Enzymes: Structure and Function | 酶：结构与功能

📚 Enzymes: Structure and Function | 酶：结构与功能

Enzymes are biological catalysts that speed up chemical reactions in living cells without being used up in the process. In the Edexcel IGCSE Science course, you need to understand how enzymes are structured, how they work, and why they are essential for processes such as digestion and industrial biotechnology.

酶是生物催化剂，能加速活细胞内的化学反应，而自身在反应中不被消耗。在爱德思 IGCSE 科学课程中，你需要理解酶的结构、作用原理，以及为什么酶对消化和工业生物技术至关重要。

1. What Are Enzymes? | 什么是酶？

Enzymes are proteins, which are long chains of amino acids folded into a specific three-dimensional shape. The exact shape of an enzyme is critical because it determines the function of the molecule.

酶是蛋白质，由氨基酸长链折叠成特定的三维形状。酶的精确形状至关重要，因为它决定了这种分子的功能。

Each enzyme contains an active site. The active site is the region where the substrate, the reactant molecule, binds. This binding site has a shape that is complementary to the substrate.

每种酶都有一个活性位点。活性位点是与底物（即反应物分子）结合的区域。该结合位点的形状与底物互补。

Enzymes are specific: each enzyme catalyses only one reaction or one type of reaction.
酶具有专一性：每种酶只催化一种或一类反应。
Enzymes are not changed permanently by the reaction, so they can be used again and again.
酶不会因反应而永久改变，因此可以反复使用。

A common example is catalase, an enzyme that breaks down hydrogen peroxide into water and oxygen. This reaction is important in cells because hydrogen peroxide is toxic.

一个常见例子是过氧化氢酶，它能把过氧化氢分解成水和氧气。这一反应在细胞中很重要，因为过氧化氢是有毒的。

2. Active Site and Specificity | 活性位点与专一性

The most widely used model for enzyme action is the lock-and-key model. In this model, the active site is the lock and the substrate is the key. Only the correctly shaped substrate can fit into the active site.

最常用的酶作用模型是“锁钥模型”。在这个模型中，活性位点是锁，底物是钥匙。只有形状正确的底物才能嵌入活性位点。

A more accurate model is the induced-fit model. Here, the active site changes shape slightly when the substrate binds, making the fit stronger and allowing the reaction to occur more efficiently.

更准确的模型是“诱导契合模型”。在这个模型中，底物结合时活性位点会发生轻微形变，使结合更紧密，从而更高效地催化反应。

When the substrate binds to the enzyme, an enzyme-substrate complex is formed. After the reaction, the product is released and the enzyme returns to its original shape.

当底物与酶结合时，会形成“酶-底物复合物”。反应结束后，产物被释放，酶恢复原有的形状。

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

The equation above shows that the enzyme is regenerated, so a small amount of enzyme can catalyse a large amount of substrate.

上面的方程式表明酶会再生，因此少量酶就能催化大量底物。

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

Temperature affects the rate of enzyme-controlled reactions because it affects the kinetic energy of the molecules. As temperature increases, particles move faster and collide with the active site more often.

温度通过影响分子的动能来影响酶促反应速率。温度升高时，粒子运动加快，与活性位点碰撞的频率更高。

The rate of reaction increases with temperature up to an optimum, which for most human enzymes is around 37°C to 40°C. This is the temperature at which the enzyme works fastest.

反应速率随温度升高而加快，直至达到最适温度。大多数人体酶的最适温度在37°C至40°C左右，此时酶作用最快。

Above the optimum temperature, the rate decreases sharply. The high temperature breaks the bonds holding the protein in its specific shape. This process is called denaturation.

超过最适温度后，反应速率急剧下降。高温会破坏维持蛋白质特定形状的化学键，这个过程称为“变性”。

Denaturation changes the shape of the active site. The substrate can no longer fit, so the enzyme cannot catalyse the reaction. Denaturation is permanent; an enzyme cannot return to its normal shape once denatured.

变性会改变活性位点的形状，底物无法结合，酶便不能再催化反应。变性是不可逆的；酶一旦变性，就无法恢复正常形状。

When drawing the temperature graph, remember to label the optimum temperature and to show a sharp drop after the optimum.

绘制温度曲线图时，记得标出最适温度，并显示最适温度之后速率的急剧下降。

4. pH and Enzyme Activity | pH 对酶活性的影响

Each enzyme has an optimum pH at which it works best. The optimum pH depends on the environment in which the enzyme normally acts.

每种酶都有其最适 pH，在最适 pH 下活性最高。最适 pH 取决于酶通常所在的环境。

If the pH is too high or too low, ionic bonds and hydrogen bonds within the enzyme can be disrupted. This changes the shape of the active site, causing denaturation.

如果 pH 过高或过低，酶内的离子键和氢键可能被破坏，这会改变活性位点的形状，导致变性。

Enzyme	Location	Optimum pH
Pepsin	Stomach	About 2
Trypsin	Small intestine	About 8
Amylase	Saliva / pancreatic juice	Around 7

In the small intestine, bile is not an enzyme, but it neutralises the acidic mixture from the stomach and emulsifies fats. This helps lipase work more effectively.

在小肠中，胆汁不是酶，但它可以中和来自胃的酸性食糜，并使脂肪乳化，从而帮助脂肪酶更有效地发挥作用。

5. Substrate and Enzyme Concentration | 底物浓度与酶浓度

The rate of an enzyme-controlled reaction also depends on the concentration of the substrate and the concentration of the enzyme.

酶促反应的速率还取决于底物浓度和酶的浓度。

At a fixed enzyme concentration, increasing substrate concentration increases the rate of reaction initially. However, all active sites become occupied, and the rate reaches a maximum plateau. Further increases in substrate concentration do not increase the rate.

在酶浓度固定时，增加底物浓度会在初期提高反应速率。但当所有活性位点都被占据后，速率达到最大值并趋于平稳。继续增加底物浓度不会提高速率。

At a fixed substrate concentration and with excess substrate, increasing enzyme concentration increases the rate of reaction proportionally. More active sites are available to catalyse the reaction.

在底物浓度固定且过量时，增加酶浓度会使反应速率成比例提高，因为可用于催化反应的活性位点更多了。

Be careful in exam questions: the plateau means the limiting factor has changed from substrate concentration to enzyme concentration or other factors.

考试时要注意：速率曲线进入平台期，说明限制因素已从底物浓度变为酶浓度或其他因素。

6. Enzyme Inhibitors | 酶抑制剂

Enzyme inhibitors are substances that reduce the rate of an enzyme-catalysed reaction. They are important in medicine and in metabolic control.

酶抑制剂是能够降低酶促反应速率的物质。它们在医学和代谢调控中很重要。

Competitive inhibitors have a similar shape to the substrate. They compete with the substrate for the active site. If a competitive inhibitor occupies the active site, the substrate cannot bind.

竞争性抑制剂与底物形状相似，会与底物竞争活性位点。如果竞争性抑制剂占据了活性位点，底物就无法结合。

Non-competitive inhibitors bind to the enzyme at a site other than the active site. This changes the shape of the enzyme, including the active site, so the substrate can no longer bind.

非竞争性抑制剂结合在酶分子上除活性位点以外的位置，这会改变酶的整体形状，包括活性位点，使底物无法结合。

Competitive inhibition can be reduced by increasing substrate concentration.
增加底物浓度可以减轻竞争性抑制。
Non-competitive inhibition cannot be overcome by adding more substrate.
非竞争性抑制不能通过增加底物浓度来解除。

7. Enzymes in Digestion | 消化中的酶

Digestion depends on enzymes that break down large insoluble food molecules into small soluble molecules that can be absorbed into the blood.

消化过程依赖酶将大而不溶的食物分子分解成可溶的小分子，以便被血液吸收。

Enzyme	Substrate	Products
Amylase	Starch	Maltose / sugars
Protease	Proteins	Amino acids
Lipase	Lipids (fats and oils)	Fatty acids and glycerol

Amylase is produced in the salivary glands and the pancreas. It breaks down starch into maltose. Proteases are produced in the stomach, pancreas and small intestine. Lipases are produced mainly in the pancreas and act in the small intestine.

淀粉酶由唾液腺和胰腺分泌，将淀粉分解为麦芽糖。蛋白酶在胃、胰腺和小肠中产生。脂肪酶主要在胰腺中产生，并在小肠中起作用。

The small intestine is the main site of digestion and absorption. Its walls are adapted by having villi, which increase the surface area for absorption.

小肠是消化和吸收的主要场所。小肠壁有小肠绒毛，可增大吸收表面积。

8. Uses of Enzymes in Industry and Medicine | 酶在工业与医学中的应用

Enzymes are used widely in industry because they are specific, work at low temperatures and are biodegradable.

酶在工业中被广泛使用，因为它们具有专一性、可在较低温度下工作，并且可生物降解。

Biological detergents contain proteases and lipases. These enzymes break down protein and fat stains such as blood, egg and grease. They allow clothes to be washed at lower temperatures, saving energy.

生物洗衣粉含有蛋白酶和脂肪酶。这些酶能分解血迹、蛋渍和油污等蛋白质和脂肪污渍，并允许较低温度洗涤，从而节约能源。

Lactase is used to produce lactose-free milk. Lactase breaks down lactose into glucose and galactose, so people who are lactose intolerant can drink the milk.

乳糖酶用于生产无乳糖牛奶。乳糖酶将乳糖分解为葡萄糖和半乳糖，使乳糖不耐受者也能饮用牛奶。

Immobilised enzymes are often used in industry. They are attached to a solid support, so they can be reused and are more stable at different temperatures and pH values.

工业中常使用固定化酶。固定化酶被附着在固体载体上，可以反复使用，并且在不同的温度和 pH 条件下更稳定。

9. Investigating Enzyme Activity | 探究酶活性

A common IGCSE practical involves measuring how quickly amylase breaks down starch at different temperatures or pH values.

一个常见的 IGCSE 实验是测量淀粉酶在不同温度或 pH 条件下分解淀粉的速度。

Place a drop of iodine solution on a spotting tile.
在点滴板上滴一滴碘液。
Mix amylase solution with starch solution in a test tube.
在试管中混合淀粉酶溶液和淀粉溶液。
Every 10 seconds, add one drop of the mixture to the iodine. If the iodine remains orange-brown, starch is no longer present.
每10秒取一滴混合液滴入碘液。如果碘液保持橙棕色，说明淀粉已消失。
The time taken for starch to disappear gives a measure of the rate of reaction.
淀粉消失所需时间可用于衡量反应速率。

For a rate measurement, shorter time means faster reaction. You can repeat the experiment at different temperatures and plot a graph of time or rate against temperature.

测定速率时，时间越短表示反应越快。你可以在不同温度下重复实验，并绘制时间或速率对温度的曲线图。

Variables should be controlled carefully: volumes, concentrations, pH and mixing time must be kept the same.

实验变量需要严格控制：体积、浓度、pH 和混合时间都应保持一致。

10. Common Exam Points and Mistakes | 常见考点与易错点

In the Edexcel IGCSE exam, questions about enzymes often ask you to describe graphs and explain denaturation. Use the correct keywords in your answers: ‘active site’, ‘substrate’, ‘denatured’ and ‘optimum’.

在爱德思 IGCSE 考试中，酶相关题目通常要求描述曲线图并解释变性。答题时要使用正确的关键词：“活性位点”“底物”“变性”“最适”等。

Do not write ‘the enzyme is killed’. The correct term is ‘denatured’.
不要写“酶被杀死”，正确说法是“变性”。
Do not say ‘the active site is destroyed’ as if it disappears. Say ‘the shape of the active site is changed’.
不要说“活性位点被破坏而消失”，要说“活性位点的形状发生改变”。
Always state that denaturation is irreversible.
务必说明变性是不可逆的。
When describing the rate graph, mention the plateau and the limiting factor.
描述速率曲线时，要提到平台期和限制因素。

Enzymes are one of the most important topics in IGCSE Science. Understanding their structure, specificity and the factors that affect them will help you answer a wide range of exam questions.

酶是 IGCSE 科学中最重要的主题之一。理解酶的结构、专一性以及影响酶活性的因素，将帮助你回答多种类型的考试题目。

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Enzymes: Structure and Function | 酶：结构与功能