的影响。
Method: add amylase to a starch solution at different pH values, then take samples every 30 seconds and test with iodine solution. A blue-black colour indicates starch is still present; no colour change means starch has been digested.
方法：在不同 pH 下将淀粉酶加入淀粉溶液中，每 30 秒取样并用碘液检测。蓝黑色表示淀粉仍存在；颜色不变表示淀粉已被消化。
Iodine + Starch → Blue-Black
碘 + 淀粉 → 蓝黑色
To measure rate, record the time taken for the blue-black colour to disappear. A shorter time means a faster rate.
为了测量速率，记录蓝黑色消失所需的时间。时间越短，速率越快。
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