📚 Enzymes and Their Functions | 酶及其功能
Enzymes are biological catalysts that speed up chemical reactions in living organisms without being used up in the process. They are essential for life because they control metabolism, digestion, respiration, and many other processes.
酶是生物催化剂，能在不自身消耗的前提下加快生物体内的化学反应。它们对生命至关重要，因为酶控制着代谢、消化、呼吸以及许多其他过程。
1. What Are Enzymes? | 什么是酶？
Enzymes are proteins made up of long chains of amino acids folded into a specific three-dimensional shape. Each enzyme has an active site, a region where the substrate (the reacting molecule) binds and reacts.
酶是由氨基酸长链折叠成特定三维结构的蛋白质。每种酶都有一个活性位点，即底物（反应分子）结合并发生反应的区域。
All enzymes are proteins, but not all proteins are enzymes.
所有酶都是蛋白质，但并非所有蛋白质都是酶。
The unique shape of the active site determines which substrate can fit.
活性位点的独特形状决定了哪种底物可以契合。
2. The Lock-and-Key Model | 锁钥模型
The lock-and-key model compares the enzyme to a lock and the substrate to a key. Only the correctly shaped key (substrate) can fit into the lock (active site) and trigger a reaction.
锁钥模型将酶比作锁，底物比作钥匙。只有形状正确的钥匙（底物）才能插入锁（活性位点）并引发反应。
Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Product
酶 + 底物 → 酶-底物复合物 → 酶 + 产物
3. The Induced-Fit Model | 诱导契合模型
The induced-fit model is more accurate. It states that the active site changes shape slightly as the substrate binds, becoming more complementary. This stresses the bonds in the substrate, lowering the activation energy.
诱导契合模型更为准确。该模型指出，底物结合时活性位点会发生轻微形变，使其更具互补性。这会拉伸底物中的化学键，降低活化能。
4. Enzyme Specificity | 酶的专一性
Enzymes are highly specific because the active site fits only one substrate (or a few closely related ones). This is why catalase only breaks down hydrogen peroxide, while amylase only breaks down starch.
酶具有高度专一性，因为活性位点只适合一种底物（或少数几种类似的底物）。因此，过氧化氢酶只能分解过氧化氢，而淀粉酶只能分解淀粉。
5. Factors Affecting Enzyme Activity | 影响酶活性的因素
Several factors change how quickly enzymes catalyse reactions:
多种因素会改变酶催化反应的速度：
Temperature – rate increases with temperature up to an optimum, then decreases.
温度 – 反应速率随温度升高而加快，达到最适温度后下降。
pH – each enzyme works best at a specific pH optimum.
pH – 每种酶都有一个特定的最适pH值。
Substrate concentration – more substrate increases rate until the enzyme is saturated.
底物浓度 – 增加底物可提高速率，直到酶饱和。
Enzyme concentration – more enzyme increases rate if substrate is plentiful.
酶浓度 – 若底物充足，增加酶可提高速率。
6. Effect of Temperature | 温度的影响
As temperature rises, molecules move faster and collide more often, so the rate of reaction increases. At the optimum temperature (usually about 37 °C in human enzymes), activity is maximum.
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。在最适温度下（人体酶通常约为 37 °C），酶活性最高。
Above the optimum, heat begins to break the bonds holding the enzyme structure together. The active site changes shape, and the enzyme is denatured. It can no longer bind its substrate.
超过最适温度后，热量开始破坏维持酶结构的化学键。活性位点发生改变，酶即变性，无法再与底物结合。
7. Effect of pH | pH 的影响
Each enzyme has an optimum pH. For example, pepsin in the stomach works best at pH 2, while salivary amylase works best at pH 7.
每种酶都有自己的最适pH。例如，胃中的胃蛋白酶在 pH 2 时活性最高，而唾液淀粉酶则在 pH 7 时活性最高。
Very high or very low pH can disrupt ionic bonds and hydrogen bonds in the enzyme, leading to denaturation. This changes the shape of the active site permanently.
过高或过低的pH会破坏酶中的离子键和氢键，导致酶变性。这会永久改变活性位点的形状。
8. Enzyme Inhibitors | 酶抑制剂
Inhibitors are molecules that reduce or stop enzyme activity. They can be competitive (binding to the active site) or non-competitive (binding elsewhere, changing the enzyme shape).
抑制剂是能降低或停止酶活性的分子。它们可以是竞争性的（结合在活性位点），也可以是非竞争性的（结合在其他部位，改变酶的形状）。
Competitive inhibitor: resembles the substrate and blocks the active site.
竞争性抑制剂：与底物相似，占用活性位点。
Non-competitive inhibitor: binds away from the active site and alters the enzyme's shape.
非竞争性抑制剂：结合在活性位点之外，改变酶的整体形状。
9. Denaturation vs. Irreversible Inhibition | 变性 vs. 不可逆抑制
Denaturation is physical damage to the enzyme structure caused by heat or extreme pH. It permanently destroys the active site. Irreversible inhibitors also permanently stop enzyme activity by forming strong covalent bonds with the enzyme.
变性是由高温或极端pH对酶结构造成的物理损伤，会永久破坏活性位点。不可逆抑制剂则通过与酶形成牢固的共价键来永久停止酶活性。
In both cases the reaction cannot go forward, even if more substrate is added.
在这两种情况下，即使添加更多底物，反应也无法进行。
10. Digestive Enzymes | 消化酶
The body uses enzymes to break down food:
人体利用酶来分解食物：
| 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 + 甘油 |
11. Enzymes in Industry and Medicine | 酶在工业和医学中的应用
Enzymes are used widely in everyday products. Biological washing powders contain proteases and lipases to break down protein and fat stains at lower temperatures.
酶被广泛用于日常产品。加酶洗衣粉含有蛋白酶和脂肪酶，可在较低温度下分解蛋白质和脂肪污渍。
In medicine, enzymes such as glucose oxidase are used in blood glucose test strips. Enzymes are also used in food production to make syrups and clarify fruit juices.
在医学中，葡萄糖氧化酶等酶用于血糖试纸。酶还用于食品工业中生产糖浆和澄清果汁。
12. Key Points for Exams | 考试要点
For IGCSE Edexcel Biology, you must be able to:
针对 Edexcel IGCSE 生物，你必须能够：
Define enzymes as biological catalysts.
定义酶为生物催化剂。
Explain the lock-and-key and induced-fit models.
解释锁钥模型和诱导契合模型。
Describe how temperature, pH, substrate and enzyme concentration affect rate.
描述温度、pH、底物浓度和酶浓度如何影响反应速率。
Explain denaturation and the importance of optimum conditions.
解释变性及最适条件的重要性。
Give examples of digestive enzymes and their functions.
举例说明消化酶及其功能。
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