Enzymes | 酶

📚 Enzymes | 酶

Enzymes are essential biological molecules that regulate nearly every chemical reaction within living organisms. Understanding how they work, what affects their activity, and how they are applied is a core part of the Edexcel IGCSE Biology syllabus.

酶是调节生物体内几乎所有化学反应的关键生物分子。理解其作用机制、影响活性的因素以及实际应用，是 Edexcel IGCSE 生物考纲的核心内容。

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1. What Are Enzymes? | 什么是酶？

Enzymes are proteins that act as biological catalysts. They speed up chemical reactions by lowering the activation energy required, without being permanently changed in the process.

酶是作为生物催化剂的蛋白质。它们通过降低反应所需的活化能来加速化学反应，而自身在过程中不发生永久性改变。

• Enzymes are specific – each enzyme catalyses only one type of reaction.
• 酶具有专一性——每种酶只催化一种类型的反应。

• Enzyme molecules have a unique 3D shape due to their amino acid sequence.
• 酶分子因其氨基酸序列而具有独特的三维形状。

Enzymes do not provide energy; they simply accelerate reactions that would otherwise occur very slowly at body temperature.

酶不提供能量；它们只是加速那些在体温下本来会非常缓慢发生的反应。

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2. Enzyme Action – Lock and Key Model | 酶的作用——锁钥模型

The active site of an enzyme is a specific region with a complementary shape to the substrate. This is described by the lock-and-key model.

酶的活性位点是一个具有与底物互补形状的特异区域，这可以用锁钥模型来描述。

In this model, the enzyme is the ‘lock’ and the substrate is the ‘key’. Only the correct key fits the lock, allowing the reaction to occur.

在该模型中，酶是“锁”，底物是“钥匙”。只有正确的钥匙才能插入锁中，从而引发反应。

Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Product

酶 + 底物 → 酶-底物复合物 → 酶 + 产物

After the reaction, the product is released, and the enzyme is free to catalyse another reaction.

反应结束后，产物被释放，酶可以继续催化下一次反应。

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3. Temperature and Enzyme Activity | 温度与酶活性

Temperature affects the kinetic energy of molecules. As temperature increases, molecules move faster, increasing the chance of enzyme-substrate collisions.

温度影响分子的动能。随着温度升高，分子运动加快，酶与底物碰撞的机会增多。

However, above the optimum temperature, the enzyme’s bonds begin to break. This causes the active site to change shape – the enzyme is denatured.

然而，超过最适温度后，酶的化学键开始断裂，导致活性位点变形——酶发生变性。

Temperature (°C)	Effect on Enzyme Activity
Low (e.g. 10 °C)	Low activity, molecule movement slow
Optimum (e.g. 37 °C)	Maximum reaction rate
High (e.g. 60 °C)	Denaturation, active site changes shape

In humans, most enzymes have an optimum temperature of about 37 °C.

在人体内，大多数酶的最适温度约为 37 °C。

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4. pH and Enzyme Activity | pH 与酶活性

Each enzyme has an optimum pH at which its activity is highest. Changes in pH affect the ionic bonds and hydrogen bonds that maintain the enzyme’s 3D structure.

每种酶都有一个最适 pH，在此 pH 下活性最高。pH 的变化会影响维持酶三维结构的离子键和氢键。

Extreme pH values can cause denaturation, just like high temperature.

极端的 pH 值如同高温一样，也可能导致变性。

• Pepsin (in stomach): optimum pH 2 – works in acidic conditions.
• 胃蛋白酶（胃内）：最适 pH 2 ——在酸性条件下工作。

• Amylase (in mouth/small intestine): optimum pH 7–8.
• 淀粉酶（口腔/小肠）：最适 pH 7–8。

Enzyme activity → pH curve: a bell-shaped relationship.

酶活性 → pH 曲线：呈钟形关系。

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5. Enzyme Concentration | 酶浓度的影响

Increasing enzyme concentration increases the rate of reaction, because more active sites are available for substrate molecules.

增加酶浓度会加快反应速率，因为有更多的活性位点可供底物分子结合。

Eventually, the rate plateaus when the substrate is entirely saturated – all active sites are occupied.

当底物完全饱和时——所有活性位点均被占据——反应速率达到平台期。

Rate of reaction ∝ Enzyme concentration (up to saturation limit)

反应速率 ∝ 酶浓度（直至饱和上限）

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6. Substrate Concentration | 底物浓度的影响

At low substrate concentrations, doubling the substrate almost doubles the rate, as more collisions occur with active sites.

在低底物浓度下，底物加倍时速率几乎加倍，因为与活性位点的碰撞增多。

At high substrate concentrations, the enzyme molecules are saturated, and adding more substrate has no further effect on the rate.

在高底物浓度下，酶分子已经饱和，此时再增加底物也不会提高速率。

This is why the rate levels off in a Michaelis-Menten style curve.

这就是反应速率在米氏曲线中逐渐趋于平坦的原因。

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7. Inhibitors – Competitive and Non-Competitive | 抑制剂——竞争性与非竞争性

Inhibitors reduce enzyme activity. A competitive inhibitor has a shape similar to the substrate and competes for the active site.

抑制剂会降低酶的活性。竞争性抑制剂具有与底物相似的形状，并与底物竞争活性位点。

• Effect of competitive inhibitor can be overcome by increasing substrate concentration.
• 增加底物浓度可以克服竞争性抑制剂的作用。

A non-competitive inhibitor binds elsewhere on the enzyme, changing the shape of the active site irreversibly.

非竞争性抑制剂与酶的其他部位结合，从而使活性位点的形状发生不可逆改变。

• Increasing substrate concentration does not reverse non-competitive inhibition.
• 增加底物浓度不能逆转非竞争性抑制作用。

Enzyme inhibition is a key control mechanism in metabolic pathways.

酶抑制是代谢途径中的关键调控机制。

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8. Industrial and Medical Uses of Enzymes | 酶在工业和医药中的应用

Enzymes are widely used in biotechnology because they are specific, efficient, and environmentally friendly.

酶在生物技术中被广泛应用，因为它们专一、高效且环保。

Use	Enzyme	Application
Biological detergents	Protease, lipase	Remove protein and fat stains
Food production	Glucose isomerase	Convert glucose to fructose syrups
Medical diagnostics	Glucose oxidase	Measure blood glucose levels

Immobilised enzymes in industry can be reused and remain stable at high temperatures.

工业中的固定化酶可以重复使用，并在高温下保持稳定。

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9. Denaturation vs. Lowered Activity | 变性 vs. 活性降低

Denaturation is permanent – the protein structure unravels and the enzyme loses function completely. It typically occurs at high temperatures or extreme pH.

变性是不可逆的——蛋白质结构解开，酶完全失去功能。其通常发生在高温或极端 pH 条件下。

Lowered activity is temporary, such as at low temperatures where collisions are fewer, but the enzyme structure remains intact and activity recovers when conditions return to optimum.

活性降低是暂时的，例如在低温下碰撞减少，但酶结构保持完整；当条件恢复正常时，活性会恢复。

Cold → reversible slowdown; Heat → irreversible denaturation.

低温 → 可逆减速；高温 → 不可逆变性。

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10. Key Equations and Summary | 关键方程与总结

The basic enzyme reaction can be summarised as:

基本酶促反应可总结为：

S + E ⇌ ES → E + P

底物 + 酶 ⇌ 酶-底物复合物 → 酶 + 产物

Remember: Enzymes are reusable, specific, and sensitive to temperature and pH. They are vital in both living organisms and industrial biotechnology.

请记住：酶可重复使用、具有专一性，并对温度和 pH 敏感。酶在生物体内和工业生物技术中都至关重要。

When revising, always draw and interpret graphs for temperature, pH, enzyme concentration, and substrate concentration – they are frequently examined in IGCSE.

复习时，务必绘制并解读温度、pH、酶浓度和底物浓度相关图形——这些都是 IGCSE 考试的高频考点。

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