Enzymes | 酶

📚 Enzymes | 酶

Enzymes are biological catalysts that speed up chemical reactions in living organisms without being used up themselves. They are essential for nearly every metabolic process, from digestion to DNA replication.

酶是生物催化剂，能在不被消耗的情况下加速生物体内的化学反应。它们几乎对每一个代谢过程都至关重要，从消化到DNA复制都离不开酶。

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1. What Are Enzymes? | 什么是酶？

Enzymes are globular proteins made of long chains of amino acids folded into a specific three-dimensional shape. This shape includes an active site, where the substrate binds.

酶是由氨基酸长链折叠成特定三维结构的球状蛋白质。这种结构包含一个活性位点，是底物结合的位置。

• Enzymes are specific – each enzyme catalyses only one type of reaction.
• 酶具有专一性——每种酶只催化一种类型的反应。
• The active site has a unique shape that matches only its substrate.
• 活性位点的独特形状只能与其底物匹配。

The lock-and-key model explains enzyme specificity: the substrate fits into the active site like a key into a lock.

锁钥模型解释了酶的专一性：底物像钥匙插入锁中一样进入活性位点。

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2. How Enzymes Work | 酶如何工作

When a substrate binds to the active site, an enzyme-substrate complex is formed. The reaction occurs, and the products are released, leaving the enzyme unchanged and ready to catalyse another reaction.

当底物与活性位点结合时，形成酶-底物复合物。反应发生，产物释放后，酶保持不变，并准备好催化下一个反应。

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

酶 + 底物 → 酶-底物复合物 → 酶 + 产物

The induced-fit model suggests that the active site slightly changes shape to bind the substrate more tightly, improving catalysis.

诱导契合模型认为，活性位点在结合底物时会轻微改变形状，从而更紧密地结合底物，提高催化效率。

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3. Factors Affecting Enzyme Activity – Temperature | 影响酶活性的因素——温度

Temperature affects the kinetic energy of molecules. As temperature increases, molecules move faster and collide more often, so the rate of reaction increases.

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，因此反应速率增加。

• Optimum temperature – the temperature at which the enzyme activity is highest (around 37 °C in human enzymes).
• 最适温度——酶活性最高时的温度（人体酶约为37 °C）。
• Denaturation – above the optimum, the bonds holding the enzyme’s shape break, the active site changes shape, and the enzyme can no longer function.
• 变性——超过最适温度后，维持酶形状的键断裂，活性位点改变形状，酶无法再发挥作用。

Rate ↑ with temperature up to optimum; then rapid ↓ due to denaturation

速率随温度升高而上升至最适点；随后因变性而迅速下降

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4. Factors Affecting Enzyme Activity – pH | 影响酶活性的因素——pH值

Each enzyme has an optimum pH. For most human enzymes, this is around pH 7, but pepsin in the stomach works best at pH 2, while trypsin in the small intestine works best at pH 8.

每种酶都有其最适pH。大多数人体酶的最适pH约为7，但胃中的胃蛋白酶在pH 2时活性最高，而小肠中的胰蛋白酶在pH 8时活性最高。

Extreme pH values alter the ionic bonds and hydrogen bonds in the enzyme, changing the shape of the active site. This is a form of denaturation.

极端pH值会改变酶中的离子键和氢键，从而改变活性位点的形状。这也是一种变性。

• Below optimum pH – enzyme may denature if the pH is too low.
• 低于最适pH —— 如果pH过低，酶可能变性。
• Above optimum pH – enzyme may also denature.
• 高于最适pH —— 酶同样可能变性。

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5. Factors Affecting Enzyme Activity – Substrate Concentration | 影响酶活性的因素——底物浓度

As substrate concentration increases, the rate of reaction increases because more substrate molecules are available to collide with active sites.

随着底物浓度增加，反应速率加快，因为更多底物分子可以与活性位点碰撞。

However, once all active sites are occupied (i.e. the enzyme is saturated), adding more substrate will no longer increase the rate.

然而，一旦所有活性位点都被占据（即酶达到饱和），继续增加底物浓度不会再提高速率。

Rate → plateaus when enzyme is saturated

酶饱和时，速率趋于平稳

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6. Enzyme Concentration | 酶浓度

Increasing enzyme concentration increases the number of active sites available, so more substrate molecules can be converted per unit time, increasing the rate of reaction.

增加酶浓度会增加可用活性位点的数量，因此单位时间内可以转化更多底物分子，从而提高反应速率。

This holds true as long as substrate is not limiting. When substrate runs out, the rate drops to zero.

只要底物不成为限制因素，这个关系就成立。当底物耗尽时，速率降至零。

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7. Digestion and Enzymes | 消化与酶

Digestive enzymes break down large insoluble food molecules into small soluble molecules that can be absorbed into the blood.

消化酶将大的不溶性食物分子分解为可以吸收进入血液的小分子可溶物质。

Enzyme 酶	Substrate 底物	Product(s) 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	Lipids (fats) 脂质（脂肪）	Fatty acids + Glycerol 脂肪酸 + 甘油

These enzymes are produced in different parts of the digestive system, including the salivary glands, stomach, pancreas, and small intestine.

这些酶在消化系统的不同部位产生，包括唾液腺、胃、胰腺和小肠。

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8. Enzymes in Industry | 工业中的酶

Enzymes are widely used in industry because they are specific, work at mild temperatures and pH, and are biodegradable.

酶在工业中被广泛使用，因为它们具有专一性，在温和的温度和pH下工作，并且可生物降解。

• Biological washing powders use proteases and lipases to remove protein and fat stains.
• 加酶洗衣粉使用蛋白酶和脂肪酶去除蛋白质和脂肪污渍。
• Proteases are used to tenderise meat and clarify beer.
• 蛋白酶用于使肉类变嫩和澄清啤酒。
• Glucose isomerase converts glucose into fructose, a sweeter sugar used in food products.
• 葡萄糖异构酶将葡萄糖转化为果糖，果糖是食品中使用的更甜的糖。

However, enzymes can be expensive and may denature under harsh industrial conditions, so careful control is needed.

然而，酶可能成本较高，并且在恶劣的工业条件下可能变性，因此需要仔细控制。

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9. Investigating Enzyme Action – Practical Skills | 探究酶的作用——实验技能

In IGCSE Biology, you may investigate the effect of temperature or pH on enzyme activity using amylase and starch.

在IGCSE生物中，你可能会使用淀粉酶和淀粉来探究温度或pH对酶活性的影响。

Starch turns blue-black when iodine solution is added. As amylase breaks down starch, the colour becomes yellow-brown, indicating starch is no longer present.

淀粉遇碘液变蓝黑色。当淀粉酶分解淀粉时，颜色变为黄棕色，表明淀粉已不存在。

Step 步骤	Procedure 操作
1	Add starch solution to a test tube and place in a water bath at a set temperature.
1	将淀粉溶液加入试管，并放入设定温度的水浴中。
2	Add amylase solution and start timing.
2	加入淀粉酶溶液并开始计时。
3	Every 30 seconds, remove a drop and test with iodine on a white tile.
3	每30秒取出一点，在白色点滴板上用碘液测试。
4	Record the time taken for the blue-black colour to disappear.
4	记录蓝黑色消失所需的时间。

Shorter time means faster enzyme activity. Repeat at different temperatures to find the optimum.

时间越短意味着酶活性越快。在不同温度下重复实验以找到最适温度。

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10. Common Exam Mistakes | 常见考试错误

• Confusing denaturation with simply slowing down – denaturation changes the active site permanently.
• 混淆变性与速率减慢——变性会永久改变活性位点。
• Stating that enzymes are used up in reactions – they are not.
• 错误地认为酶在反应中被消耗——实际上不会。
• Writing that all enzymes have the same optimum temperature – they differ.
• 错误地认为所有酶具有相同的最适温度——它们各不相同。
• Describing the lock-and-key model as changing shape – that is the induced-fit model.
• 用“形状改变”来描述锁钥模型——那是诱导契合模型。

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Published by TutorHao | Biology Revision Series | aleveler.com

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