Enzymes: Catalysts of Life | 酶：生命的催化剂

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

Enzymes are the molecular machinery that keeps cells alive. In IGCSE Biology, you need to know how enzymes work, what affects their activity, and some of their roles in organisms and industry.

酶是维持细胞存活的分子机器。在 IGCSE 生物中，你需要掌握酶如何运作、哪些因素影响其活性，以及它们在生物体与工业中的部分作用。

1. What Are Enzymes? | 什么是酶？

An enzyme is a biological catalyst. It speeds up a chemical reaction but remains chemically unchanged at the end. Most enzymes are globular proteins that fold into a precise three-dimensional shape. The region where reactants attach is called the active site. Because the active site has a specific shape, each enzyme can usually catalyse only one type of reaction. This is called enzyme specificity.

酶是一种生物催化剂。它加快化学反应，但在反应结束时其化学性质不发生改变。大多数酶是折叠成精确三维形状的球状蛋白质。反应物结合的部位称为活性位点。由于活性位点具有特定形状，每种酶通常只能催化一种类型的反应，这称为酶的专一性。

Enzymes lower the activation energy of a reaction, making it happen faster.

酶降低反应的活化能，使其更快发生。
Enzymes remain unchanged and can be reused many times.

酶不会被消耗，可重复使用多次。
The active site determines which substrate molecule can fit.

活性位点决定了哪个底物分子可以契合。

2. How Enzymes Work | 酶如何工作

The simplest model is the lock-and-key model. The enzyme is the lock and the substrate is the key. Only the correct substrate fits the active site. When it binds, it forms an enzyme-substrate complex. The reaction takes place and products leave, leaving the enzyme unchanged.

锁钥模型是最简单的解释。酶就像锁，底物就像钥匙。只有正确的底物才能嵌入活性位点。底物结合后形成酶-底物复合物，反应随即发生，产物离开，酶保持不变。

A more accurate model is induced fit. The active site is not completely rigid. When the substrate binds, the shape of the active site changes slightly to hold the substrate more tightly and strain the bonds, lowering the activation energy.

更精确的是诱导契合模型。活性位点并非完全刚硬。底物结合时，活性位点形状会稍微改变，以更紧密地结合底物并拉紧化学键，从而降低活化能。

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

3. Activation Energy | 活化能

All chemical reactions need an initial input of energy to break existing bonds. This energy barrier is the activation energy. Without enzymes, many biological reactions would be far too slow to sustain life. An enzyme provides an alternative pathway with a lower activation energy, so the same products can form more quickly at body temperature.

所有化学反应都需要初始能量来断裂原有化学键。这个能量障碍就是活化能。没有酶，许多生物反应会太慢而无法维持生命。酶提供了一条活化能更低的反应路径，使同样的产物能在体温下更快生成。

Think of it like a mountain pass: the enzyme digs a tunnel, so substances do not have to climb over the highest peak before reacting.

可以把酶想象成在山中挖了一条隧道：物质不先翻越最高峰就能发生反应。

4. Effect of Temperature | 温度的影响

Temperature affects both collisions and enzyme structure. As temperature rises, molecules gain kinetic energy and move faster. The frequency of successful collisions between enzyme and substrate increases, so the rate of reaction increases. For most human enzymes, the optimum temperature is about 37°C. Between low temperature and the optimum, the rate roughly doubles for every 10°C rise.

温度影响分子碰撞和酶的结构。温度升高，分子获得动能并运动加快，酶与底物之间的有效碰撞频率增加，因此反应速率升高。大多数人类酶的最适温度约为 37°C。在低温至最适温度之间，温度每升高 10°C，速率大约翻倍。

Above the optimum, the extra energy causes vibrations that break the hydrogen bonds holding the enzyme’s structure together. The active site changes shape and the enzyme can no longer bind to its substrate. The enzyme is said to be denatured. Denaturation is permanent because the high temperature alters the protein’s three-dimensional shape.

超过最适温度后，额外的能量导致分子剧烈振动，令维持酶结构的氢键断裂，活性位点形状改变，酶无法再结合底物，这就是酶的变性。高温造成的变性不可逆转，因为蛋白质三维结构已被破坏。

Low temperature: molecules move slowly, fewer collisions, rate is low.

低温：分子运动缓慢，碰撞少，速率低。
Optimum temperature: rate is maximum.

最适温度：速率达到最大。
High temperature: enzyme denatures, rate falls to zero.

高温：酶变性，速率下降至零。

5. Effect of pH | pH 的影响

Every enzyme has an optimum pH. Changes in pH affect the charges on amino acids in the enzyme. If the pH is too high or too low, hydrogen and ionic bonds in the protein break, so the active site changes shape and the enzyme becomes denatured.

每种酶都有最适 pH。pH 变化会影响酶中氨基酸的电荷。若 pH 过高或过低，蛋白质中的氢键和离子键断裂，活性位点形状改变，酶发生变性。

Two important examples: pepsin in the stomach works best at pH 2, while amylase in the mouth works best near neutral pH 7.

两个重要例子：胃中的胃蛋白酶在 pH 2 时活性最高；口腔中的淀粉酶在中性 pH 7 附近活性最高。

Enzyme	Site	Optimum pH
Pepsin	Stomach	2
Amylase	Saliva / pancreatic juice	7 or slightly higher
Catalase	Most cells	7

6. Effect of Enzyme Concentration | 酶浓度的影响

If substrate is present in excess, increasing enzyme concentration increases the rate because there are more active sites available. The reaction rate is directly proportional to enzyme concentration until another factor becomes limiting.

如果底物过量，增加酶浓度会提高反应速率，因为可供使用的活性位点更多。在另一因素成为限制因素之前，反应速率与酶浓度成正比。

If substrate is not in excess, the effect is less clear. Some enzymes are idle because there is no available substrate, so adding more enzyme may not increase the rate.

如果底物不过量，效果就不明显。部分酶因为没有底物可结合而处于空闲状态，因此增加酶量可能不会提高反应速率。

7. Effect of Substrate Concentration | 底物浓度的影响

At low substrate concentration, enzymes spend part of the time with empty active sites, so as substrate concentration rises, the rate rises. At high substrate concentration, all enzyme active sites are occupied; adding more substrate has no effect. This maximum rate is called Vmax.

低底物浓度时，酶的活性位点部分时间处于空闲状态，因此底物浓度上升，反应速率随之上升。高底物浓度时，所有活性位点都被占据，再增加底物也不会提高速率，这个最大速率称为 Vmax。

To increase the rate beyond Vmax, you must add more enzyme, not more substrate.

要突破 Vmax 进一步提高速率，必须增加酶量，而不是再增加底物。

8. Inhibitors | 抑制剂

An inhibitor is any substance that reduces the rate of an enzyme-controlled reaction. Some inhibitors are competitive: they have a shape similar to the substrate and compete for the active site. Their effect can be overcome by increasing substrate concentration.

抑制剂是任何能降低酶促反应速率的物质。竞争性抑制剂的形状与底物相似，会与底物竞争活性位点，其抑制作用可通过增加底物浓度来缓解。

Non-competitive inhibitors bind to a different site on the enzyme, called an allosteric site, and change the shape of the active site. Adding more substrate cannot reverse their effect.

非竞争性抑制剂结合在酶的其他部位，即别构位点，并改变活性位点的

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Enzymes: Catalysts of Life | 酶：生命的催化剂