Enzymes | 酶

📚 Enzymes | 酶

Enzymes are biological catalysts that accelerate chemical reactions within living organisms. They are essential to nearly every metabolic process, from digestion to DNA replication, and understanding their behaviour is a core requirement of the Edexcel IGCSE Science syllabus.

酶是生物催化剂，能够加速生物体内的化学反应。它们几乎参与所有代谢过程，从消化到 DNA 复制，理解酶的行为方式是 Edexcel IGCSE 科学课程的核心要求之一。

1. What Are Enzymes? | 什么是酶？

Enzymes are globular proteins made up of long chains of amino acids folded into specific three-dimensional shapes. Each enzyme has an active site — a region with a unique shape that allows only specific substrate molecules to bind. This specificity is often described as the ‘lock and key’ mechanism, where the enzyme is the lock and the substrate is the key.

酶是由氨基酸长链折叠成特定三维形状的球状蛋白质。每种酶都有一个活性位点——一个形状独特的区域，只允许特定的底物分子结合。这种特异性常被形容为“锁钥机制”，其中酶是锁，底物是钥匙。

The overall reaction catalysed by an enzyme can be written as:

酶催化的总体反应可写成：

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

酶 + 底物 → 酶-底物复合物 → 酶 + 产物

The enzyme is not consumed during the reaction; it can be reused repeatedly, making enzymes highly efficient biological machines.

酶在反应中不会被消耗，可以反复使用，这使得酶成为高效的生物机器。

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2. The Active Site and Lock-and-Key Model | 活性位点与锁钥模型

The active site is a small pocket or groove on the enzyme surface. Its shape is complementary to the substrate molecule — only the correct substrate fits perfectly, like a key fitting into a lock. This explains why each enzyme catalyses only one type of reaction.

活性位点是酶表面的一个小口袋或凹槽。其形状与底物分子互补——只有正确的底物才能完美契合，就像钥匙插入锁孔。这解释了为什么每种酶只催化一种类型的反应。

An alternative explanation is the induced-fit model, which suggests that the active site slightly changes shape when the substrate binds, becoming more complementary to the substrate. This model is now widely accepted because it better explains how enzymes work with flexibility.

另一种解释是诱导契合模型，该模型认为当底物结合时，活性位点的形状会发生轻微改变，使其与底物更加互补。该模型目前被广泛接受，因为它更好地解释了酶的柔性工作机制。

The key points about the active site:

关于活性位点的关键要点：

• The shape of the active site determines substrate specificity | 活性位点的形状决定了底物特异性
• If the active site is damaged (e.g., by high temperature or extreme pH), the enzyme cannot function | 如果活性位点受损（例如高温或极端 pH），酶将无法发挥作用
• Enzyme-substrate complexes are temporary and very short-lived | 酶-底物复合物是暂时的，寿命很短

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3. How Enzymes Speed Up Reactions | 酶如何加速反应

Enzymes speed up reactions by lowering the activation energy — the minimum energy required for a reaction to occur. By providing an alternative pathway with a lower energy barrier, enzymes allow reactions to proceed rapidly at body temperature (37 °C) instead of requiring high temperatures.

酶通过降低活化能——反应发生所需的最低能量——来加速反应。通过提供一条能量屏障更低的替代路径，酶使反应能在体温（37 °C）下快速进行，而无需高温。

There are two main ways enzymes work:

酶主要通过两种方式发挥作用：

• Bringing substrates together, increasing the chance of successful collisions | 将底物聚集在一起，增加有效碰撞的机会
• Weakening chemical bonds within the substrate, making it easier for bonds to break and form | 削弱底物内部的化学键，使化学键更容易断裂和形成

In a reaction without an enzyme, the activation energy (Eₐ) is high. With an enzyme, the activation energy is significantly reduced:

在没有酶的反应中，活化能（Eₐ）很高。有了酶，活化能显著降低：

Eₐ (uncatalysed) > Eₐ (catalysed)

Eₐ（无催化剂） > Eₐ（有催化剂）

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4. Effect of Temperature on Enzyme Activity | 温度对酶活性的影响

Temperature has a dramatic effect on enzyme activity. As temperature rises from 0 °C, the kinetic energy of molecules increases, causing more collisions between enzymes and substrates. Enzyme activity therefore increases with temperature up to an optimum point — typically around 37 °C in the human body.

温度对酶活性有显著影响。当温度从 0 °C 升高时，分子的动能增加，酶与底物之间的碰撞增多。因此，酶活性随温度升高而增强，直到最适温度——人体内通常约为 37 °C。

At the optimum temperature, the rate of reaction is at its maximum. However, if the temperature continues to rise beyond this point, the enzyme begins to denature. Denaturation involves the breaking of bonds that hold the enzyme’s three-dimensional structure together, causing the active site to lose its specific shape.

在最适温度下，反应速率达到最大值。然而，如果温度继续升高超过此点，酶开始变性。变性涉及维持酶三维结构的化学键断裂，导致活性位点失去其特定形状。

Key values to remember:

需要记住的关键数值：

Temperature (°C) | 温度（°C）	Effect on Enzyme | 对酶的影响
0	Very slow reaction; enzyme is inactive but not denatured | 反应极慢；酶失活但未变性
37 (optimum) | 37（最适）	Maximum rate of reaction | 反应速率最大
50+	Denaturation occurs; active site permanently damaged | 发生变性；活性位点永久受损

It is important to note that denaturation is permanent and irreversible — cooling the enzyme back down will not restore its function.

需要特别注意，变性是永久性的、不可逆的——将酶冷却回原温度并不能恢复其功能。

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5. Effect of pH on Enzyme Activity | pH 对酶活性的影响

Each enzyme has an optimum pH at which it works most efficiently. For most enzymes in the human body, this optimum is around pH 7 (neutral). However, some enzymes operate in highly acidic or alkaline environments. For example, pepsin in the stomach works best at pH 2, while trypsin in the small intestine works best at pH 8.

每种酶都有一个最适 pH，在此 pH 下工作效率最高。人体内的大多数酶最适 pH 约为 7（中性）。然而，某些酶在高度酸性或碱性环境中发挥作用。例如，胃中的胃蛋白酶在 pH 2 时活性最强，而小肠中的胰蛋白酶在 pH 8 时活性最强。

Changes in pH affect enzyme activity by altering the ionic bonds and hydrogen bonds that maintain the enzyme’s structure. Extreme pH values can cause denaturation, permanently changing the shape of the active site.

pH 变化通过改变维持酶结构的离子键和氢键来影响酶活性。极端 pH 值会导致变性，永久改变活性位点的形状。

When plotting a pH vs. rate of reaction graph, you will see a bell-shaped curve. The peak of the curve represents the optimum pH. A common exam question asks you to identify the optimum pH from a graph or to explain why the enzyme activity decreases at pH values above and below the optimum.

绘制 pH 与反应速率关系图时，你会看到一条钟形曲线。曲线的峰值代表最适 pH。常见的考试题要求你从图表中识别最适 pH，或解释为什么在高于和低于最适 pH 时酶活性下降。

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6. Enzyme Concentration and Substrate Concentration | 酶浓度与底物浓度

The rate of an enzyme-catalysed reaction also depends on the concentration of both the enzyme and the substrate. As enzyme concentration increases, the rate of reaction increases proportionally, provided that substrate is available in excess. This is because more active sites are available to bind substrate molecules.

酶催化反应的速率还取决于酶和底物的浓度。当底物过量时，随着酶浓度增加，反应速率成比例增加。这是因为有更多的活性位点可供底物分子结合。

Similarly, increasing substrate concentration initially increases the rate of reaction. However, once all enzyme active sites are occupied — the point of saturation — further increases in substrate concentration have no effect on the reaction rate. The enzyme has reached its maximum turnover rate.

类似地，增加底物浓度最初会提高反应速率。然而，一旦所有酶的活性位点都被占满——即饱和点——继续增加底物浓度不再影响反应速率。酶已达到其最大转化速率。

In a graph of substrate concentration vs. reaction rate, the curve rises steeply at first and then levels off to a plateau. The plateau indicates that the enzyme is the limiting factor.

在底物浓度与反应速率的图中，曲线先急剧上升，然后趋于平缓形成平台。平台表明酶成为限制因素。

Common misconception: adding more enzyme at saturation will not increase rate. Actually, adding more enzyme will raise the plateau — it is adding more substrate that has no effect at saturation.

常见误区：在饱和时增加酶量不会提高速率。实际上，增加酶量会抬高平台——在饱和时没有效果的是增加底物量。

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7. Uses of Enzymes in Industry and Medicine | 酶在工业和医学中的应用

Enzymes are widely used in biotechnology and industry because they are specific, efficient, and work under mild conditions. In the food industry, enzymes such as amylase and protease are used to break down starch and proteins in products like bread, beer, and baby food.

酶因其特异性、高效性和温和的反应条件，被广泛应用于生物技术和工业中。在食品工业中，淀粉酶和蛋白酶用于分解面包、啤酒和婴儿食品等产品中的淀粉和蛋白质。

Biological washing powders contain lipases and proteases that break down fat and protein stains at low temperatures, saving energy. In medicine, enzymes are used in diagnostic tests — for example, glucose oxidase is used in test strips to measure blood glucose levels in diabetic patients.

生物洗衣粉含有脂肪酶和蛋白酶，可在低温下去除油脂和蛋白质污渍，从而节约能源。在医学中，酶用于诊断测试——例如，葡萄糖氧化酶用于试纸检测糖尿病患者的血糖水平。

Key industrial examples for the exam:

考试中需要掌握的关键工业实例：

Enzyme | 酶	Source | 来源	Use | 用途
Amylase | 淀粉酶	Fungi/bacteria | 真菌/细菌	Converting starch to sugar in brewing | 酿造中将淀粉转化为糖
Protease | 蛋白酶	Bacteria | 细菌	Removing protein stains in washing powder | 去除洗衣粉中的蛋白质污渍
Lipase | 脂肪酶	Fungi | 真菌	Removing fatty stains; making cheese | 去除油脂污渍；制作奶酪

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8. Enzyme Inhibitors | 酶抑制剂

Enzyme inhibitors are substances that reduce or stop enzyme activity. They can be competitive — meaning they have a similar shape to the substrate and compete for the active site — or non-competitive, meaning they bind elsewhere on the enzyme and change its overall shape.

酶抑制剂是降低或阻止酶活性的物质。它们可以是竞争性的——即形状与底物相似，与底物竞争活性位点——也可以是非竞争性的，即结合在酶的其他位置并改变其整体形状。

Competitive inhibition can be reversed by increasing the substrate concentration, because the substrate will outcompete the inhibitor for the active site. Non-competitive inhibition cannot be reversed by adding more substrate, because the inhibitor is not competing for the active site — the shape of the enzyme has already been altered.

竞争性抑制可以通过增加底物浓度来逆转，因为底物会与抑制剂竞争活性位点并占据优势。非竞争性抑制无法通过添加更多底物来逆转，因为抑制剂并非竞争活性位点——酶的形状已经发生改变。

Some inhibitors are irreversible, such as heavy metal ions (e.g., lead and mercury) and cyanide. These permanently denature the enzyme. In medicine, drugs often act as inhibitors — for example, aspirin inhibits an enzyme involved in inflammation, which produces pain and swelling.

一些抑制剂是不可逆的，例如重金属离子（如铅和汞）和氰化物。这些物质会使酶永久变性。在医学中，药物通常作为抑制剂发挥作用——例如，阿司匹林抑制与炎症相关的一种酶，从而减轻疼痛和肿胀。

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9. Required Practical: Investigating Enzyme Activity | 必做实验：探究酶活性

The Edexcel IGCSE specification includes a required practical where you investigate how one factor (temperature or pH) affects the rate of an enzyme-controlled reaction. A common setup uses amylase to break down starch, with iodine solution used to test for the presence of starch.

Edexcel IGCSE 大纲要求完成一个必做实验：探究一个因素（温度或 pH）如何影响酶促反应速率。一个常见的实验装置是用淀粉酶分解淀粉，并用碘液检测淀粉是否存在。

Method outline:

方法概要：

• Place starch solution and amylase solution in a water bath set to a specific temperature | 将淀粉溶液和淀粉酶溶液放入设定好温度的水浴中
• Mix the two solutions and start a timer | 混合两种溶液并启动计时器
• At regular intervals, remove a drop of the mixture and test it with iodine solution | 每隔一定时间，取一滴混合液用碘液测试
• When the iodine remains brown (no blue-black colour), all starch has been broken down | 当碘液保持棕色（不出现蓝黑色）时，说明所有淀粉已被分解
• Repeat at different temperatures or pH values to compare rates | 在不同温度或 pH 下重复实验以比较速率

The shorter the time taken for the iodine to remain brown, the faster the reaction rate. You could also measure the rate by using a colorimeter to measure the intensity of the blue-black colour over time.

碘液保持棕色所需的时间越短，反应速率越快。你也可以使用比色计来测量蓝黑色随时间变化的强度，从而测定速率。

When answering practical questions, remember to state variables: the independent variable (temperature or pH), the dependent variable (time for starch to be broken down), and control variables (concentration of amylase, volume of starch, pH buffer, etc.).

回答实验题时，记得说明变量：自变量（温度或 pH）、因变量（淀粉被分解所需的时间）以及控制变量（淀粉酶浓度、淀粉体积、pH 缓冲液等）。

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10. Common Exam Questions and Revision Tips | 常见考题与复习建议

Enzyme questions are among the most frequently tested topics in IGCSE Science exams. Typical questions include drawing and interpreting graphs of reaction rate against temperature or pH, explaining why denaturation occurs, and describing experiments to test enzyme activity.

酶相关题目是 IGCSE 科学考试中最常考的主题之一。典型题目包括绘制和解读反应速率随温度或 pH 变化的图表、解释变性发生的原因，以及设计实验来测试酶活性。

To gain full marks on graph questions, follow this rule: use the data, describe the trend, and then explain the biology. For example: “The rate increases up to 37 °C because kinetic energy increases collisions, but above 40 °C the rate decreases because the enzyme denatures and the active site changes shape.”

要在图表题中得满分，遵循这一原则：引用数据、描述趋势、然后解释生物学原理。例如：“反应速率在 37 °C 前不断升高，因为动能增加使碰撞增多；但超过 40 °C 后速率下降，因为酶变性，活性位点形状改变。”

Memorise these key terms and always use them precisely: active site, denature, optimum, substrate, product, catalyst, specificity. Examiners award marks for key terminology, so be precise and avoid vague language.

牢记这些关键术语并准确使用：活性位点、变性、最适、底物、产物、催化剂、特异性。考官会根据关键术语给分，所以用语要精确，避免模糊表达。

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