Enzymes: Nature, Action and Factors | 酶：本质、作用与影响因素

📚 Enzymes: Nature, Action and Factors | 酶：本质、作用与影响因素

Enzymes are biological catalysts that control nearly every chemical reaction in living organisms. They are essential for digestion, respiration, photosynthesis and many industrial processes. In the Edexcel IGCSE Science Double Award course, you need to understand the structure of enzymes, how they work, and how temperature, pH and concentration affect the rate of reaction.

酶是控制生物体内几乎所有化学反应的生物催化剂。它们对消化、呼吸、光合作用以及众多工业过程都至关重要。在 Edexcel IGCSE 科学（双奖）课程中，你需要理解酶的结构、作用方式，以及温度、pH 和浓度如何影响反应速率。

1. What Are Enzymes? | 什么是酶？

Enzymes are proteins made from long chains of amino acids. The chain folds into a unique three-dimensional shape, and a small region called the active site forms a pocket or groove on the surface of the enzyme. The active site has a specific shape that can only bind to one particular molecule, called the substrate.

酶是由氨基酸长链构成的蛋白质。这条链折叠成独特的三维形状，在酶表面形成一个小的凹陷区域，称为活性位点。活性位点具有特定的形状，只能与一种特定的分子——底物——结合。

Because each enzyme recognises only one substrate, enzymes are described as specific. For example, the enzyme amylase only breaks down starch; it cannot break down protein. This specificity is the reason cells need thousands of different enzymes to carry out all their reactions. Any change in the shape of the active site, however small, will prevent the substrate from binding.

由于每种酶只识别一种底物，酶被称为具有专一性。例如，淀粉酶只分解淀粉，不能分解蛋白质。这种专一性正是细胞需要成千上万种不同酶来完成全部反应的原因。活性位点的形状一旦发生任何微小的改变，底物便无法与之结合。

2. The Lock and Key Model | 锁钥模型

The most common way to explain enzyme action is the lock and key model. In this model, the enzyme is the lock and the substrate is the key. Only the correct key fits into the lock. When the substrate binds to the active site, an enzyme-substrate complex is formed.

解释酶作用最常用的模型是锁钥模型。在这个模型中，酶是锁，底物是钥匙。只有正确的钥匙才能插入锁孔。当底物与活性位点结合时，就形成了酶-底物复合物。

Once the complex is formed, the reaction takes place and the product is released. The enzyme remains unchanged and is free to bind to another substrate molecule. This is why a single enzyme molecule can process millions of substrate molecules per second. The shape of the active site is perfectly complementary to the substrate, just as a key fits a specific lock.

复合物一旦形成，反应便发生并释放产物。酶本身保持不变，可以继续与下一个底物分子结合。这就是为什么一个酶分子每秒能处理数百万个底物分子。活性位点的形状与底物完全互补，正如一把钥匙对应一把特定的锁。

3. Lowering Activation Energy | 降低活化能

Every chemical reaction needs an initial input of energy to start, called the activation energy. Without a catalyst, most biological reactions require too much energy, so they would happen extremely slowly at body temperature. Enzymes reduce the activation energy by holding the substrate in exactly the right position and stressing certain bonds inside it.

每一个化学反应都需要一个初始的能量输入才能启动，称为活化能。没有催化剂时，大多数生物反应所需的能量太高，在体温下进行得极其缓慢。酶通过将底物固定在正确的位置并拉伸其内部的某些化学键，从而降低活化能。

Think of a hill: a reaction without an enzyme must climb a high hill, but with an enzyme the hill is much lower, so the reaction can proceed quickly and easily. As catalysts, enzymes are not used up and do not change the products of the reaction; they only change the speed.

可以把这想象成一座山丘：没有酶的反应必须翻越很高的山丘，而有了酶，山丘变得很低，反应就能快速、轻松地进行。作为催化剂，酶不会被消耗，也不改变反应的产物；它们只改变反应速度。

Substrate + Enzyme → Enzyme-Substrate Complex → Product + Enzyme

底物 + 酶 → 酶-底物复合物 → 产物 + 酶

4. Effect of Temperature | 温度的影响

Temperature affects the movement of molecules. As temperature rises, molecules move faster and collide more often with the active site, so the rate of reaction increases. For most human enzymes, the rate roughly doubles for every 10 °C rise up to the optimum temperature of about 37 °C.

温度影响分子的运动。温度升高时，分子运动加快，与活性位点的碰撞更加频繁，反应速率随之增大。对人体的大多数酶而言，在到达约 37 °C 的最适温度之前，温度每升高 10 °C，反应速率大约翻一番。

Above the optimum temperature, the vibrations inside the enzyme molecule become violent. The hydrogen bonds that hold the protein shape break, and the active site changes shape permanently. The enzyme is denatured, meaning it can no longer bind to its substrate. The rate of reaction therefore falls sharply to zero. This drop is not gradual and cannot be reversed by cooling.

超过最适温度后，酶分子内部的振动变得剧烈。维持蛋白质形状的氢键断裂，活性位点的形状被永久改变，酶就变性了，无法再与底物结合，反应速率因此急剧下降至零。这种下降不是渐进的，也不能通过冷却来逆转。

In the required practical, you can test this using amylase and starch at different temperatures. Drops of the mixture are added to iodine solution; iodine turns blue-black in the presence of starch. The time taken for the blue-black colour to stop appearing shows how fast the enzyme works. Alternatively, catalase from potato can be mixed with hydrogen peroxide, and the rate is measured by counting bubbles of oxygen gas produced.

在必做实验中，可以用淀粉酶和淀粉在不同温度下测试。将混合液滴加到碘液中；碘遇淀粉变蓝黑色。蓝黑色不再出现所需的时间可以

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