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

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

Enzymes are biological molecules that act as catalysts, speeding up chemical reactions inside living organisms without being used up themselves. They are essential for every metabolic process, from digestion and respiration to DNA replication and photosynthesis. In the Edexcel IGCSE Biology syllabus, enzymes appear in many topics, and questions about their properties, the factors that affect their activity, and their applications in industry are a regular feature of both Paper 1 and Paper 2.

酶是作为催化剂起作用的生物分子，能够加速生物体内的化学反应，而自身不会在反应中被消耗。酶对每一个代谢过程都至关重要，从消化、呼吸到 DNA 复制和光合作用。在 Edexcel IGCSE 生物课程中，酶出现在多个章节中，关于酶的性质、影响酶活性的因素以及酶在工业中应用的题目，在 Paper 1 和 Paper 2 中都非常常见。

1. What Are Enzymes? | 什么是酶？

Enzymes are proteins made from long chains of amino acids folded into a specific three-dimensional shape. The unique folding of each enzyme creates a region called the active site, which is the part of the enzyme where the substrate molecule binds and the reaction takes place.

酶是由氨基酸长链折叠成特定三维结构而形成的蛋白质。每种酶独特的折叠方式会形成一个称为活性位点的区域，即底物分子结合并发生反应的部位。

Enzymes are highly specific. Because the active site has a particular shape, only one type of substrate (or a small group of related substrates) can fit into it. For example, the enzyme amylase can only catalyse the breakdown of starch, not proteins or fats.

酶具有高度的专一性。由于活性位点有特定的形状，只有一种底物（或一小类相关底物）能够与之结合。例如，淀粉酶只能催化淀粉的分解，而不能催化蛋白质或脂肪的分解。

Enzyme 酶	Substrate 底物	Product 产物
Amylase 淀粉酶	Starch 淀粉	Maltose 麦芽糖
Protease 蛋白酶	Protein 蛋白质	Amino acids 氨基酸
Lipase 脂肪酶	Fats 脂肪	Fatty acids + glycerol 脂肪酸和甘油

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

The lock and key model is a simple way to understand 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, just as only the correct key can open the lock. When the substrate binds, an enzyme-substrate complex is formed.

锁钥模型是理解酶专一性的简单方式。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，就像只有正确的钥匙才能打开锁一样。当底物结合后，就形成了酶-底物复合物。

Enzyme + Substrate ⇌ Enzyme-Substrate Complex → Enzyme + Products

酶 + 底物 ⇌ 酶-底物复合物 → 酶 + 产物

Enzymes work by lowering the activation energy of a reaction, meaning that less energy is needed for the reaction to start. The reaction therefore proceeds much more quickly. After the reaction, the products are released from the active site, and the enzyme is unchanged and ready to be reused.

酶通过降低反应的活化能来发挥作用，这意味着反应起动所需的能量更少，反应因此进行得更快。反应结束后，产物从活性位点释放出来，酶本身保持不变，可以继续被重复利用。

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

Temperature has a major effect on the rate of enzyme-controlled reactions. As temperature increases from a low value, molecules gain kinetic energy and move faster. This increases the frequency of successful collisions between enzymes and substrates, so the rate of reaction rises steadily.

温度对酶促反应速率有很大影响。随着温度从较低值逐渐升高，分子获得更多动能并运动加快，酶与底物之间有效碰撞的频率增加，因此反应速率稳步上升。

For most human enzymes, the optimum temperature is about 37°C, which is normal body temperature. At this point the reaction rate is at its maximum. Beyond the optimum, the rate falls sharply because high temperatures break the hydrogen bonds and other bonds that maintain the enzyme’s three-dimensional shape.

大多数人体酶的最适温度约为 37°C，即正常体温。此时反应速率达到最大。超过最适温度后，速率急剧下降，因为高温会破坏维持酶三维形状的氢键和其他化学键。

When the shape of the active site is changed, the substrate can no longer fit into it. The enzyme is described as denatured. Denaturation is permanent and irreversible. At low temperatures, however, enzymes are not denatured; they are simply inactive or less active because the molecules have less kinetic energy. This is why food is kept in a refrigerator to slow down enzyme-controlled decay.

当活性位点的形状发生改变后，底物无法再与之结合，此时称酶发生了变性。变性是永久且不可逆的。但在低温下，酶并不会变性，只是活性降低或暂时失去活性，因为分子动能减少。这就是为什么把食物放入冰箱可以减缓酶催化的腐败过程。

4. pH and Enzyme Activity | pH与酶活性

Each enzyme has an optimum pH at which its activity is highest. A change in pH affects the charges on amino acid residues in the enzyme and can break the bonds that hold the protein in its correct shape. Extreme pH values cause denaturation, just as high temperatures do.

每种酶都有其活性最高的最适 pH。pH 的改变会影响酶分子中氨基酸残基的电荷，并可能破坏维持蛋白质正确形状的化学键。极端 pH 值会像高温一样导致酶变性。

Different enzymes have different optimum pH values depending on where they work in the body. Pepsin, which digests protein in the stomach, works best at pH 2 because the stomach contains hydrochloric acid. Amylase in the mouth and small intestine works best at pH 7, which is neutral. Trypsin, which works in the small intestine, has an optimum pH of about 8 to 9.

不同酶的最适 pH 各不相同，这取决于它们在体内的工作位置。在胃中消化蛋白质的胃蛋白酶，因为胃内含有盐酸，所以在 pH 2 时活性最高。口腔和小肠中的淀粉酶在 pH 7（中性）时活性最高。在小肠中发挥作用的胰蛋白酶，其最适 pH 约为 8 至 9。

In exams, you may be shown a curve of enzyme activity against pH. You should state the optimum pH from the curve and explain that the activity falls on both sides of the optimum because the enzyme becomes denatured at values far from it.

考试中，你可能会看到酶活性随 pH 变化的曲线图。你应该从图中读出最适 pH，并解释在远离最适 pH 的两侧活性下降，是因为酶发生了变性。

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

Substrate concentration and enzyme concentration both affect the rate of reaction. When the substrate concentration increases with a fixed amount of enzyme, the rate of reaction increases because more substrate molecules are available to collide with the active sites. However, this only continues until all active sites are occupied. Once this saturation point is reached, adding more substrate cannot speed up the reaction, and the graph becomes a horizontal line.

底物浓度和酶浓度都会影响反应速率。当酶量固定而底物浓度升高时，反应速率加快，因为有更多底物分子可与活性位点发生碰撞。但这种趋势只持续到所有活性位点都被占满为止。一旦达到饱和点，继续增加底物无法加快反应，曲线变为水平线。

Similarly, if the substrate is in excess, increasing the enzyme concentration increases the rate of reaction because there are more active sites available for the substrate to bind to. If the substrate is limited, however, adding more enzyme will have no further effect, because the substrate runs out first.

类似地，当底物充足时，增加酶浓度会加快反应速率，因为有更多活性位点可供底物结合。但如果底物有限，继续增加酶也不会产生更多效果，因为底物会先被耗尽。

Rate of reaction ∝ Enzyme concentration (when substrate is in excess)

反应速率 ∝ 酶浓度（当底物足量时）

6. Enzymes in Digestion | 消化中的酶

Digestion depends on enzymes that break large, insoluble food molecules into small, soluble molecules that can be absorbed into the blood. Different digestive enzymes act on different types of food.

消化过程依赖酶将大分子、不溶性的食物分解为可吸收进入血液的小分子、可溶性物质。不同的消化酶作用于不同类型的食物。

Carbohydrases break down carbohydrates. Amylase is produced by the salivary glands and the pancreas. It breaks starch into maltose in the mouth and small intestine. Maltase, found on the cell surface membrane of the small intestine, breaks maltose into glucose, which is readily absorbed.

糖酶分解碳水化合物。淀粉酶由唾液腺和胰腺产生，在口腔和小肠中将淀粉分解为麦芽糖。小肠细胞表面膜上的麦芽糖酶将麦芽糖分解为葡萄糖，葡萄糖很容易被吸收。

Proteases break down proteins into amino acids. Pepsin is secreted by the stomach wall and works at low pH. Trypsin, made by the pancreas, continues the digestion of proteins and peptides in the small intestine at a slightly alkaline pH.

蛋白酶将蛋白质分解为氨基酸。胃壁分泌的胃蛋白酶在低 pH 环境中工作。胰腺产生的胰蛋白酶在小肠的微碱性环境中继续将蛋白质和多肽分解为氨基酸。

Lipases break down fats into fatty acids and glycerol. Lipase is produced by the pancreas and acts in the small intestine. Bile, produced by the liver and stored in the gall bladder, does not contain enzymes, but it emulsifies fats into tiny droplets. This increases the surface area of the fat, making digestion by lipase much faster and more efficient.

脂肪酶将脂肪分解为脂肪酸和甘油。脂肪酶由胰腺产生，在小肠中发挥作用。肝脏产生、胆囊储存的胆汁不含酶，但它能将脂肪乳化成微小液滴，增大脂肪的表面积，从而使脂肪酶的消化更快、更高效。

7. Industrial Applications of Enzymes | 酶的工业应用

Enzymes are widely used in industry because they are highly specific, work at moderate temperatures and pressures, and are biodegradable, which makes them more environmentally friendly than many chemical catalysts. They are also effective in very small quantities.

酶在工业中被广泛使用，因为它们专一性高，在温和的温度和压力下即可工作，且可生物降解，因此比许多化学催化剂更环保。此外，酶的用量很少却效果显著。

Biological washing powders contain proteases and lipases. These enzymes break down protein stains such as blood and fat stains such as grease, allowing fabrics to be cleaned at temperatures below 40°C. Washing at lower temperatures saves energy and reduces damage to delicate fabrics.

加酶洗衣粉含有蛋白酶和脂肪酶。这些酶能够分解血液等蛋白质污渍和油脂类污渍，使衣物能在低于 40°C 的温度下被清洗干净。低温洗涤可以节约能源，并减少对精细面料的损伤。

In the food industry, enzymes are used to extract and clarify fruit juices. For example, pectinase breaks down pectin, a substance found in plant cell walls, which helps release more juice and makes the juice clear. In medicine, enzymes are used in biosensors, such as the glucose biosensor used by people with diabetes to monitor their blood sugar level.

在食品工业中，酶被用于果汁的提取和澄清。例如，果胶酶能分解植物细胞壁中的果胶，从而提取更多果汁并使果汁澄清。在医学领域，酶被用于生物传感器，例如糖尿病患者用来监测血糖水平的葡萄糖生物传感器。

8. Experiments with Enzymes | 酶的实验

Practical work on enzymes is a common source of exam questions. The most common experiment investigates the effect of temperature on the rate of amylase activity using starch and iodine solution. A drop of reaction mixture is removed at intervals and tested with iodine: iodine solution turns from orange-brown to blue-black in the presence of starch. The time taken for the iodine to stop turning blue-black indicates how quickly the starch was digested.

有关酶的实验是常见的考试出题来源。最常见的实验是用淀粉和碘液研究温度对淀粉酶活性的影响。每隔一段时间取一滴反应混合液，用碘液检测：当有淀粉存在时，碘液从橙棕色变为蓝黑色。碘液不再变蓝黑色所需的时间，反映了淀粉被消化的快慢。

You should also be able to identify the variables in such an experiment. The independent variable is the factor being changed, for example temperature. The dependent variable is the rate of reaction, often measured as the time for the starch to be digested. Controlled variables include pH, enzyme concentration, substrate concentration, and the total volume of the mixture.

你还需要能够识别实验中的变量。自变量是被改变的因素，例如温度。因变量是反应速率，通常用淀粉被消化完所需的时间来度量。控制变量包括 pH、酶浓度、底物浓度和混合液的总体积。

Benedict’s test is also useful in enzyme practicals: it tests for reducing sugars such as glucose and maltose. A blue Benedict’s solution changes to orange or brick-red when heated with a reducing sugar. This test can be used to confirm that a carbohydrate has been digested into sugars.

本尼迪特试验在酶实验中也很有用：它用于检测葡萄糖、麦芽糖等还原糖。蓝色的本尼迪特溶液与还原糖混合加热后会变为橙色或砖红色。这个试验可用于确认碳水化合物已被消化为糖类。

9. Common Exam Mistakes and Tips | 常见考试错误与备考提示

One of the most common mistakes is saying that an enzyme is killed or dies at high temperatures. The correct term is denatured. Denaturation is a permanent change in the shape of the active site, not a biological death.

最常见的错误之一是说出”酶被杀死了”或”酶死了”。正确的说法是变性。变性是活性位点形状的永久改变，而不是生物学意义上的死亡。

A second common error is forgetting that low temperatures do not denature enzymes. At 0°C, enzyme activity is very low but the enzyme remains intact and will regain full activity when warmed up. Another mistake is confusing the lock and key model with the induced fit model; for IGCSE, the lock and key description is normally sufficient and expected.

第二个常见错误是忘记低温不会使酶变性。在 0°C 时，酶活性极低，但酶的结构保持完整，升温后会恢复全部活性。另一个错误是混淆锁钥模型

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