Enzymes: Biological Catalysts | 酶：生物催化剂

📚 Enzymes: Biological Catalysts | 酶：生物催化剂

Enzymes are biological catalysts that accelerate chemical reactions within living organisms without being used up in the process. Every living cell contains hundreds of different enzymes, each specialised for a particular reaction. In the Edexcel IGCSE Science specification, enzymes appear in topics covering nutrition, digestion, respiration and biotechnology, making them one of the most frequently examined concepts in the paper.

酶是在生物体内加速化学反应而不被消耗的生物催化剂。每个活细胞都含有数百种不同的酶，每种酶专门催化一种特定的反应。在Edexcel IGCSE科学大纲中，酶出现在营养、消化、呼吸和生物技术等章节中，是考卷中最常考查的概念之一。

1. What Are Enzymes? | 什么是酶？

Enzymes are globular proteins made from long chains of amino acids. These chains fold into a unique three-dimensional shape, held together by hydrogen bonds and ionic bonds between the amino acids. The most important part of an enzyme is the active site, a pocket or cleft whose shape is complementary to that of the substrate, the molecule on which the enzyme acts.

酶是由氨基酸长链构成的球状蛋白质。这些链折叠成独特的三维形状，由氨基酸之间的氢键和离子键维持。酶最重要的部分是活性位点，即一个形状与底物（酶作用的分子）互补的口袋或凹槽。

Enzymes are specific: one enzyme catalyses only one reaction or one type of reaction. | 酶具有专一性：一种酶只催化一种或一类反应。
The active site is the region where the substrate binds. | 活性位点是底物结合的区域。
Enzyme names usually end in ‘-ase’, e.g. amylase, protease, lipase. | 酶的名称通常以“-ase”结尾，例如淀粉酶、蛋白酶、脂肪酶。
Enzymes are not changed or destroyed by the reaction they catalyse. | 酶不会被其催化的反应改变或破坏。

2. How Enzymes Work: The Lock-and-Key Model | 酶如何工作：锁钥模型

The lock-and-key model explains enzyme specificity. The enzyme is the lock and the substrate is the key. Only a substrate with the correct shape can fit into the active site. When the substrate binds, an enzyme-substrate complex forms. The reaction then takes place, the substrate is converted into products, and the products leave the active site. The enzyme remains unchanged and can catalyse another reaction.

锁钥模型解释了酶的专一性。酶是锁，底物是钥匙。只有形状正确的底物才能进入活性位点。当底物结合时，形成酶-底物复合物。随后反应发生，底物被转化为产物，产物离开活性位点。酶保持不变，可以继续催化下一个反应。

Substrate + Enzyme → Enzyme–Substrate Complex → Product + Enzyme

底物 + 酶 → 酶-底物复合物 → 产物 + 酶

A more detailed model, the induced fit model, suggests that the active site changes shape slightly when the substrate binds, making the fit even tighter. This is an extension of the lock-and-key idea and is worth knowing for higher-mark questions.

更为精细的诱导契合模型认为，底物结合时活性位点会发生轻微形变，使结合更加紧密。这是锁钥模型的延伸，对高分值的题目很有价值。

3. Factor: Temperature | 影响因素：温度

Temperature affects the kinetic energy of molecules. As temperature increases, molecules move faster and collide more frequently, so the rate of reaction increases. However, enzymes are proteins. Above a certain temperature, the optimum temperature, the bonds holding the enzyme’s three-dimensional structure begin to break. The active site changes shape and the substrate can no longer bind. The enzyme is said to be denatured.

温度影响分子的动能。随着温度升高，分子运动加快，碰撞更频繁，反应速率增大。然而，酶是蛋白质。超过某个温度，即最适温度后，维持酶三维结构的化学键开始断裂。活性位点形状改变，底物无法再结合。这时称酶发生了变性。

Rate increases as temperature rises towards the optimum. | 温度升至最适温度前，反应速率不断增大。
The optimum temperature for most human enzymes is 37 °C. | 大多数人体酶的最适温度为37°C。
Above the optimum, rate falls sharply because enzymes denature. | 超过最适温度后，速率急剧下降，因为酶变性失活。
Denaturation is permanent and irreversible. | 变性是永久性的、不可逆的。

4. Factor: pH | 影响因素：pH

Every enzyme has an optimum pH at which its activity is highest. Most human enzymes work best at pH 7, which is neutral. Pepsin, an enzyme in the stomach, works best at pH 2 because the stomach contains hydrochloric acid. Extreme pH values change the charges on the amino acids in the enzyme and disrupt the ionic bonds that maintain the active site. This causes denaturation.

每种酶都有其活性最高的最适pH。大多数人体酶在pH 7（中性）时活性最高。胃中的胃蛋白酶在pH 2时活性最高，因为胃内含有盐酸。极端pH值会改变酶中氨基酸的电荷，破坏维持活性位点的离子键，导致变性。

Enzyme 酶	Location 位置	Optimum pH 最适pH
Pepsin 胃蛋白酶	Stomach 胃	2
Amylase 淀粉酶	Saliva / Pancreas 唾液 / 胰脏	7
Lipase 脂肪酶	Pancreas / Small intestine 胰脏 / 小肠	7 – 8

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

When the substrate is present in excess, increasing the concentration of enzyme increases the rate of reaction because more active sites are available for substrate molecules. If the enzyme concentration is kept constant, increasing the substrate concentration increases the rate up to a maximum. Beyond this point, all active sites are occupied at any given moment, so adding more substrate has no further effect.

当底物过量时，增加酶浓度会增大反应速率，因为可用的活性位点增多。如果酶浓度保持不变，增加底物浓度会使速率增大至最大值。超过这一点后，所有活性位点在任意时刻都已被占据，继续增加底物不再产生任何影响。

Condition 条件	Effect on rate 对速率的影响
Enzyme ↑ (substrate excess) 酶浓度增加（底物过量）	Rate increases proportionally 速率成正比增大
Substrate ↑ (enzyme constant) 底物浓度增加（酶恒定）	Rate increases until saturation 速率增大直至饱和
Vmax reached 达到最大速率	Further substrate has no effect 继续增加底物无影响

6. Enzyme Inhibitors | 酶抑制剂

An inhibitor is a substance that slows down or stops the action of an enzyme. A competitive inhibitor has a shape similar to the substrate and competes for the active site. Its effect can be reduced by increasing the substrate concentration. A non-competitive inhibitor binds to a site other than the active site, changing the enzyme’s shape so that the active site no longer works. Its effect cannot be overcome by adding more substrate.

抑制剂是减慢或阻止酶作用的物质。竞争性抑制剂具有与底物相似的形状，与底物竞争活性位点。增加底物浓度可以减弱其抑制作用。非竞争性抑制剂结合在活性位点以外的部位，改变酶的形状，使活性位点失效。增加底物浓度不能消除其抑制作用。

Competitive: binds to the active site; substrate competes. | 竞争性：结合活性位点；底物与之竞争。
Non-competitive: binds elsewhere; changes the shape of the active site. | 非竞争性：结合其他部位；改变活性位点形状。
Inhibitors can be reversible or irreversible. | 抑制剂可以是可逆的或不可逆的。

7. Enzymes in Digestion | 消化中的酶

Digestion relies on enzymes to break down large, insoluble food molecules into small, soluble molecules that can be absorbed into the blood. Each digestive enzyme targets a specific food group and produces specific products.

消化过程依赖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。每种消化酶针对特定的食物类别并产生特定的产物。

Enzyme 酶	Source 来源	Substrate 底物	Products 产物
Amylase 淀粉酶	Salivary glands, pancreas 唾液腺、胰腺	Starch 淀粉	Maltose 麦芽糖
Protease 蛋白酶	Stomach, pancreas 胃、胰腺	Protein 蛋白质	Amino acids 氨基酸
Lipase 脂肪酶	Pancreas 胰腺	Lipids (fats) 脂质（脂肪）	Fatty acids + glycerol 脂肪酸 + 甘油

8. Enzymes in Industry and Medicine | 酶在工业与医学中的应用

Enzymes are used widely outside the body because they work at moderate temperatures, are specific and biodegrade safely. Biological washing powders contain proteases and lipases that remove protein-based and fat-based stains at low temperatures, saving energy. In medicine, lactase is used to produce lactose-free milk for people who cannot digest lactose, and enzymes are used in biosensors to measure blood glucose levels.

酶在体外应用广泛，因为它们在温和的温度下工作、具有专一性且可安全生物降解。生物洗衣粉含有蛋白酶和脂肪酶，可在低温下去除蛋白质和脂肪污渍，节约能源。医学上，乳糖酶用于生产无乳糖牛奶，供不能消化乳糖的人群食用；酶还用于生物传感器测量血糖水平。

Biological washing powders: proteases and lipases break down stains. | 生物洗衣粉：蛋白酶和脂肪酶分解污渍。
Lactase converts lactose into glucose and galactose. | 乳糖酶将乳糖转化为葡萄糖和半乳糖。
Immobilised enzymes in industry reduce cost and allow reuse. | 工业中固定化酶降低成本并可再生利用。

9. Core Practical: Investigating Amylase Activity | 核心实验：探究淀粉酶活性

A classic IGCSE practical is to investigate the effect of temperature on the rate of starch digestion by amylase. Starch gives a blue-black colour with iodine solution. As amylase breaks down starch into maltose, the iodine colour disappears. Using a water bath at different temperatures, a drop of the mixture is removed every 30 seconds and tested on a white tile with iodine.

IGCSE经典实验是探究温度对淀粉酶分解淀粉速率的影响。淀粉遇碘溶液呈蓝黑色。随着淀粉酶将淀粉分解为麦芽糖，碘的颜色消失。使用不同温度的水浴，每隔30秒取一滴混合液，在白瓷板上用碘液测试。

Independent variable: temperature. | 自变量：温度。
Dependent variable: time for iodine colour to disappear. | 因变量：碘颜色消失所需时间。
Control variables: pH, enzyme concentration, starch concentration, volume. | 控制变量：pH、酶浓度、淀粉浓度、体积。
Iodine turns blue-black in the presence of starch; brown/orange when starch is absent. | 碘遇淀粉变蓝黑色；无淀粉时呈棕橙色。

10. Exam Tips and Common Misconceptions | 考试技巧与常见误区

Students often lose marks by confusing key terms. Denaturation is not “death” and it is not a temporary slowing down; it is a permanent change in the shape of the active site. Another common error is to say that pH or temperature changes the substrate; in fact, these factors act on the enzyme’s structure. When describing graphs, name the variable, state the trend, then explain using the terms ‘active site’, ‘optimum’ and ‘denatured’.

学生常因混淆关键术语而失分。变性不是酶“死亡”，也不是暂时性速度减慢；它是活性位点形状的永久性改变。另一个常见错误是认为pH或温度改变了底物；实际上这些因素作用于酶的结构。在描述图表时，先说明变量，再陈述趋势，然后用“活性位点”“最适”“变性”等术语进行解释。

Denaturation changes the active site permanently. | 变性永久改变活性位点。
Lock-and-key: the active site has a fixed shape. | 锁钥模型：活性位点形状固定。
Induced fit: the active site adjusts upon binding. | 诱导契合：结合时活性位点微调。
Enzymes are reused — they are not consumed in the reaction. | 酶可重复使用——反应中不被消耗。

Published by TutorHao | Science Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Enzymes: Biological Catalysts | 酶：生物催化剂