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

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

Enzymes are biological catalysts that speed up chemical reactions in living organisms without being used up in the process. They are protein molecules essential for survival, controlling everything from the digestion of food in the gut to the replication of DNA inside our cells. In the IGCSE Edexcel Science syllabus, enzymes form a core topic that connects biology, chemistry and practical investigation skills.

酶是生物催化剂，能够在不被消耗的情况下加速生物体内的化学反応。它们是维持生命所必需的蛋白质分子，控制着从肠道中食物的消化到细胞内DNA复制的一切生命过程。在Edexcel IGCSE科学课程中，酶是连接生物学、化学和实验技能的核心主题。

1. The Structure and Nature of Enzymes | 酶的结构与性质

All enzymes are proteins made from long chains of amino acids folded into a precise three-dimensional shape. Each enzyme has a unique shape that allows it to recognise and bind to one specific reactant molecule, called the substrate. The region of the enzyme where the substrate binds is known as the active site, and its shape is complementary to the substrate.

所有酶都是由氨基酸长链折叠而成的蛋白质，具有精确的三维形状。每种酶都有独特的形状，使其能够识别并结合一种特定的反应物分子，即底物。酶上底物结合的区域称为活性位点，其形状与底物互补。

The key distinguishing properties of enzymes are:

酶的关键特性包括：

• Specificity: each enzyme catalyses only one reaction because only its correct substrate fits the active site. / 专一性：由于只有正确的底物才能嵌合活性位点，每种酶仅催化一种反应。
• Reusability: enzymes are not consumed during a reaction and can be used repeatedly. / 可重复使用性：酶在反应中不被消耗，可反复使用。
• Susceptibility: enzyme activity is strongly affected by temperature, pH and inhibitor concentration. / 敏感性：酶活性受温度、pH和抑制剂浓度的显著影响。
• Denaturability: extreme heat or extreme pH permanently destroys enzyme shape and function. / 可变性：极端温度或pH会永久破坏酶的形状和功能。

2. How Enzymes Work – Lowering Activation Energy | 酶如何工作——降低活化能

In any chemical reaction, reactant particles must absorb a minimum amount of energy, called the activation energy, before bonds can break and new bonds form. In biological systems, this energy barrier is often too high for reactions to occur at normal body temperature. Enzymes overcome this problem by providing an alternative reaction pathway with a much lower activation energy.

在任何化学反应中，反应物粒子必须吸收被称为活化能的最小能量，才能断裂旧键并形成新键。在生物系统中，这一能量屏障通常过高，使得反应无法在正常体温下进行。酶通过提供一条活化能低得多的替代反应途径来解决这个问题。

The action of an enzyme can be expressed simply as:

酶的作用可简单表示为：

Substrate + Enzyme → Enzyme-Substrate Complex → Product + Enzyme
底物 + 酶 → 酶-底物复合物 → 产物 + 酶

The enzyme holds the substrate molecules in the correct orientation, which lowers the activation energy and greatly increases the rate of reaction. This process is vital for reactions such as the breakdown of hydrogen peroxide (H₂O₂) into water and oxygen, catalysed by catalase.

酶使底物分子保持正确取向，从而降低活化能并显著加快反应速率。这一过程对许多反应至关重要，例如过氧化氢酶催化的过氧化氢（H₂O₂）分解为水和氧气的反应。

3. The Lock and Key and Induced Fit Models | 锁钥模型与诱导契合模型

Two models are used to explain how enzymes and substrates interact. The lock and key model, proposed by Emil Fischer, suggests that the active site has a rigid shape that exactly matches the substrate, like a key fitting a lock. Once the substrate binds, the reaction occurs and the product is released.

有两种模型用来解释酶与底物的相互作用。Emil Fischer提出的锁钥模型认为，活性位点具有固定形状，恰好与底物匹配，如同钥匙配锁。底物结合后，反应随即发生，产物被释放。

The induced fit model, which is more widely accepted today, proposes that the active site is flexible. When the substrate binds, the enzyme changes shape slightly to wrap around the substrate, creating a tighter fit and making the reaction more efficient.

如今更广为接受的诱导契合模型则认为活性位点是柔性的。当底物结合时，酶的构象会轻微改变以包裹底物，形成更紧密的结合，从而提高反应效率。

For IGCSE, you need to be able to describe both models and explain that the induced fit model better accounts for the high catalytic power and fine regulation of enzymes.

在IGCSE考试中，你需要能够描述这两种模型并解释为什么诱导契合模型更能体现酶的高催化效率和精细调控能力。

4. Effect of Temperature on Enzyme Activity | 温度对酶活性的影响

Temperature affects enzyme activity through two opposing factors. As temperature rises,

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