📚 Enzymes: Structure, Function and Factors | 酶：结构、功能与影响因素
Enzymes are biological catalysts that speed up chemical reactions in living organisms without being used up. Understanding their structure, mode of action, and the factors that affect them is essential for IGCSE Biology.
酶是生物催化剂，能加快生物体内的化学反应，但自身不会被消耗。理解酶的结构、作用方式以及影响其活性的因素，是IGCSE生物学的核心内容。
1. What Are Enzymes? | 什么是酶？
Enzymes are proteins made of chains of amino acids. Each enzyme has a unique three-dimensional shape, which is critical to its function. They catalyse specific reactions by lowering the activation energy required for the reaction to proceed.
酶是由氨基酸链构成的蛋白质。每种酶都有独特的三维结构，这对功能至关重要。它们通过降低反应所需活化能来催化特定反应。
- Enzymes remain unchanged at the end of a reaction and can be reused.
- 酶在反应结束后保持不变，可以重复使用。
- Enzymes are specific to their substrate due to the active site shape.
- 由于活性位点形状，酶对其底物具有专一性。
2. Lock and Key Model | 锁钥模型
The lock and key model explains enzyme specificity. The enzyme is the lock, and the substrate is the key. Only the correctly shaped substrate can fit into the enzyme’s active site, forming an enzyme-substrate complex.
锁钥模型解释了酶的专一性。酶是锁，底物是钥匙。只有形状匹配的底物才能进入酶活性位点，形成酶-底物复合物。
Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Product
酶 + 底物 → 酶-底物复合物 → 酶 + 产物
3. Induced Fit Model | 诱导契合模型
In the induced fit model, the active site is not exactly complementary to the substrate initially. When the substrate binds, the enzyme changes shape slightly to achieve a better fit, stressing bonds in the substrate and facilitating the reaction.
在诱导契合模型中，活性位点起初并非与底物完全互补。当底物结合时，酶形状发生微小变化，以更好地契合，使底物化学键受力并促进反应。
This model explains why enzymes can also catalyse reactions on large molecules and how inhibitors can distort the active site.
该模型解释了酶为何也能催化大分子反应，以及抑制剂如何扭曲活性位点。
4. Factors Affecting Enzyme Activity | 影响酶活性的因素
Enzyme activity is influenced by temperature, pH, substrate concentration, enzyme concentration, and inhibitors. In IGCSE Biology, you should be able to describe and explain graphs of these relationships.
酶活性受温度、pH、底物浓度、酶浓度和抑制剂影响。在IGCSE生物中，你需要能够描述并解释这些关系的曲线图。
- Temperature alters kinetic energy and can denature enzymes.
- 温度改变动能，并可使酶变性。
- pH affects the charges and shape of the active site.
- pH影响活性位点的电荷和形状。
- Substrate concentration increases rate until saturation.
- 底物浓度增加速率直至饱和。
5. Effect of Temperature | 温度的影响
As temperature increases from low values, the rate of reaction increases because molecules move faster and collide more frequently. The optimum temperature is usually around 37°C for human enzymes. Above the optimum, bonds break, the active site loses its shape, and the enzyme is denatured.
随着温度从低温升高，反应速率加快，因为分子运动更快、碰撞更频繁。人体酶的最适温度通常约为37°C。超过最适温度后，化学键断裂，活性位点改变形状，酶变性。
Rate ↑ with temperature until optimum, then ↓ sharply due to denaturation.
速率随温度升高至最适，然后因变性急剧下降。
6. Effect of pH | pH的影响
Each enzyme has an optimum pH. Pepsin works best in the stomach at pH 2, while most cytoplasmic enzymes have an optimum around pH 7. Changes in pH alter the ionisation of amino acid side groups, disrupting the hydrogen bonds and ionic interactions that maintain the active site’s shape.
每种酶都有最适pH。胃蛋白酶在胃中pH 2时活性最高，而大多数细胞质酶的最适pH在7左右。pH变化会改变氨基酸侧链基团的离子化状态，破坏维持活性位点形状的氢键和离子键。
| Enzyme 酶 | Optimum pH 最适pH |
| Pepsin 胃蛋白酶 | 2 |
| Amylase 唾液淀粉酶 | 6.8–7 |
| Trypsin 胰蛋白酶 | 8–9 |
7. Effect of Substrate Concentration | 底物浓度的影响
At a fixed enzyme concentration, increasing substrate concentration initially increases the rate of reaction. However, once all active sites are occupied, the enzyme is saturated, and adding more substrate has no further effect on the rate.
在酶浓度固定时，增加底物浓度最初会提高反应速率。然而，当所有活性位点都被占用时，酶达到饱和，继续添加底物不再影响速率。
Vmax is reached when all active sites are busy.
当所有活性位点被占用时，达到最大速率Vmax。
8. Effect of Enzyme Concentration and Inhibitors | 酶浓度与抑制剂的影响
With excess substrate, increasing enzyme concentration increases the rate proportionally, because more active sites are available. Inhibitors reduce the rate. Competitive inhibitors block the active site, while non-competitive inhibitors bind elsewhere and change the enzyme’s shape.
底物过量时，增加酶浓度会成比例提高反应速率，因为可用活性位点增加。抑制剂降低速率。竞争性抑制剂占据活性位点，非竞争性抑制剂结合在其他位置并改变酶的形状。
9. Denaturation | 变性
Denaturation is a permanent change in the enzyme’s tertiary structure. High temperatures and extreme pH disrupt hydrogen bonds and ionic bonds, causing the active site to lose its specific shape. The enzyme cannot bind to its substrate and becomes inactive.
变性是酶三级结构的永久性改变。高温和极端pH破坏氢键和离子键，导致活性位点失去特定形状。酶无法结合底物而失活。
Denaturation is irreversible. A denatured enzyme cannot function, even if normal conditions are restored.
变性不可逆。即使恢复正常条件，变性酶也无法发挥功能。
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