Enzymes: Catalysts of Life | 酶：生命的催化剂

📚 Enzymes: Catalysts of Life | 酶：生命的催化剂

Enzymes are biological molecules, usually proteins, that accelerate chemical reactions in living organisms. Without enzymes, most metabolic reactions would be too slow to sustain life.

酶是生物分子，通常是蛋白质，能加速生物体内的化学反应。若没有酶，大多数代谢反应将慢到无法维持生命。

1. What Are Enzymes? | 什么是酶

Enzymes are specific proteins that act as biological catalysts. A catalyst is a substance that increases the rate of a chemical reaction without being used up in the process.

酶是充当生物催化剂的特定蛋白质。催化剂是指在化学反应中能提高反应速率但自身不消耗的物质。

Each enzyme is folded into a unique three-dimensional shape, which includes an active site. The active site is the region where the substrate binds and the reaction occurs.

每种酶都折叠成独特的三维形状，其中包括一个活性位点。活性位点是底物结合并发生反应的区域。

2. Enzyme Specificity | 酶的专一性

Enzymes are highly specific. This means each enzyme only catalyses one reaction, or one type of reaction, because only certain substrates fit the active site.

酶具有高度专一性。这意味着每种酶只催化一种反应或一类反应，因为只有特定底物能契合其活性位点。

This is often described by the lock-and-key model. The enzyme is the lock, and the substrate is the key. Only the correct key can fit into the lock.

这通常用“锁钥模型”来描述。酶是锁，底物是钥匙，只有正确的钥匙才能插入锁中。

Some enzymes use the induced-fit model, where the active site changes shape slightly to allow the substrate to bind more tightly.

有些酶采用“诱导契合模型”，即活性位点会稍微改变形状，使底物结合得更紧密。

3. The Catalytic Cycle | 催化循环

An enzyme-catalysed reaction can be summarised in a simple cycle:

酶催化反应可概括为一个简单循环：

Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Product
酶 + 底物 → 酶-底物复合物 → 酶 + 产物

E + S ⇌ ES → E + P

The enzyme remains unchanged at the end of the reaction, so it can catalyse the next substrate molecule.

反应结束时酶保持不变，因此它可继续催化下一个底物分子。

4. Factors Affecting Enzyme Activity | 影响酶活性的因素

Several factors change how fast an enzyme works. The most important are temperature, pH, substrate concentration, and enzyme concentration.

多种因素会改变酶的工作速率，最重要的是温度、pH、底物浓度和酶浓度。

Temperature | 温度

As temperature rises, molecules move faster, so enzymes and substrates collide more often. The rate of reaction increases up to an optimum temperature.

温度升高时，分子运动加快，酶与底物碰撞更频繁，反应速率随之上升，直至达到最适温度。

Above the optimum, the enzyme denatures. The vibrations break bonds in the protein, changing the shape of the active site irreversibly.

超过最适温度后，酶将变性。振动会破坏蛋白质内的化学键，使活性位点形状发生不可逆改变。

Optimum temperature for most human enzymes: 37 °C

大多数人体酶的最适温度：37 °C

5. Effect of pH | pH的影响

Every enzyme has an optimum pH. Many intracellular enzymes work best at pH 7, while pepsin in the stomach works best at pH 2.

每种酶都有最适pH。许多细胞内酶在pH 7时活性最高，而胃中的胃蛋白酶在pH 2时活性最高。

If pH moves too far from the optimum, hydrogen ions interfere with the ionic and hydrogen bonds that hold the enzyme in shape. The enzyme denatures.

若pH偏离最适值过远，氢离子会干扰维持酶形状的离子键和氢键，导致酶变性。

Enzyme	Optimum pH
Pepsin (胃蛋白酶)	2
Salivary amylase (唾液淀粉酶)	7
Trypsin (胰蛋白酶)	8

6. Substrate and Enzyme Concentration | 底物与酶浓度

When substrate concentration increases, the rate of reaction increases because more substrate molecules are available to collide with enzymes. However, this only happens until all active sites are occupied.

底物浓度增加时，反应速率提高，因为更多底物分子可与酶碰撞。但仅当所有活性位点都被占据之前有效。

After this point, adding more substrate has no effect on the rate. The enzyme is working at its maximum rate.

在这一点之后，继续增加底物不再影响速率，因为酶已以最大速率工作。

Similarly, increasing enzyme concentration increases the rate of reaction, provided there is sufficient substrate.

类似地，在底物充足时，增加酶浓度可提高反应速率。

7. Enzyme Inhibitors | 酶抑制剂

An inhibitor is a substance that slows down or stops an enzyme-catalysed reaction. Competitive inhibitors have a similar shape to the substrate and compete for the active site.

抑制剂是能减慢或停止酶催化反应的物质。竞争性抑制剂与底物形状相似，会竞争活性位点。

Non-competitive inhibitors bind elsewhere on the enzyme, altering 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，改变活性位点形状，使底物无法再结合。

Competitive inhibition: rate can be restored by adding more substrate

竞争性抑制：增加底物浓度可恢复反应速率

Non-competitive inhibition: adding more substrate does not restore rate

非竞争性抑制：增加底物不能恢复速率

8. Enzymes in Everyday Life | 酶在日常生活中的应用

Enzymes are widely used in industry and medicine. In food production, enzymes help make bread, cheese, and fruit juice.

酶在工业和医学中广泛应用。在食品生产中，酶帮助制作面包、奶酪和果汁。

Amylase is used to break down starch into sugars in brewing and baking.
淀粉酶在酿造和烘焙中用于将淀粉分解为糖类。
Proteases are used in biological washing powders to remove protein stains.
蛋白酶用于生物洗涤剂中去除蛋白质污渍。
Lactase is used to make lactose-free milk for people who are lactose intolerant.
乳糖酶用于生产无乳糖牛奶，供乳糖不耐受者饮用。

9. Investigating Enzyme Activity | 探究酶活性

A common IGCSE experiment uses amylase to break down starch at different temperatures or pH values. The reaction is followed by testing a drop of the mixture with iodine solution.

IGCSE常见实验用淀粉酶在不同温度或pH下分解淀粉。通过向混合物滴加碘液来追踪反应进程。

Iodine solution turns blue-black in the presence of starch

碘液遇淀粉变蓝黑色

The faster the blue-black colour disappears, the faster the enzyme works. This method allows you to find the optimum temperature or pH.

蓝黑色消失越快，说明酶作用越快。该方法可帮助你找到最适温度或pH。

10. Summary | 小结

Enzymes are essential biological catalysts with high specificity. Their activity is affected by temperature, pH, and concentrations of substrate and enzyme.

酶是具有高度专一性的重要生物催化剂，其活性受温度、pH、底物浓度和酶浓度的影响。

Understanding enzymes helps us explain digestion, metabolism, and many industrial processes. Mastering these concepts is key for your Edexcel IGCSE Biology exam.

理解酶有助于我们解释消化、代谢和许多工业过程。掌握这些概念是你在Edexcel IGCSE生物考试中取得好成绩的关键。

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Enzymes: Catalysts of Life | 酶：生命的催化剂