📚 Enzymes and Their Role in Biological Reactions | 酶及其在生物反应中的作用
Enzymes are proteins that act as biological catalysts, speeding up chemical reactions inside living organisms. They are essential for every aspect of life, from digestion to DNA replication, and without them, many reactions would be too slow to sustain life. This article explores the structure, function, and practical applications of enzymes, aligned with the Edexcel IGCSE Biology specification.
酶是充当生物催化剂的蛋白质，能够加速生物体内的化学反应。它们对生命的各个方面都至关重要，从消化到DNA复制，没有它们，许多反应将慢到无法维持生命。本文围绕Edexcel IGCSE生物学大纲，探讨酶的结构、功能及实际应用。
1. What Are Enzymes? | 什么是酶？
An enzyme is a globular protein with a specific three-dimensional shape. It lowers the activation energy of a chemical reaction, allowing the reaction to proceed more quickly at normal body temperatures. Each enzyme is specific to one substrate – the molecule it acts upon.
酶是一种具有特定三维形状的球状蛋白质。它能降低化学反应的活化能，从而让反应在正常体温下更快地进行。每种酶只针对一种底物——即它所作用的分子。
- Enzymes remain unchanged after a reaction – they can be reused many times.
- Enzymes are specific – each enzyme catalyses only one type of reaction.
- Enzymes are denaturable – extreme conditions permanently alter their shape.
- 反应后酶保持不变——它们可以被重复使用多次。
- 酶具有专一性——每种酶只能催化一种类型的反应。
- 酶可被变性——极端条件会永久改变其形状。
2. Structure of an Enzyme | 酶的结构
Enzymes are made of chains of amino acids folded into a unique shape. The active site is a small pocket or groove on the enzyme surface, specifically shaped to fit the substrate molecule. The rest of the enzyme molecule maintains the stability of the active site.
酶由氨基酸链折叠成独特的形状。活性位点是酶表面的一个小口袋或凹槽，其形状恰好与底物分子匹配。酶分子的其余部分用于维持活性位点的稳定性。
Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Product
酶 + 底物 → 酶-底物复合物 → 酶 + 产物
This equation shows that the enzyme is not consumed; it is available for further reactions after the product is released.
这个方程式表明酶本身不被消耗；在产物释放后，酶可继续参与下一轮反应。
3. The Lock and Key Hypothesis | 锁钥假说
The lock and key hypothesis explains enzyme specificity. The active site is the "lock" and the substrate is the "key". Only the correct key can fit into the lock. When the substrate binds, an enzyme-substrate complex is formed, and the reaction occurs.
锁钥假说解释了酶的专一性。活性位点好比“锁”，底物好比“钥匙”。只有正确的钥匙才能插入锁中。当底物结合时，形成酶-底物复合物，反应随即发生。
Some enzymes undergo induced fit – the active site changes shape slightly to accommodate the substrate. However, the Edexcel IGCSE course mainly expects you to describe the lock and key model.
有些酶会发生诱导契合——活性位点会轻微改变形状以适配底物。不过，Edexcel IGCSE课程主要要求你描述锁钥模型。
4. Factors Affecting Enzyme Activity | 影响酶活性的因素
4.1 Temperature | 温度
As temperature increases, kinetic energy increases, so molecules move faster and collisions between enzyme and substrate become more frequent. The rate of reaction rises up to the optimum temperature. Beyond this, the enzyme begins to denature. Denaturation breaks the bonds that maintain the enzyme's shape, especially the active site, so the enzyme can no longer catalyse the reaction.
随着温度升高，动能增加，分子运动加快，酶与底物之间碰撞更频繁。反应速率上升直到最适温度。超过最适温度后，酶开始变性。变性会破坏维持酶形状的化学键，尤其是活性位点，导致酶无法再催化反应。
| Temperature range | Effect on enzyme activity |
| Low (0–10 °C) | Low activity, but enzyme is not damaged |
| Optimum (around 37 °C in humans) | Maximum activity |
| High (above 60 °C) | Rapid denaturation |
| 温度区间 | 对酶活性的影响 |
| 低温（0–10 °C） | 活性低，但酶不受损伤 |
| 最适温度（人体约37 °C） | 活性最大 |
| 高温（超过60 °C） | 迅速变性 |
4.2 pH | pH值
Each enzyme has an optimum pH. Most human enzymes work best near pH 7, but pepsin in the stomach works best at pH 2. Changes in pH can affect the charges on amino acids and break bonds, leading to denaturation.
每种酶都有最适pH值。多数人体酶在pH 7附近活性最高，但胃中的胃蛋白酶最适pH为2。pH改变会影响氨基酸的电荷并破坏化学键，导致变性。
5. Enzymes in Digestion | 消化中的酶
Digestive enzymes break large insoluble molecules into small soluble molecules that can be absorbed into the blood.
消化酶将大分子不溶性食物分解为可被血液吸收的小分子可溶性物质。
- Amylase – produced in the salivary glands and pancreas; breaks starch into maltose.
- Protease – produced in the stomach, pancreas and small intestine; breaks proteins into amino acids.
- Lipase – produced in the pancreas and small intestine; breaks fats into fatty acids and glycerol.
- 淀粉酶 – 由唾液腺和胰腺分泌；将淀粉分解为麦芽糖。
- 蛋白酶 – 由胃、胰腺和小肠分泌；将蛋白质分解为氨基酸。
- 脂肪酶 – 由胰腺和小肠分泌；将脂肪分解为脂肪酸和甘油。
Bile, produced by the liver, is not an enzyme, but it emulsifies fats to increase the surface area for lipase action.
肝脏分泌的胆汁并不是酶，但它能乳化脂肪，增加脂肪与脂肪酶接触的表面积。
6. Uses of Enzymes in Industry | 酶在工业中的应用
Enzymes are widely used in food manufacturing, medicine and biological detergents.
酶广泛应用于食品制造、医药和生物洗涤剂中。
- Proteases and lipases in biological washing powders remove protein and fat stains at low temperatures, saving energy.
- Carbohydrases (e.g. isomerase) convert glucose into fructose, which is sweeter, so less sugar is needed in foods.
- Lactase breaks down lactose in milk, producing lactose-free milk for people who are lactose intolerant.
- 蛋白酶和脂肪酶 在生物洗衣粉中，可在低温下去除蛋白质和脂肪污渍，节约能源。
- 糖酶（例如异构酶）将葡萄糖转化为果糖，果糖更甜，因此食品中可少用糖。
- 乳糖酶 分解牛奶中的乳糖，为乳糖不耐受人群生产无乳糖牛奶。
Advantages of enzymes: they are specific, work at moderate temperatures and pH, and are environmentally friendly. Disadvantages: they can be denatured by heat or pH extremes, may be expensive, and some people have allergies to enzyme powders.
酶的优势：专一性高，在温和的温度和pH下工作，且环境友好。劣势：可能受热或极端pH影响而变性，成本高，部分人可能对酶粉过敏。
7. Enzyme Inhibitors | 酶抑制剂
Inhibitors are molecules that reduce enzyme activity. Competitive inhibitors have a similar shape to the substrate and block the active site. Non-competitive inhibitors bind elsewhere on the enzyme, changing its shape so the active site no longer works.
抑制剂是降低酶活性的分子。竞争性抑制剂与底物形状相似，占据活性位点。非竞争性抑制剂结合在酶的其他部位，改变酶的形状，使活性位点失效。
In everyday life, some drugs and poisons act as inhibitors. For example, aspirin inhibits an enzyme involved in inflammation.
在日常生活中，一些药物和毒物就是抑制剂。例如，阿司匹林可抑制与炎症相关的酶。
8. Investigating Enzyme Activity | 探究酶活性
In the laboratory, you can investigate the effect of temperature or pH on enzyme activity. A common experiment uses amylase and starch:
在实验室中，你可以探究温度或pH对酶活性的影响。一个常见实验使用淀粉酶和淀粉：
- Place a drop of iodine solution into each well of a spotting tile.
- Add starch solution and amylase to a test tube, and place it in a water bath at a set temperature.
- Every 30 seconds, remove a drop of the mixture and add it to the iodine solution.
- Iodine turns blue-black in the presence of starch. Record the time taken for the iodine to stay orange-brown, indicating that all starch has been digested.
- 在每个白瓷板凹槽中滴入一滴碘液。
- 在试管中加入淀粉溶液和淀粉酶，放入设定温度的水浴中。
- 每30秒取出一滴混合液，滴入碘液中。
- 碘遇淀粉变蓝黑色。记录碘液保持橙棕色所需的时间，说明淀粉已被完全消化。
The shorter the time, the higher the enzyme activity. Repeat at different temperatures to plot a graph of rate against temperature.
时间越短，酶活性越高。在不同温度下重复实验，可绘制反应速率对温度的曲线图。
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