📚 Enzymes and Their Role in Biological Reactions | 酶及其在生物反应中的作用
Enzymes are biological catalysts that speed up chemical reactions inside living organisms without being used up themselves. They are essential for nearly every process in the body, from digestion to DNA replication.
酶是生物催化剂，能在生物体内加速化学反应，而自身不会被消耗。从消化到 DNA 复制，体内几乎每一个过程都离不开酶。
1. What Are Enzymes? | 什么是酶？
Enzymes are globular proteins made of long chains of amino acids folded into a specific three-dimensional shape. This shape includes an active site, a small region where the substrate binds and reacts.
酶是由长链氨基酸折叠成的特定三维形状的球状蛋白质。其形状中包含一个活性位点，这是底物结合并发生反应的小区域。
- Each enzyme is specific to one substrate because the active site fits only that substrate.
- 每种酶只对一种底物具有特异性，因为活性位点只与该底物匹配。
- If the shape of the enzyme changes, it can no longer work.
- 如果酶的形状发生改变，它便无法再发挥作用。
2. The Lock and Key Model | 锁钥模型
The classic model for enzyme action is the lock and key model. The enzyme is the lock, the substrate is the key, and only the correctly shaped key can fit into the lock to start the reaction.
酶的经典作用模型是锁钥模型。酶相当于锁，底物相当于钥匙，只有形状匹配的钥匙才能插入锁中并启动反应。
Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Product
酶 + 底物 → 酶-底物复合物 → 酶 + 产物
3. Induced Fit Model | 诱导契合模型
A more accurate model is the induced fit model. Here, the active site is not perfectly rigid; when the substrate binds, the enzyme changes shape slightly to make the fit tighter and to stress the substrate bonds.
更精确的模型是诱导契合模型。在该模型中，活性位点并非完全刚性；当底物结合时，酶会稍微改变形状，使结合更紧密，并拉伸底物的化学键。
- This distortion lowers the activation energy needed for the reaction.
- 这种形变降低了反应所需的活化能。
- The product leaves the active site, and the enzyme returns to its original shape.
- 产物离开活性位点后，酶恢复其原始形状。
4. Activation Energy | 活化能
Every chemical reaction needs a certain amount of energy to begin, called the activation energy. Enzymes reduce this energy barrier, allowing reactions to happen rapidly at body temperature.
每个化学反应都需要一定能量才能启动，这就是活化能。酶降低了这一能量屏障，使反应能在体温条件下快速进行。
ΔG = Energy of products - Energy of reactants
ΔG = 产物能量 - 反应物能量
Without enzymes, many reactions would be too slow to sustain life.
没有酶，许多反应将慢到无法维持生命。
5. Factors Affecting Enzyme Activity: Temperature | 影响酶活性的因素：温度
Temperature affects the kinetic energy of molecules. As temperature rises, molecules move faster and collide more often, so enzyme activity increases up to an optimum temperature.
温度影响分子的动能。温度升高时，分子运动加快、碰撞更频繁，因此酶活性会随温度升高而增强，直至达到最适温度。
| Temperature / 温度 | Effect / 影响 |
| Below optimum / 低于最适温度 | Activity increases slowly / 活性缓慢增加 |
| Optimum / 最适温度 | Maximum activity / 活性最大 |
| Above optimum / 高于最适温度 | Activity rapidly decreases / 活性迅速下降 |
6. Denaturation | 变性
At high temperatures (usually above 45–50°C for human enzymes), the vibrations break the bonds holding the enzyme's structure. The active site changes shape and the enzyme becomes denaturated.
在高温下（人体酶通常超过 45–50°C），振动会破坏维持酶结构的键。活性位点的形状发生改变，酶便发生变性。
- Denaturation is usually permanent.
- 变性通常是不可逆的。
- The substrate can no longer bind, so the reaction stops.
- 底物无法再结合，反应因此停止。
7. Factors Affecting Enzyme Activity: pH | 影响酶活性的因素：pH
Each enzyme has an optimum pH. In the stomach, pepsin works best at pH 2; in the small intestine, trypsin works best at pH 8.
每种酶都有其最适 pH。在胃中，胃蛋白酶在 pH 2 时活性最高；在小肠中，胰蛋白酶在 pH 8 时活性最高。
Extreme pH changes alter ionic bonds and cause denaturation.
极端的 pH 变化会破坏离子键并导致变性。
A buffer solution can maintain a constant pH during an experiment.
实验中可用缓冲液保持 pH 恒定。
8. Enzyme Concentration and Substrate Concentration | 酶浓度与底物浓度
At low enzyme concentration, the rate of reaction is directly proportional to enzyme concentration because more active sites are available.
酶浓度较低时，反应速率与酶浓度成正比，因为可用的活性位点更多。
As substrate concentration increases, the rate rises until all active sites are occupied. Beyond that point, adding more substrate has no effect.
随着底物浓度增加，反应速率上升，直到所有活性位点都被占满。超过这一点后，再增加底物也不会改变速率。
- Limiting factor: substrate concentration when enzyme is in excess.
- 限制因素：酶过量时的底物浓度。
- Limiting factor: enzyme concentration when substrate is in excess.
- 限制因素：底物过量时的酶浓度。
9. Inhibitors | 抑制剂
Inhibitors are substances that reduce enzyme activity. Competitive inhibitors bind to the active site and block the substrate. Non-competitive inhibitors bind elsewhere, changing the enzyme's shape.
抑制剂是降低酶活性的物质。竞争性抑制剂结合在活性位点上，阻断底物。非竞争性抑制剂结合在其他位置，改变酶的形状。
| Type / 类型 | Binding site / 结合位点 | Effect of increasing substrate / 增加底物的效果 |
| Competitive / 竞争性 | Active site / 活性位点 | Can be reversed / 可被逆转 |
| Non-competitive / 非竞争性 | Other site / 其他位点 | Cannot be reversed / 不可被逆转 |
10. Enzymes in Digestion | 消化中的酶
Digestive enzymes break down large food molecules into small soluble molecules that can be absorbed.
消化酶将大分子食物分解为可被吸收的小分子可溶性物质。
- Amylase breaks starch into maltose.
- 淀粉酶将淀粉分解为麦芽糖。
- Protease breaks proteins into amino acids.
- 蛋白酶将蛋白质分解为氨基酸。
- Lipase breaks lipids into fatty acids and glycerol.
- 脂肪酶将脂质分解为脂肪酸和甘油。
11. Investigating Enzymes: Practical Skills | 探究酶：实验技能
A common experiment uses amylase and starch. You can measure the time for starch to disappear using iodine solution, which turns blue-black in the presence of starch.
常见实验使用淀粉酶和淀粉。可以用碘液测定淀粉消失的时间，因为碘液遇淀粉会变蓝黑色。
To test the effect of temperature, place the enzyme and substrate in water baths at different temperatures, then add iodine every 30 seconds.
要测试温度的影响，可将酶和底物分别置于不同温度的水浴中，然后每 30 秒滴加碘液。
Enzyme activity ∝ 1 / time taken for starch to disappear
酶活性 ∝ 1 / 淀粉消失所需时间
12. Enzymes in Daily Life and Industry | 酶在日常与工业中的应用
Enzymes are widely used in food, medicine, and biotechnology. Biological washing powders contain proteases and lipases to remove protein and fat stains.
酶广泛应用于食品、医药和生物技术领域。加酶洗衣粉含有蛋白酶和脂肪酶，用于去除蛋白质和脂肪污渍。
- Lactase is used to make lactose-free milk.
- 乳糖酶用于生产无乳糖牛奶。
- Immobilised enzymes are used in industrial reactors because they can be reused.
- 固定化酶在工业反应器中可重复使用。
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