Enzymes and Their Action | 酶及其作用

📚 Enzymes and Their Action | 酶及其作用

Enzymes are biological catalysts that speed up chemical reactions in living organisms. They are essential for metabolism, enabling reactions such as digestion, respiration, and photosynthesis to occur rapidly at body temperature.

酶是生物催化剂，可加快生物体内化学反应的速度。它们对代谢至关重要，使消化、呼吸和光合作用等反应能够在体温下快速进行。

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 unique region that binds to a specific substrate molecule.

酶是由氨基酸长链折叠成特定三维形状的蛋白质。每种酶都有一个活性位点，这是一个能够与特定底物分子结合的特殊区域。

Enzymes are specific, meaning each enzyme catalyses only one type of reaction or acts on one particular substrate.

酶具有专一性，即每种酶只能催化一种类型的反应或作用于一种特定的底物。

However, some enzymes, such as amylase, act on several similar substrates. In IGCSE biology, we focus on the idea of one enzyme, one substrate.

然而，有些酶（如淀粉酶）可以作用于几种相似的底物。在IGCSE生物课程中，我们重点关注“一种酶对应一种底物”的概念。

2. The Lock and Key Model | 锁钥模型

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

锁钥模型解释了酶的专一性：底物像钥匙一样插入活性位点，就像钥匙插入锁中一样。

When the substrate binds, an enzyme-substrate complex is formed. The reaction then occurs, and the products leave the active site.

当底物结合时，形成酶-底物复合物。随后反应发生，产物离开活性位点。

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

酶 + 底物 → 酶-底物复合物 → 酶 + 产物

The enzyme remains unchanged and can be reused. This is why enzymes are effective in very small amounts.

酶在反应后保持不变，可以重复使用。因此，极少量酶也能发挥高效作用。

3. Induced Fit Model (Advanced) | 诱导契合模型（进阶）

Some scientists prefer the induced fit model. In this model, the active site changes shape slightly to fit the substrate more tightly once binding occurs.

一些科学家更倾向于诱导契合模型。在该模型中，当底物结合时，活性位点的形状会发生轻微改变，从而更紧密地匹配底物。

This helps to explain why enzymes can catalyse reactions on molecules that are not perfect fits at first contact.

这有助于解释为什么酶能够催化那些初接触时并非完全匹配的分子发生反应。

For IGCSE, the lock and key model is sufficient, but understanding induced fit helps you answer extension questions.

对于IGCSE考试，锁钥模型已经足够，但理解诱导契合有助于回答拓展性问题。

4. Factors Affecting Enzyme Activity: Temperature | 影响酶活性的因素：温度

Temperature increases the kinetic energy of molecules, causing more frequent collisions between enzyme and substrate. As temperature rises, the rate of reaction increases up to a point.

温度升高会增加分子的动能，使酶与底物之间的碰撞更加频繁。随着温度上升，反应速率会逐渐增加，直到达到某一点。

The optimum temperature for most human enzymes is around 37°C. At this temperature, the enzyme works fastest.

大多数人酶的最适温度约为37°C。在该温度下，酶的活性最高，反应速率最快。

Above the optimum temperature, the enzyme denatures. The active site changes shape, so the substrate can no longer bind.

超过最适温度后，酶会发生变性。活性位点形状改变，底物无法再与之结合。

Low temperature: reaction rate decreases because molecules move slowly.

低温：分子运动缓慢，反应速率降低。
Optimum temperature: maximum rate of reaction.

最适温度：反应速率达到最大值。
High temperature: enzyme denatures irreversibly.

高温：酶发生不可逆变性。

5. Factors Affecting Enzyme Activity: pH | 影响酶活性的因素：酸碱度（pH）

Each enzyme has an optimum pH. Most human enzymes work best at pH 7, which is neutral.

每种酶都有其最适pH。大多数人酶在中性pH 7时活性最高。

However, pepsin in the stomach has an optimum pH of 2, whereas trypsin in the small intestine works best at pH 8.

然而，胃中的胃蛋白酶最适pH为2，而小肠中的胰蛋白酶在pH 8时活性最强。

Extreme pH values disrupt the ionic bonds and hydrogen bonds that hold the enzyme in its specific shape, causing denaturation.

极端pH值会破坏维持酶特定形状的离子键和氢键，导致酶变性。

Optimum pH varies for different enzymes | 不同酶具有不同的最适pH

6. Factors Affecting Enzyme Activity: Substrate Concentration | 影响酶活性的因素：底物浓度

At a fixed enzyme concentration, increasing substrate concentration increases the rate of reaction, provided there are free active sites available.

在酶浓度恒定时，只要还有空余的活性位点，增加底物浓度就会提高反应速率。

When all active sites are occupied, the enzyme is working at its maximum rate. Adding more substrate has no further effect.

当所有活性位点都被占据时，酶已达到最大反应速率。此时再增加底物也不会提高速率。

Rate rises steeply at first → then plateaus.

速率先迅速上升 → 然后达到平台期。
Plateau is called Vmax (maximum velocity).

平台期称为Vmax（最大速率）。

7. Factors Affecting Enzyme Activity: Enzyme Concentration | 影响酶活性的因素：酶浓度

With excess substrate, increasing enzyme concentration increases the rate of reaction linearly. More enzyme means more active sites for substrate to bind to.

在底物过量时，增加酶浓度会使反应速率呈线性上升。酶越多，可供底物结合的活性位点就越多。

In theory, the rate continues to rise as long as substrate is in excess. In practice, other factors may become limiting.

理论上，只要底物过量，速率就会持续上升。但实际中，其他因素可能成为限制因素。

This relationship is important in understanding metabolic control in cells.

理解这一关系对掌握细胞内的代谢调控非常重要。

8. Enzyme Denaturation | 酶的变性

Denaturation is a permanent change in the three-dimensional structure of an enzyme. It can be caused by high temperature or extreme pH.

变性是酶三维结构的永久性改变，可由高温或极端pH引起。

During denaturation, the bonds within the protein break, and the active site loses its shape. The substrate can no longer fit.

变性过程中，蛋白质内部的键断裂，活性位点失去原有形状，底物无法再与之结合。

Denatured enzymes cannot catalyse reactions. This is a key concept in many exam questions about cooking, infection control, and industrial processes.

变性的酶无法催化反应。这是许多关于烹饪、感染控制和工业过程的考题中的关键概念。

Denaturation is irreversible | 变性是不可逆的

9. Enzymes in Digestion | 消化中的酶

Digestive enzymes are secreted by glands and break down large insoluble food molecules into small soluble ones that can be absorbed.

消化酶由腺体分泌，将不溶的大分子食物分解为可吸收的可溶性小分子。

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	Lipids 脂肪	Fatty acids + glycerol 脂肪酸 + 甘油

Amylase is produced in the salivary glands and pancreas. Protease is made in the stomach, pancreas, and small intestine. Lipase is mainly produced by the pancreas.

淀粉酶由唾液腺和胰腺分泌。蛋白酶在胃、胰腺和小肠中产生。脂肪酶主要由胰腺分泌。

10. Enzymes in Biotechnology | 酶在生物技术中的应用

Enzymes are widely used in industry and medicine. For example, biological washing powders contain proteases and lipases to break down stains.

酶广泛应用于工业和医学领域。例如，加酶洗衣粉含有蛋白酶和脂肪酶，用于分解污渍。

Lactase is used to break down lactose in milk, producing lactose-free products for people who are lactose intolerant.

乳糖酶用于分解牛奶中的乳糖，为乳糖不耐受人群生产无乳糖产品。

In brewing, enzymes convert starch into sugars for fermentation. In medicine, enzymes are used to diagnose diseases and in some treatments.

在酿造中，酶将淀粉转化为糖，用于发酵。在医学中，酶可用于疾病诊断和治疗。

11. Enzyme Experiments (Core Practical) | 酶实验（核心实践）

In IGCSE biology, you may need to investigate the effect of pH, temperature, or substrate concentration on enzyme activity. A common method uses amylase and starch.

在IGCSE生物考试中，你可能需要探究pH、温度或底物浓度对酶活性的影响。常见方法使用淀粉酶和淀粉。

Steps include adding iodine solution to test for starch. If starch remains, the iodine turns blue-black. If all starch has been digested, the iodine stays orange-brown.

步骤包括加入碘液检测淀粉。如果淀粉存在，碘液变蓝黑色；如果淀粉已被完全消化，碘液保持棕黄色。

Use equal volumes of enzyme and substrate for fair testing.

使用等体积的酶和底物以保证公平测试。
Buffer solutions control pH accurately.

使用缓冲溶液精确控制pH。
Temperature is controlled using a water bath.

使用水浴控制温度。

Measure the time taken for the iodine to stop changing colour, or the rate of colour disappearance, to compare enzyme activity.

测量碘液颜色不再改变所需的时间，或颜色消失的速率，以比较酶活性。

12. Common Exam Mistakes and Tips | 常见考试错误与提示

Many students confuse denaturation with ‘being killed’ or ‘dying’. Enzymes are not alive; they are proteins.

许多学生将变性误解为“被杀死”或“死亡”。酶没有生命，它们是蛋白质。

Remember that temperature and pH affect the active site shape, while substrate concentration only affects the frequency of collisions.

记住：温度和pH影响活性位点的形状，而底物浓度只影响碰撞频率。

Always state ‘active site changes shape’ when explaining denaturation. Avoid saying ‘enzyme is destroyed’ unless the question mentions breakdown of the protein.

解释变性时，务必说明“活性位点形状改变”。除非题目提到蛋白质分解，否则不要说“酶被破坏”。

Use the word ‘specific’ when describing enzyme-substrate reactions. Mention the lock and key model to earn full marks on mechanism questions.

描述酶-底物反应时使用“专一性”。在机制题中提及锁钥模型可以获得满分。

For graph questions, describe the trend clearly and refer to the saturation effect at high substrate concentrations.

对于图表题，要清晰描述趋势，并提及高底物浓度下的饱和效应。

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Enzymes and Their Action | 酶及其作用