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

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

Enzymes are proteins that act as biological catalysts, speeding up chemical reactions in living organisms by lowering the activation energy. They are essential for processes like respiration, photosynthesis, and digestion.

酶是作为生物催化剂的蛋白质，通过降低活化能来加速生物体内的化学反应。它们对呼吸、光合作用和消化等过程至关重要。

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1. What Are Enzymes? | 什么是酶？

Enzymes are globular proteins made up of chains of amino acids folded into specific three-dimensional shapes. This specific shape is critical to their function.

酶是由氨基酸链折叠成特定三维形状的球状蛋白质。这种特定形状对其功能至关重要。

Each enzyme has an active site, which is the region where the substrate binds. The rest of the enzyme molecule provides structural support and stability.

每种酶都有一个活性位点，即底物结合的区域。酶分子的其余部分提供结构支撑和稳定性。

Enzymes are not consumed in a reaction; they can be used repeatedly. This makes them efficient catalysts in cells.

酶在反应中不会被消耗，可重复使用。这使它们成为细胞内高效的催化剂。

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

The active site of an enzyme is a small pocket or groove with a shape complementary to the substrate molecule. Because only the correct substrate can fit perfectly, enzymes are highly specific.

酶的活性位点是一个形状与底物分子互补的小口袋或凹槽。由于只有正确的底物才能完美契合，酶具有高度的专一性。

For example, amylase only catalyses the hydrolysis of starch, not proteins or fats. This specificity is often described as a “lock and key” mechanism.

例如，淀粉酶只催化淀粉的水解，不作用于蛋白质或脂肪。这种专一性常被描述为”锁钥”机制。

If the shape of the active site changes, the enzyme may no longer bind its substrate. This is important when considering denaturation.

如果活性位点的形状发生变化，酶可能无法再结合其底物。这在讨论变性时很重要。

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3. Lock-and-Key vs Induced-Fit | 锁钥模型与诱导契合

The lock-and-key model suggests that the active site is rigid and the substrate fits exactly, like a key into a lock. Many GCSE and IGCSE courses teach this simple model.

锁钥模型认为活性位点刚性固定，底物恰好嵌入，就像钥匙插入锁孔。许多GCSE和IGCSE课程教授这个简单模型。

The induced-fit model is a more up-to-date description. It states that the active site is flexible and changes shape slightly when the substrate binds, ensuring a snug fit.

诱导契合模型是更新的描述。它指出活性位点具有柔性，当底物结合时会轻微改变形状，确保更紧密的契合。

Both models explain the specificity and catalytic activity of enzymes. The induced-fit model also explains how some inhibitors can distort the active site.

两种模型都解释了酶的专一性和催化活性。诱导契合模型还解释了某些抑制剂如何使活性位点变形。

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4. Enzyme-Substrate Complex | 酶-底物复合物

When a substrate binds to an enzyme’s active site, they form an enzyme-substrate complex. This temporary structure lowers the activation energy of the reaction.

当底物与酶的活性位点结合时，形成酶-底物复合物。这种临时结构降低了反应的活化能。

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

During the reaction, bonds in the substrate are broken or formed. After the products are released, the enzyme is unchanged and ready for another substrate molecule.

反应过程中，底物中的化学键被断裂或形成。产物释放后，酶保持原样，准备好与下一个底物分子结合。

In condensation reactions, water is removed; in hydrolysis reactions, water is added. Enzymes catalyse both types of reactions.

在缩合反应中会去除水；在水解反应中会加入水。酶对这两类反应均有催化作用。

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

Temperature affects the kinetic energy of molecules. As temperature rises, molecules move faster, so the rate of enzyme-catalysed reactions increases up to an optimum.

温度影响分子的动能。随着温度升高，分子运动加快，因此酶催化反应的速率上升，直到达到最适温度。

For most human enzymes, the optimum temperature is about 37 °C. Beyond this, the rate drops sharply because the enzyme begins to denature.

大多数人体酶的最适温度约为37 °C。超过该温度后，反应速率急剧下降，因为酶开始变性。

Denaturation involves the breaking of bonds that maintain the enzyme’s shape. The active site changes shape and the substrate can no longer bind.

变性涉及破坏维持酶形状的化学键。活性位点形状改变，底物无法再结合。

At very high temperatures, denaturation is permanent. At low temperatures, enzymes are inactive but not denatured; they resume activity when warmed.

在极高温下，变性是不可逆的。在低温下，酶没有活性但未变性；升温后恢复活性。

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6. Factors: pH | 因素：pH

Each enzyme has an optimum pH at which it works fastest. Most enzymes in the human body have an optimum near pH 7, but some are adapted to acidic or alkaline conditions.

每种酶都有一个最适pH值，在此pH下活性最高。人体内大多数酶的最适pH接近7，但有些酶适应酸性或碱性环境。

Pepsin, a digestive enzyme in the stomach, works best at pH 1.5–2.0. Trypsin, in the small intestine, works best at pH 8.

胃蛋白酶是胃中的消化酶，在pH 1.5–2.0时活性最高。胰蛋白酶在小肠中，最适pH约为8。

If the pH changes too far from the optimum, the hydrogen ion concentration alters the charges on amino acid side groups. This disrupts ionic and hydrogen bonds that hold the enzyme’s shape.

如果pH偏离最适值太远，氢离子浓度会改变氨基酸侧链基团的电荷。这会破坏维持酶形状的离子键和氢键。

Thus, extreme pH leads to denaturation. Buffer solutions are used in experiments to maintain a constant pH.

因此，极端pH会导致变性。实验中常使用缓冲液来维持恒定的pH。

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7. Factors: Substrate and Enzyme Concentration | 因素：底物与酶浓度

At a constant enzyme concentration, increasing substrate concentration increases the rate of reaction, but only up to a limit. Once all active sites are occupied, the rate reaches a plateau.

在酶浓度恒定时，增加底物浓度可提高反应速率，但仅到一个限度。一旦所有活性位点都被占据，速率达到平台。

The limiting factor changes as concentration increases. Initially substrate is limiting; later, enzyme concentration becomes the limiting factor.

随着浓度增加，限制因素会改变。最初底物是限制因素；之后酶浓度成为限制因素。

Similarly, at a fixed substrate concentration, increasing enzyme concentration increases the rate linearly, because more active sites are available to bind substrate molecules.

类似地，在底物浓度固定时，增加酶浓度会使速率线性上升，因为更多活性位点可用于结合底物分子。

In living cells, enzymes are usually present in small amounts, so substrate availability and inhibitors often control the overall reaction rate.

在活细胞中，酶通常含量较少，因此底物可用性和抑制剂常常控制整体反应速率。

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8. Inhibitors | 抑制剂

Inhibitors are substances that reduce the rate of an enzyme-catalysed reaction. They can be competitive or non-competitive.

抑制剂是降低酶催化反应速率的物质。它们可以是竞争性的或非竞争性的。

Competitive inhibitors have a similar shape to the substrate. They compete with the substrate for the active site, but they do not react.

竞争性抑制剂具有与底物相似的形状。它们与底物竞争活性位点，但不参与反应。

Increasing substrate concentration can overcome competitive inhibition, because more substrate molecules outcompete the inhibitor.

增加底物浓度可以克服竞争性抑制，因为更多底物分子会胜过抑制剂。

Non-competitive inhibitors bind to another part of the enzyme. This changes the shape of the active site, making it non-functional. Increasing substrate concentration does not help.

非竞争性抑制剂结合在酶的其他部位。这会改变活性位点的形状，使其失去功能。增加底物浓度没有帮助。

Some inhibitors are reversible; others are irreversible. Heavy metal ions, such as lead and mercury, are irreversible non-competitive inhibitors.

有些抑制剂是可逆的；有些是不可逆的。重金属离子，如铅和汞，是不可逆的非竞争性抑制剂。

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9. Uses of Enzymes in Biotechnology | 酶在生物技术中的应用

Enzymes are used in industry because they are specific, work at moderate temperatures, and are biodegradable. Common examples include biological washing powders and food production.

酶在工业中被广泛使用，因为它们具有专一性、在温和温度下工作且可生物降解。常见例子包括生物洗衣粉和食品生产。

Biological washing powders contain proteases and lipases to break down protein and fat stains. They allow clothes to be washed at lower temperatures.

生物洗衣粉含有蛋白酶和脂肪酶，用于分解蛋白质和脂肪污渍。它们允许在较低温度下洗涤衣物。

In bread making, amylase is added to flour to break down starch into sugars. Yeast then ferments these sugars to produce carbon dioxide, making bread rise.

在面包制作中，淀粉酶被加入面粉中，将淀粉分解为糖。酵母随后发酵这些糖产生二氧化碳，使面包膨胀。

Enzymes are also used to make fruit juice clearer. Pectinase breaks down pectin, which helps release more juice and reduces cloudiness.

酶也用于使果汁更清澈。果胶酶分解果胶，有助于释放更多果汁并减少浑浊。

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10. Experiments to Investigate Enzyme Activity | 研究酶活性的实验

In the IGCSE course, a common experiment uses amylase and starch. Add iodine solution to test for starch: it changes from orange-brown to blue-black in the presence of starch.

在IGCSE课程中，一个常见实验使用淀粉酶和淀粉。加入碘液检测淀粉：有淀粉时，碘液从橙棕色变为蓝黑色。

To investigate the effect of temperature, you can place amylase and starch in water baths at different temperatures. Take samples at regular intervals and test with iodine to see when starch disappears.

要研究温度的影响，可将淀粉酶和淀粉置于不同温度的水浴中。定期取样并用碘液测试，观察淀粉何时消失。

Another experiment uses catalase, which breaks down hydrogen peroxide into water and oxygen. The rate can be measured by collecting the oxygen produced in a gas syringe.

另一个实验使用过氧化氢酶，它将过氧化氢分解为水和氧气。可通过气体注射器收集产生的氧气来测量速率。

Controls are essential. Use a buffer to keep pH constant, and denatured enzyme as a control to show that no reaction occurs without active enzyme.

对照组至关重要。使用缓冲液保持pH恒定，并利用变性酶作为对照，以证明没有活性酶时不会发生反应。

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11. Enzymes in Digestion | 消化中的酶

Digestion is the breakdown of large insoluble food molecules into small soluble molecules that can be absorbed into the bloodstream. Enzymes are essential for this process.

消化是将大的不溶性食物分子分解为可被吸收到血液中的小可分性分子的过程。酶对此过程至关重要。

Amylase is secreted in the saliva and pancreas. It breaks down starch into maltose in the mouth and small intestine.

淀粉酶由唾液腺和胰腺分泌。它在口腔和小肠中把淀粉分解为麦芽糖。

Proteases, such as pepsin and trypsin, digest proteins into amino acids. Pepsin works in the stomach; trypsin works in the small intestine.

蛋白酶，如胃蛋白酶和胰蛋白酶，将蛋白质消化为氨基酸。胃蛋白酶在胃中起作用；胰蛋白酶在小肠中起作用。

Lipases break down fats into fatty acids and glycerol. Bile, produced by the liver, helps emulsify fats to increase the surface area for lipase action.

脂肪酶将脂肪分解为脂肪酸和甘油。肝脏产生的胆汁有助于乳化脂肪，增大脂肪酶作用的表面积。

Here is a summary table of digestive enzymes:

以下是消化酶的汇总表：

Enzyme | 酶	Substrate | 底物	Product(s) | 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	Fat | 脂肪	Fatty acids + Glycerol | 脂肪酸 + 甘油

Remember that digestion involves both mechanical and chemical processes. Enzymes perform the chemical breakdown, while teeth and muscle contractions perform the mechanical breakdown.

请记住，消化既涉及机械过程也涉及化学过程。酶完成化学分解，而牙齿和肌肉收缩完成机械分解。

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12. Summary and Exam Tips | 总结与考试提示

Enzymes are biological catalysts that speed up reactions without being used up. They are specific due to their active site shape, and are affected by temperature, pH, and concentrations.

酶是生物催化剂，在加速反应的同时自身不被消耗。由于活性位点形状而具有专一性，并受温度、pH和浓度的影响。

Denaturation is a permanent change in the enzyme’s shape at high temperature or extreme pH. Denatured enzymes can no longer function.

变性是酶在高温或极端pH下发生的永久性形状改变。变性的酶无法再起作用。

For the exam, always use the term “enzyme-substrate complex” when explaining how enzymes work. Quote the optimum temperature and pH for named enzymes.

在考试中，解释酶的工作原理时务必使用”酶-底物复合物”这一术语。说明指定酶的最适温度和pH。

When describing experiments, mention controlling variables such as temperature, pH, and concentration. State how you measure the reaction rate, for example by timing the colour change.

描述实验时，要提及控制变量，如温度、pH和浓度。说明如何测量反应速率，例如通过计时颜色变化。

Practice drawing graphs: rate versus temperature should show an upward curve then a sharp drop after the optimum. Rate versus pH shows a bell-shaped curve.

练习作图：速率-温度曲线应显示上升曲线，最适温度后急剧下降。速率-pH曲线呈钟形。

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