📚 Enzymes and Their Properties | 酶及其特性
Enzymes are biological catalysts that speed up chemical reactions in living organisms without being used up themselves. They are essential for processes such as digestion, respiration, and photosynthesis.
酶是生物催化剂，能在活生物体内加速化学反应，而自身不会被消耗。它们对消化、呼吸和光合作用等过程至关重要。
1. What Are Enzymes? | 什么是酶？
Enzymes are proteins made of long chains of amino acids folded into a specific three-dimensional shape. This shape includes an active site where substrate molecules bind.
酶是由氨基酸长链折叠成特定三维形状的蛋白质。该形状包含一个活性位点，底物分子在此结合。
- Active site: A region complementary to the substrate.
- 催化特性: Enzymes remain unchanged after the reaction.
The unique shape of each enzyme means it can only catalyse one type of reaction. This is called enzyme specificity.
每种酶独特的形状意味着它只能催化一种类型的反应，这称为酶的专一性。
2. The Lock and Key Model | 锁钥模型
The lock and key model describes how an enzyme and substrate interact. The substrate fits into the active site like a key into a lock.
锁钥模型描述了酶与底物的相互作用方式。底物像钥匙插入锁一样进入活性位点。
Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Product
Weak bonds hold the substrate in place while the reaction occurs. After the reaction, products leave the active site, and the enzyme can be reused.
弱键在反应发生时将底物固定。反应结束后，产物离开活性位点，酶可重复使用。
3. Induced Fit Model | 诱导契合模型
Modern evidence supports the induced fit model. The active site is not completely rigid; it changes shape slightly to fit the substrate more tightly.
现代证据支持诱导契合模型。活性位点并非完全刚性，它会轻微改变形状以更紧密地贴合底物。
This conformational change stresses the substrate bonds, lowering the activation energy needed for the reaction.
这种构象变化会拉伸底物的化学键，降低反应所需的活化能。
4. Factors Affecting Enzyme Activity – Temperature | 影响酶活性的因素 – 温度
As temperature increases, molecules move faster and collide more often, so enzyme activity rises up to an optimum temperature.
随着温度升高，分子运动加快，碰撞频率增加，酶活性升至最适温度。
- Optimum temperature for human enzymes: approximately 37 °C.
- 人体酶最适温度: 约 37 °C。
Above the optimum, heat breaks the hydrogen bonds holding the enzyme’s structure together. The active site changes shape, so the substrate no longer fits. The enzyme is denatured.
超过最适温度后，热能破坏维持酶结构的氢键。活性位点形状改变，底物无法再结合，酶发生变性。
5. Factors Affecting Enzyme Activity – pH | 影响酶活性的因素 – pH
Each enzyme has an optimum pH. Changes in pH alter the charge and shape of the active site.
每种酶都有最适 pH。pH 变化会改变活性位点的电荷和形状。
| Enzyme | Optimum pH | Location |
|---|---|---|
| Pepsin | 2 | Stomach |
| Salivary amylase | 7 | Mouth |
| Pancreatic enzymes | 8–9 | Small intestine |
Extreme pH causes denaturation. The enzyme loses its biological activity permanently.
极端 pH 会导致酶变性，酶永久失去生物学活性。
6. Factors Affecting Enzyme Activity – Substrate Concentration | 影响酶活性的因素 – 底物浓度
Increasing substrate concentration increases the rate of reaction, because more substrate molecules collide with the active site per second.
增加底物浓度会提高反应速率，因为每秒有更多底物分子与活性位点碰撞。
However, at a certain point, all active sites are occupied. Further increases in substrate concentration have no effect on the rate.
然而，当所有活性位点都被占据后，继续增加底物浓度不会影响反应速率。
7. Enzyme Concentration | 酶浓度
More enzyme molecules mean more active sites available, so the rate of reaction increases as enzyme concentration rises, assuming excess substrate.
在底物足量的前提下，酶分子越多，可用的活性位点越多，反应速率随酶浓度升高而加快。
Eventually, the substrate becomes limiting, and the rate stops increasing.
最终底物成为限制因素，反应速率不再增加。
8. Denaturation | 变性
Denaturation is a permanent change in the three-dimensional structure of an enzyme, caused by high temperature or extreme pH.
变性是由高温或极端 pH 引起的酶三维结构永久性改变。
- The peptide bonds stay intact.
- The active site is destroyed.
- The enzyme cannot catalyse the reaction.
肽键仍然完整，但活性位点被破坏，酶无法催化反应。
9. Catalase – A Common Enzyme | 过氧化氢酶 – 一种常见酶
Catalase is found in many tissues. It breaks down hydrogen peroxide (H₂O₂) into water and oxygen.
过氧化氢酶存在于许多组织中，能将过氧化氢（H₂O₂）分解为水和氧气。
2H₂O₂ → 2H₂O + O₂
This reaction is important for protecting cells from oxidative damage.
该反应对保护细胞免受氧化损伤非常重要。
10. Enzymes in Digestion | 消化中的酶
Digestive enzymes break down large insoluble food molecules into small soluble molecules that can be absorbed.
消化酶将大的不溶性食物分子分解为可吸收的小的可溶性分子。
| Enzyme | Substrate | Product(s) |
|---|---|---|
| Amylase | Starch | Maltose |
| Protease | Protein | Amino acids |
| Lipase | Lipids | Fatty acids + glycerol |
Amylase is produced in the salivary glands and pancreas. Protease and lipase are mainly produced in the pancreas.
淀粉酶由唾液腺和胰腺产生。蛋白酶和脂肪酶主要由胰腺产生。
11. Enzymes in Industry | 工业中的酶
Enzymes are widely used in industry because they are specific, work at low temperatures, and reduce energy costs.
酶在工业中广泛应用，因为它们具有专一性、在较低温度下工作且降低能源成本。
- Biological washing powders: contain proteases and lipases to remove stains.
- 加酶洗衣粉: 含有蛋白酶和脂肪酶以去除污渍。
- Food industry: enzymes are used to make cheese, bread, and fruit juice.
- 食品工业: 酶用于制作奶酪、面包和果汁。
Some enzymes are immobilised to allow easier recovery and reuse.
有些酶会被固定化，以便更容易回收和重复使用。
12. Core Practical – Investigating Enzyme Activity | 核心实验 – 探究酶活性
The Edexcel IGCSE core practical often involves measuring the rate of catalase activity using potato discs or liver tissue.
爱德思 IGCSE 核心实验通常使用马铃薯薄片或肝脏组织测量过氧化氢酶活性。
Oxygen gas is collected in a measuring cylinder, and the volume is recorded over time under different conditions.
用排水集气法收集氧气，并记录不同条件下单位时间内的气体体积。
Rate = Volume of gas ÷ Time (cm³/s)
Variables such as temperature, pH, and enzyme concentration can be investigated. Control experiments must be set up carefully.
可探究温度、pH 和酶浓度等变量，同时需要仔细设置对照实验。
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