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

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

Enzymes are essential proteins that speed up chemical reactions in living organisms without being used up themselves. They are highly specific and work under particular conditions. Understanding enzymes is a core requirement of the Edexcel IGCSE Biology syllabus, especially for questions involving metabolism, digestion, and practical investigations.

酶是生命体中加速化学反应而不被消耗的蛋白质。它们具有高度专一性，且需要在特定条件下发挥作用。理解酶是 Edexcel IGCSE 生物考纲的核心要求，尤其涉及代谢、消化和实验探究类题目。

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. The unique shape includes an active site, where the substrate binds. Only molecules with a complementary shape can fit into this active site, which is why enzymes are specific to one reaction or a group of similar reactions.

酶是由氨基酸长链折叠成特定三维结构的球形蛋白质。其独特形状包含一个活性位点，底物在此结合。只有形状互补的分子才能进入活性位点，因此酶只对某一种反应或一组相似反应具有专一性。

Enzymes reduce the activation energy needed for a reaction, allowing metabolic reactions to occur quickly at body temperature. Without enzymes, many reactions would be too slow to sustain life.

酶能降低反应所需的活化能，使代谢反应在体温条件下快速进行。没有酶，许多反应将慢到无法维持生命。

2. The Lock and Key Model | 锁钥模型

The lock and key model explains enzyme specificity. The enzyme is the ‘lock’ and the substrate is the ‘key’. The shape of the substrate must fit exactly into the active site of the enzyme. When they combine, an enzyme-substrate complex is formed, and the reaction proceeds.

锁钥模型解释了酶的专一性。酶是“锁”，底物是“钥匙”。底物的形状必须与酶的活性位点完全匹配。当它们结合时，形成酶-底物复合物，反应随即进行。

This model is simplistic but useful. A modern refinement is the induced fit model, where the active site changes shape slightly to accommodate the substrate, like a glove moulding around a hand. Both models emphasise that the active site must be complementary to the substrate.

该模型虽然简化，但很有用。更现代的修正是“诱导契合模型”，即活性位点会轻微改变形状以容纳底物，如同手套包裹手一样。两个模型都强调活性位点必须与底物互补。

3. Enzyme Specificity | 酶的专一性

Enzymes are highly specific because their active sites have a unique shape. For example, the enzyme amylase only breaks down starch, not protein or lipids. Similarly, lipase acts on fats, and protease acts on proteins. This specificity is determined by the enzyme’s amino acid sequence and its folded three-dimensional structure.

酶具有高度专一性，因为其活性位点具有独特形状。例如，淀粉酶只分解淀粉，而不分解蛋白质或脂质。同样，脂肪酶作用于脂肪，蛋白酶作用于蛋白质。这种专一性由酶的氨基酸序列及其折叠形成的三维结构决定。

Biological washing powders use a mixture of enzymes: proteases to break down protein stains and lipases to break down fat stains. The specificity allows each enzyme to target a particular type of stain without damaging the fabric.

生物洗衣粉使用多种酶的混合物：蛋白酶分解蛋白质污渍，脂肪酶分解脂肪污渍。这种专一性使每种酶只针对特定类型的污渍，而不损伤织物。

4. Factors Affecting Enzyme Action: Temperature | 影响酶作用的因素：温度

Temperature has a major effect on enzyme activity. As temperature increases, particles gain kinetic energy and collide more frequently, so the rate of reaction increases. This continues up to the optimum temperature, usually around 37 °C in the human body.

温度对酶活性有重大影响。随着温度升高，粒子获得更多动能，碰撞更加频繁，因此反应速率加快。这个过程持续至最适温度，人体内通常约为 37 °C。

Above the optimum temperature, the enzyme’s bonds begin to break. The active site changes shape, so the substrate can no longer bind. The enzyme is said to be denatured. Denaturation is irreversible in most cases. At very low temperatures, enzymes are inactive but not denatured; they simply work very slowly.

超过最适温度后，酶的化学键开始断裂，活性位点形状改变，底物无法再结合，此时称酶已变性。大多数情况下变性是不可逆的。在极低温度下，酶活性降低但未变性，只是反应非常缓慢。

Rate of reaction increases with temperature until optimum, then falls sharply after denaturation.

反应速率随温度升高而加快，达到最适温度后，因变性而急剧下降。

5. Factors Affecting Enzyme Action: pH | 影响酶作用的因素：pH

Every enzyme has an optimum pH. Most human enzymes work best at pH 7 (neutral). However, pepsin in the stomach works best at pH 2, which is highly acidic. Changes in pH alter the charges and hydrogen bonds within the enzyme molecule, causing the active site to change shape and possibly denature the enzyme.

每种酶都有最适 pH。大多数人体酶在中性 pH 7 时活性最高。然而，胃中的胃蛋白酶在 pH 2 的强酸环境下活性最佳。pH 变化会改变酶分子内部的电荷和氢键，导致活性位点形状改变，甚至使酶变性。

In practical investigations, buffer solutions are used to maintain a constant pH so that only the tested variable is changed. This allows a fair test.

在实验探究中，使用缓冲液以维持恒定 pH，从而只改变被测变量，保证实验公平。

6. Factors Affecting Enzyme Action: Substrate and Enzyme Concentration | 影响酶作用的因素：底物浓度和酶浓度

When substrate concentration increases, the rate of reaction increases because there are more particles to collide with active sites. However, once all active sites are occupied, adding more substrate has no effect. The enzyme concentration also affects the rate: more enzymes provide more active sites, so the rate increases proportionally, unless substrate is limited.

当底物浓度增加时，反应速率加快，因为更多底物分子与活性位点碰撞。然而，一旦所有活性位点都被占用，再增加底物也不会提高速率。酶浓度同样影响速率：酶越多，活性位点越多，速率按比例增加，除非底物不足。

Two important calculations are often tested: rate = 1000 ÷ time (for enzyme experiments using a substrate that produces a coloured product), and the effect of inhibitors. Competitive inhibitors block the active site, while non-competitive inhibitors bind elsewhere and change the enzyme’s shape.

两个重要计算常被考查：速率 = 1000 ÷ 时间（用于底物产生有色产物的酶实验），以及抑制剂的作用。竞争性抑制剂占据活性位点，非竞争性抑制剂结合在其他位置并改变酶的形状。

7. Enzymes in Digestion | 消化中的酶

The human digestive system uses enzymes to break down large insoluble molecules into small soluble ones. Amylase is produced in the salivary glands and pancreas and breaks down starch into maltose. Proteases such as pepsin and trypsin break down proteins into amino acids. Lipase, produced in the pancreas, breaks down fats into fatty acids and glycerol.

人体消化系统利用酶将大分子不溶性物质分解为小分子可溶性物质。淀粉酶由唾液腺和胰腺分泌，将淀粉分解为麦芽糖。蛋白酶如胃蛋白酶和胰蛋白酶将蛋白质分解为氨基酸。脂肪酶由胰腺分泌，将脂肪分解为脂肪酸和甘油。

Bile, produced by the liver, is not an enzyme but helps emulsify fats into tiny droplets, increasing the surface area for lipase to work on. The small intestine also produces maltase, sucrase, and peptidase to complete digestion.

胆汁由肝脏产生，虽然不是酶，但能乳化脂肪形成微小液滴，增大脂肪酶作用的表面积。小肠还会产生麦芽糖酶、蔗糖酶和肽酶以完成消化。

8. Enzymes in Respiration and Metabolism | 呼吸与代谢中的酶

Enzymes control every step of respiration. In aerobic respiration, a series of enzymes in the cytoplasm and mitochondria convert glucose and oxygen into carbon dioxide, water, and ATP. Without these enzymes, energy release would be impossible at normal body temperature.

酶控制呼吸作用的每一步。在有氧呼吸中，细胞质和线粒体中的一系列酶将葡萄糖和氧气转化为二氧化碳、水和 ATP。没有这些酶，在正常体温下不可能释放能量。

Enzymes also regulate photosynthesis. For example, rubisco is an enzyme that fixes carbon dioxide in the Calvin cycle. Metabolic pathways are chains of enzyme-controlled reactions, where the product of one reaction becomes the substrate of the next.

酶还调节光合作用。例如，Rubisco 是卡尔文循环中固定二氧化碳的酶。代谢途径是由酶控制的连锁反应，其中一种反应的产物成为下一种反应的底物。

9. Practical Application: Investigating Enzyme Activity | 实际应用：探究酶活性

A common IGCSE practical involves measuring the rate of amylase breaking down starch. You can use iodine solution, which turns blue-black in the presence of starch. At regular time intervals, a drop of mixture is tested with iodine. The time taken for the iodine to stop changing colour indicates how quickly the starch has been digested.

一个常见的 IGCSE 实验是测量淀粉酶分解淀粉的速率。可使用碘液，碘液遇淀粉变蓝黑色。每隔固定时间，取一滴混合液与碘液测试。碘液不再变色的时间即表示淀粉被消化完的速率。

Another experiment uses catalase from plant or animal tissue to decompose hydrogen peroxide. The volume of oxygen gas collected over time gives the rate of reaction. In both experiments, variables such as temperature, pH, and concentration must be controlled.

另一个实验使用植物或动物组织中的过氧化氢酶分解过氧化氢。收集氧气的体积随时间的变化可得出反应速率。在两个实验中，必须控制温度、pH 和浓度等变量。

Rate = 1 ÷ time taken (for a measurable endpoint)

速率 = 1 ÷ 所用时间（对于可测量的终点）

10. Enzymes in Industry and Medicine | 酶在工业和医药中的应用

Enzymes are used in many industrial processes. In the food industry, pectinase is used to clarify fruit juices, and lactase is used to make lactose-free milk. In medicine, enzymes are used in diagnostic tests, such as glucose oxidase strips for measuring blood sugar levels.

酶在许多工业过程中得到应用。在食品工业中，果胶酶用于澄清果汁，乳糖酶用于生产无乳糖牛奶。在医学上，酶用于诊断测试，如测量血糖水平的葡萄糖氧化酶试纸。

Precision of enzymes makes them valuable in biotechnology. Genetically engineered enzymes can be produced in large quantities by microorganisms. However, enzymes can be expensive and are often sensitive to temperature and pH, which limits some industrial applications.

酶的专一性使其在生物技术中极具价值。通过基因工程改造的微生物可以大量生产酶。然而，酶可能昂贵且对温度和 pH 敏感，这限制了某些工业应用。

11. Summary of Key Points | 关键要点总结

Enzymes are biological catalysts that are specific, reusable, and affected by temperature, pH, and concentration. They lower activation energy and are essential for all metabolic reactions. Denaturation is permanent damage to an enzyme’s active site.

酶是生物催化剂，具有专一性、可重复使用性，并受温度、pH 和浓度影响。它们降低活化能，是所有代谢反应所必需的。变性是对酶活性位点的永久性损伤。

Active site – region where substrate binds | 活性位点 — 底物结合的区域
Enzyme-substrate complex – temporary combination | 酶-底物复合物 — 临时结合体
Denaturation – irreversible change in shape | 变性 — 形状的不可逆改变
Optimum – conditions where enzyme works fastest | 最适条件 — 酶活性最高的条件

Remember to always state the independent variable, dependent variable, and controlled variables when writing about enzyme experiments. Practice interpreting graphs of enzyme activity; they are frequently tested.

在书写酶相关实验时，务必说明自变量、因变量和控制变量。练习解读酶活性曲线图，这是高频考点。

12. Examination Tips | 考试技巧

When answering exam questions, use precise scientific terms. Say ‘denatured’ rather than ‘killed’ or ‘destroyed’. Use ‘active site’ rather than ‘special part’. Always refer to the shape change in the active site when explaining high temperature or extreme pH effects.

回答考试问题时，要使用精确的科学术语。说“变性”而非“杀死”或“破坏”。使用“活性位点”而非“特殊部位”。在解释高温或极端 pH 影响时，务必提及活性位点形状的变化。

For rate calculations, always show your working and include units. If a question asks why the rate stops increasing, explain that all active sites are occupied, not that the enzyme has ‘run out’. This distinction is important for full marks.

对于速率计算，务必展示计算过程并包含单位。如果题目问为什么速率不再增加，应解释所有活性位点都已占满，而不是“酶耗尽了”。这种区分对于满分很重要。

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