Enzymes: Structure, Function and Applications | 酶：结构、功能与应用

📚 Enzymes: Structure, Function and Applications | 酶：结构、功能与应用

Enzymes are essential biological molecules that control almost every chemical reaction inside living organisms. In your IGCSE Edexcel Science course, understanding how enzymes work, what affects their activity, and where they are used is a core topic that appears regularly in both Paper 1 and Paper 2. This revision guide breaks down everything you need to know into clear, exam-focused sections.

酶是控制生物体内几乎一切化学反应的必需生物分子。在IGCSE爱德思科学课程中，理解酶如何工作、什么因素影响其活性以及酶的应用，是Paper 1和Paper 2中反复出现核心考点。本复习指南将你需要掌握的全部内容划分为清晰、紧扣考点的章节。

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1. What Are Enzymes? | 什么是酶？

Enzymes are proteins that act as biological catalysts. A catalyst is a substance that speeds up a chemical reaction without being used up in the process. Because enzymes are proteins, their shape is determined by the sequence of amino acids that makes them up. Each enzyme has a specific three-dimensional shape with a region called the active site.

酶是充当生物催化剂的蛋白质。催化剂是一种能加快化学反应速率但自身在反应过程中不会被消耗的物质。由于酶是蛋白质，其形状由构成它的氨基酸序列决定。每种酶都具有特定的三维结构，其中有一个称为活性位点的区域。

The substance that an enzyme acts upon is called the substrate. The substrate binds to the active site, and the enzyme converts it into one or more products. A useful way to write this is:

酶作用的物质称为底物。底物与活性位点结合，酶将其转化为一种或多种产物。一个有用的书写方式是：

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

The enzyme remains unchanged after the reaction, so one enzyme molecule can catalyse thousands of reactions per second. This is why enzymes are described as being reusable.

酶在反应后保持不变，因此一个酶分子每秒钟可以催化数千次反应。这就是酶被称为可重复使用的原因。

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2. The Lock and Key Model | 锁钥模型

The lock and key model explains enzyme specificity. In this model, the enzyme is like a lock and the substrate is like a key. Only a substrate with the correct shape can fit into the active site, just as only the correct key can fit into a lock. This is why enzymes are specific to one substrate or a small group of similar substrates.

锁钥模型解释了酶的专一性。在该模型中，酶好比一把锁，底物好比一把钥匙。只有形状正确的底物才能嵌入活性位点，就像只有正确的钥匙才能插入锁一样。这就是酶只对一种底物或一小类相似底物具有专一性的原因。

For example, the enzyme amylase can only break down starch; it cannot break down proteins or fats. If the shape of the active site changes, the substrate can no longer bind, and the enzyme stops working.

例如，淀粉酶只能分解淀粉，而不能分解蛋白质或脂肪。如果活性位点的形状改变，底物便无法再结合，酶就会停止发挥作用。

In the Edexcel specification, you should also be aware of the induced fit model, where the active site changes shape slightly to fit the substrate more closely. However, the lock and key model is the one most often described in exam answers.

在爱德思考纲中，你也需要了解诱导契合模型，即活性位点会稍微改变形状以更紧密地贴合底物。然而，锁钥模型是考试答案中最常使用的模型。

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

Four main factors affect the rate of an enzyme-catalysed reaction: temperature, pH, enzyme concentration and substrate concentration. Each of these can be investigated using simple experiments, so you need to be able to describe the methods and interpret the graphs.

影响酶促反应速率的主要因素有四个：温度、pH、酶浓度和底物浓度。每一个都可以通过简单实验进行研究，因此你需要能够描述实验方法并解读曲线图。

• Temperature | 温度 — affects kinetic energy and can cause denaturation | 影响动能并可能导致变性

• pH | pH值 — each enzyme has an optimum pH | 每种酶都有最适pH

• Enzyme concentration | 酶浓度 — more enzymes, faster reaction up to a limit | 酶越多反应越快，但有上限

• Substrate concentration | 底物浓度 — limited by the number of active sites | 受活性位点数量限制

In examinations, you are often asked to draw or label graphs for these factors. Make sure your graph axes are labelled with the correct variables and that you describe each region of the curve precisely.

在考试中，你经常会被要求绘制或标注这些因素的曲线图。务必确保坐标轴标有正确的变量，并准确描述曲线的每个区域。

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4. Temperature and Enzyme Activity | 温度与酶活性

As temperature increases from low values, the rate of reaction increases because the molecules gain kinetic energy. This means more collisions occur between the enzyme and the substrate per second, so more enzyme–substrate complexes form.

当温度从低值升高时，反应速率加快，因为分子获得更多动能。这意味着酶与底物每秒碰撞的次数增加，因此形成更多酶–底物复合物。

However, above a certain point, the rate of reaction begins to decrease sharply. This is because the high temperature breaks the weak bonds that maintain the enzyme’s three-dimensional shape. The active site changes shape, and the substrate can no longer fit. The enzyme is said to be denatured. Denaturation is permanent — the enzyme cannot recover even if the temperature is lowered again.

然而，超过某一温度点后，反应速率急剧下降。这是因为高温破坏了维持酶三维结构的弱键。活性位点形状改变，底物无法再嵌入。此时称酶已变性。变性是不可逆的——即使温度再次降低，酶也无法恢复活性。

The temperature at which the rate of reaction is highest is called the optimum temperature. For most human enzymes, the optimum temperature is around 37 °C, which is normal body temperature. For enzymes in plants, the optimum is often lower, around 20–30 °C.

反应速率达到最高时的温度称为最适温度。大多数人体酶的最适温度约为37 °C，即正常体温。植物中酶的最适温度通常较低，约为20–30 °C。

Key graph shape: ascending curve → peak at optimum → sharp fall after denaturation | 关键图形：曲线上升 → 最适温度处达峰 → 变性后急剧下降

A common exam question asks you to explain why the rate falls after the optimum temperature. Always write ‘the enzyme denatures’ and ‘the active site changes shape’ in your answer to earn full marks.

常见的考试题会要求你解释为什么最适温度之后反应速率下降。作答时务必写出“酶变性”和“活性位点形状改变”以获得满分。

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5. pH and Enzyme Activity | pH与酶活性

Each enzyme has an optimum pH at which its rate of reaction is greatest. For most enzymes in the human body, the optimum pH is around 7 (neutral). However, pepsin, a digestive enzyme found in the stomach, has an optimum pH of around 2 because the stomach contains hydrochloric acid.

每种酶都有使其反应速率最大的最适pH。人体内大多数酶的最适pH约为7（中性）。然而，胃中的消化酶胃蛋白酶的最适pH约为2，因为胃中含有盐酸。

If the pH moves too far from the optimum, in either direction, the rate of reaction decreases. Very high or very low pH values break the bonds holding the enzyme’s structure together, so the active site changes shape and the enzyme becomes denatured. Like temperature denaturation, pH denaturation is permanent.

如果pH偏离最适值太远，无论偏高还是偏低，反应速率都会下降。过高或过低的pH会破坏维持酶结构的键，导致活性位点形状改变，酶发生变性。与温度变性一样，pH变性也是不可逆的。

Note that raising the temperature increases the rate of reaction up to the optimum, but changing pH always decreases the rate either side of the optimum. This is a useful contrast for exam questions.

请注意，升高温度在达到最适温度前会加快反应速率，但pH无论偏离最适值偏高还是偏低，都只会使反应速率下降。这是一个在考试题目中很有用的对比。

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

When the substrate concentration is low, the rate of reaction increases as substrate concentration increases. This is because with more substrate molecules available, more collisions with the active sites occur, so more enzyme–substrate complexes form per second.

当底物浓度较低时，反应速率随底物浓度增加而加快。这是因为底物分子越多，与活性位点的碰撞越频繁，每秒形成的酶–底物复合物就越多。

Eventually, the graph levels off. At this point, all the active sites are occupied, and the reaction is running at its maximum rate. Adding more substrate will not increase the rate further because the limiting factor is no longer substrate concentration but the number of enzymes available.

最终曲线趋于水平。此时所有活性位点都已被占用，反应以达到最大速率运行。继续增加底物不会使速率进一步加快，因为限制因素不再是底物浓度，而是可用酶的数量。

Rate rises linearly → plateaus at Vₘₐₓ (all active sites full) | 速率线性上升 → 在Vₘₐₓ处趋于平台（所有活性位点饱和）

The same reasoning applies to enzyme concentration: as enzyme concentration increases, the rate increases proportionally, provided substrate is in excess. Since there are more active sites available, more substrate can be converted per second.

同样的道理适用于酶浓度：在底物过量的情况下，随着酶浓度增加，反应速率成比例加快。因为可利用的活性位点更多，每秒能转化的底物也更多。

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7. Enzymes in Digestion | 消化中的酶

Digestion relies on specific enzymes to break down large, insoluble food molecules into small, soluble molecules that can be absorbed into the blood. In the Edexcel IGCSE course, you need to know three main digestive enzymes: amylase, protease and lipase.

消化过程依赖特异性酶将大而不溶的食物分子分解为可被血液吸收的小而可溶的分子。在IGCSE爱德思课程中，你需要掌握三种主要的消化酶：淀粉酶、蛋白酶和脂肪酶。

• Amylase | 淀粉酶 breaks starch into maltose. It is produced in the salivary glands and the pancreas. | 将淀粉分解为麦芽糖，由唾液腺和胰腺产生。

• Protease | 蛋白酶 breaks proteins into amino acids. Pepsin acts in the stomach; trypsin acts in the small intestine. | 将蛋白质分解为氨基酸。胃蛋白酶在胃中起作用，胰蛋白酶在小肠中起作用。

• Lipase | 脂肪酶 breaks fats (lipids) into fatty acids and glycerol. It is produced in the pancreas and acts in the small intestine. | 将脂肪（脂质）分解为脂肪酸和甘油。由胰腺产生，在小肠中起作用。

Bile, produced by the liver and stored in the gallbladder, is not an enzyme. However, it emulsifies fats into tiny droplets, increasing the surface area so that lipase can work faster. Bile also neutralises the acidic mixture from the stomach, providing the alkaline pH that intestinal enzymes need.

胆汁由肝脏产生、储存于胆囊，它并不是酶。但胆汁能将脂肪乳化为微小液滴，增大表面积，使脂肪酶能更快发挥作用。胆汁还能中和来自胃的酸性食糜，为肠道酶提供所需的碱性pH环境。

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8. Industrial and Medical Applications | 酶在工业与医学中的应用

Enzymes are widely used in industry because they are specific, efficient and work at moderate temperatures, which reduces energy costs. You should be able to give at least three named applications.

酶在工业中被广泛使用，因为酶具有专一性、高效性，并且在温和温度下工作，从而降低能源成本。你应当能够列举至少三个具体应用。

• Biological detergents | 生物洗涤剂 contain protease and lipase to break down protein and fat stains on clothing. They work best at low temperatures, saving energy. | 含有蛋白酶和脂肪酶，用于分解衣物上的蛋白质和脂肪污渍。它们在低温下效果最好，节约能源。

• Food industry | 食品工业 — enzymes are used to convert starch into sugar syrups, to clarify fruit juices, and in cheese making. | 酶用于将淀粉转化为糖浆、澄清果汁以及制作奶酪。

• Medicine | 医学 — enzymes such as glucose oxidase are used in biosensors to measure blood glucose levels in diabetic patients. | 葡萄糖氧化酶等酶被用于生物传感器中，用于测量糖尿病患者的血糖水平。

Enzymes also have disadvantages in industry. They are easily denatured by high temperatures and unsuitable pH, and they can be expensive to produce. Some people also develop skin allergies to enzymes in biological detergents.

酶在工业中也有缺点。它们容易因高温和不适宜的pH而变性，生产成本可能较高。部分人对生物洗涤剂中的酶也会出现皮肤过敏。

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9. Enzyme Inhibitors | 酶抑制剂

An inhibitor is a substance that slows down or stops the action of an enzyme. At IGCSE level, you mainly need to know about competitive inhibitors, which have a similar shape to the substrate and compete with it for the active site.

抑制剂是减慢或阻止酶发挥作用的物质。在IGCSE阶段，你主要需要了解竞争性抑制剂，其形状与底物相似，会与底物竞争活性位点。

When a competitive inhibitor occupies the active site, the substrate cannot bind. This can be overcome by increasing the substrate concentration, because more substrate molecules will successfully outcompete the inhibitor. The maximum rate can still be reached if enough substrate is added.

当竞争性抑制剂占据活性位点时，底物便无法结合。这种抑制可以通过增加底物浓度来克服，因为更多的底物分子会成功胜过抑制剂。如果加入足够多的底物，反应仍能达到最大速率。

Non-competitive inhibitors bind to the enzyme away from the active site, changing the enzyme’s overall shape so that the active site no longer works. Adding more substrate does not overcome this type of inhibition. Heavy metal ions such as mercury and lead are examples of non-competitive inhibitors. They are a useful example in exam questions about food poisoning or pollution.

非竞争性抑制剂结合在远离活性位点的位置，改变酶的整体形状，使活性位点无法正常工作。增加底物浓度不能克服这种抑制。汞、铅等重金属离子就是非竞争性抑制剂的例子。在关于食物中毒或污染的考题中，它们是很有用的例子。

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

The Edexcel specification requires you to know how to investigate the effect of pH on the rate of reaction of amylase. A common method is described below.

爱德思考纲要求你掌握如何探究pH对淀粉酶反应速率的影响。常用方法如下所述。

• Place a drop of iodine solution into each well of a spotting tile. | 在点滴板的每个凹孔中滴入一滴碘液。

• Add starch solution and amylase solution to a test tube, along with a buffer solution of a known pH. | 在试管中加入淀粉溶液、淀粉酶溶液以及已知pH的缓冲溶液。

• Start a stopwatch and immediately remove a drop of the mixture to test with iodine. Record the time taken for the iodine to stop turning blue-black. | 启动秒表，立即取一滴混合液用碘液测试。记录碘液不再变蓝黑色所需的时间。

• Repeat with buffers of different pH values (e.g. pH 3, 5, 7, 9). | 使用不同pH的缓冲液（如pH 3、5、7、9）重复实验。

The shorter the time taken for the iodine to stop turning blue-black, the faster the enzyme is working. This is because iodine turns blue-black in the presence of starch; when all the starch has been broken down, the iodine remains orange-brown. The faster the starch disappears, the faster the rate of reaction. You should be able to identify the optimum pH as the one with the shortest reaction time.

碘液停止变蓝黑色的时间越短，说明酶的工作速率越快。这是因为碘遇淀粉会变蓝黑色；当所有淀粉都被分解后，碘液保持橙棕色。淀粉消失越快，反应速率越快。你应当能够根据最短反应时间确定最适pH。

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11. Common Exam Questions and Key Words | 常见考题与关键术语

Examiners use a limited set of command words in enzyme questions. Knowing how to respond to each one will help you score marks confidently.

考官在酶相关题目中使用的指令词是有限的。了解如何回应每一种指令词，能帮助你自信得分。

Key terms you must spell correctly in exams: enzyme, catalyst, substrate, active site, product, denature, optimum. These words should appear in almost every answer you write about enzyme action.

考试中必须拼写正确的关键术语：enzyme（酶）、catalyst（催化剂）、substrate（底物）、active site（活性位点）、product（产物）、denature（变性）、optimum（最适）。在你书写的几乎所有酶相关答案中，都应出现这些词。

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12. Revision Summary | 复习总结

Enzymes are specific protein catalysts that speed up reactions without being used up. Their activity depends on temperature and pH, and each enzyme has an optimum for both. Extremes of either condition cause denaturation, which permanently changes the shape of the active site.

酶是特异性蛋白质催化剂，能加快反应速率而不被消耗。其活性取决于温度和pH，每种酶对两者都有最适值。任一条件达到极端都会导致变性，永久改变活性位点形状。

Reaction rate also increases with enzyme concentration and substrate concentration until all active sites are saturated. Digestion, industrial detergents and medical biosensors all depend on enzymes. Finally, inhibitors reduce enzyme activity, and competitive inhibition can be overcome by raising substrate concentration.

反应速率也随酶浓度和底物浓度增加而加快，直到所有活性位点饱和为止。消化过程、工业洗涤剂和医学生物传感器都依赖酶。最后，抑制剂会降低酶活性，而竞争性抑制可以通过提高底物浓度来克服。

Use the table below to review the digestive enzymes quickly before your exam:

考试前使用下表快速复习消化酶：

Practice drawing the temperature and pH graphs from memory, and make sure you can explain each part of the curve in full sentences. This will prepare you for the highest-mark questions in the paper.

练习凭记忆绘制温度和pH曲线图，确保你能用完整句子解释曲线的每个部分。这将帮助你应对试卷中分值最高的题目。

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