Enzymes: Nature's Biological Catalysts | 酶：天然生物催化剂

📚 Enzymes: Nature’s Biological Catalysts | 酶：天然生物催化剂

Enzymes are essential proteins that control almost every chemical reaction in living organisms. In the Edexcel IGCSE Science specification, understanding how enzymes work is one of the most important ideas in biology, and it is frequently tested through graph interpretation and practical investigations.

酶是控制生物体内几乎一切化学反应的关键蛋白质。在 Edexcel IGCSE 科学大纲中，理解酶的作用方式是生物学最重要的内容之一，而且经常通过图表分析和实验探究来考查。

1. What Are Enzymes? | 什么是酶？

Enzymes are biological catalysts. A catalyst is a substance that speeds up a chemical reaction without being used up or permanently changed in the process. Because enzymes are never consumed, a single enzyme molecule can catalyse many reactions per second.

酶是生物催化剂。催化剂是在化学反应中加快反应速度，同时自身不会被消耗或永久改变的物质。由于酶不会被消耗，一个酶分子每秒钟可以催化成千上万次反应。

Enzymes are made of protein, so their structure is a long chain of amino acids folded into a precise three-dimensional shape. This shape is responsible for the enzyme’s function and for its ability to bind only to specific molecules.

酶由蛋白质构成，所以它的结构是氨基酸长链折叠成精确的三维形状。这个形状决定了酶的功能，也决定了它只能与特定分子结合。

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

The part of an enzyme where the reaction takes place is called the active site. The active site has a particular shape, and only the correct substrate — the molecule that the enzyme acts on — can fit into it. Because the active site is a specific shape, each enzyme only catalyses one particular reaction or group of closely related reactions.

酶上发生反应的部位叫做活性位点。活性位点具有特定的形状，只有正确的底物——也就是酶作用的分子——才能进入并与之结合。由于活性位点形状固定，每种酶只能催化一种特定的反应或一组非常相似的反应。

For example, the enzyme catalase found in living cells catalyses the breakdown of hydrogen peroxide into water and oxygen. The equation for this reaction is:

例如，存在于活细胞中的过氧化氢酶催化过氧化氢分解为水和氧气，反应方程式为：

2H₂O₂ → 2H₂O + O₂

Catalase cannot break down other substances, such as starch or protein, because those molecules do not fit its active site.

过氧化氢酶不能分解淀粉或蛋白质等其他物质，因为这些分子无法与它的活性位点匹配。

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

The simplest way to describe enzyme specificity is the lock-and-key model. In this model, the active site is a rigid ‘lock’ and the substrate is the ‘key’. Only the correctly shaped key can enter the lock, just as only the correct substrate can bind to the active site.

描述酶的专一性最简单的方式是锁钥模型。在这个模型中，活性位点像一个固定的“锁”，底物像“钥匙”。只有形状正确的钥匙才能插入锁中，就像只有正确的底物才能结合到活性位点上。

The more modern induced-fit model suggests that the active site is not completely rigid. When the substrate binds, the shape of the active site changes slightly to fit more closely around the substrate. This causes the enzyme to ‘wrap around’ the substrate and puts extra stress on the substrate, which helps the reaction to go faster.

更现代的诱导契合模型认为，活性位点并非完全固定。当底物结合时，活性位点的形状会略微改变，更紧密地包裹底物。这使酶能“包住”底物，并对底物产生额外张力，从而加快反应。

Both models explain why enzymes are so specific, but the induced-fit model is closer to what actually happens in cells.

这两种模型都能解释酶为何具有高度的专一性，但诱导契合模型更接近细胞内真实发生的情况。

4. How Enzymes Lower Activation Energy | 酶如何降低活化能

Every chemical reaction needs a certain amount of energy to begin; this is called the activation energy. Without enzymes, many biological reactions would require temperatures that are far too high for cells to survive.

每个化学反应都需要一定能量才能开始，这个能量叫做活化能。如果没有酶，许多生物反应所需的温度会远远高于细胞能承受的范围。

Enzymes lower the activation energy by holding the substrate correctly and destabilising the chemical bonds that need to be broken. This means that fewer random collisions between molecules are needed for the reaction to occur. The substrate enters the active site, the reaction happens, and the product leaves, allowing the enzyme to be reused.

酶通过正确固定底物并削弱需要断裂的化学键来降低活化能。这意味着反应不需要分子间发生那么多次随机碰撞就能进行。底物进入活性位点，反应发生，产物离开，酶随即被回收再利用。

This overall catalytic cycle can be written as:

这个催化循环的总过程可以写成：

E + S ⇌ ES → E + P

Here, E represents the enzyme, S is the substrate, ES is the enzyme-substrate complex, and P is the product.

其中 E 代表酶，S 代表底物，ES 是酶-底物复合物，P 是产物。

5. Effect of Temperature | 温度对酶的影响

Temperature is one of the most commonly tested factors affecting enzyme activity. At low temperatures, the particles have less kinetic energy, so they move more slowly. Fewer successful collisions occur between enzymes and substrates, so the rate of reaction is low.

温度是考查酶活性时最常见的因素之一。在低温下，分子动能较小，运动速度较慢，酶与底物之间发生有效碰撞的次数较少，因此反应速率较低。

As the temperature increases, particles gain kinetic energy and move faster. More successful collisions happen, so the rate of reaction increases. For most enzymes, the activity approximately doubles for every 10 °C rise until a certain point called the optimum temperature.

随着温度升高，分子获得更多动能，运动加快，有效碰撞增多，反应速率随之升高。对大多数酶来说，每升高 10 °C，活性大约翻倍，直至某个特定点——最适温度。

For human enzymes such as amylase, the optimum temperature is usually around 37 °C, which is normal body temperature. For enzymes from some bacteria, the optimum may be much higher because these bacteria live in hot springs.

对于人体酶（例如淀粉酶），最适温度通常在 37 °C 左右，也就是正常体温。而对于某些细菌的酶，最适温度可能高得多，因为这些细菌生活在温泉中。

The effect of temperature on an enzyme-catalysed reaction is shown by a characteristic curve: the rate rises steadily, reaches a peak, then drops sharply after the optimum temperature. It is important to describe this curve from left to right in an exam.

酶催化反应速率随温度变化的曲线很有特点：速率逐渐上升，达到峰值，然后超过最适温度后急剧下降。在考试中，你需要从左到右准确描述这条曲线的变化趋势。

6. Effect of pH | pH 对酶的影响

Each enzyme also has an optimum pH at which its activity is highest. Most enzymes in the human body work best at a pH of around 7, but some are adapted to more acidic or alkaline environments.

每种酶也有一个使其活性最高的最适 pH 值。人体内大多数酶在 pH 值约为 7 的环境中活性最高，但有些酶适应更酸或更碱的环境。

For example, pepsin is a digestive enzyme in the stomach and works best at pH 2, because the stomach contains hydrochloric acid. In contrast, the pancreatic enzyme lipase works best at pH 8, which is slightly alkaline, matching the conditions in the small intestine.

例如，胃蛋白酶是胃中的消化酶，最适 pH 为 2，因为胃中含有盐酸。相反，胰腺脂肪酶在小肠中工作，最适 pH 为 8，呈弱碱性，与小肠环境相适应。

Changes in pH alter the charges on the amino acid molecules in the enzyme. This disrupts the hydrogen bonds and ionic bonds that hold the protein in its precise shape. If the pH moves far from the optimum, the active site changes shape and becomes unable to bind the substrate.

pH 值的变化会改变酶分子中氨基酸上的电荷，从而破坏维持蛋白质精确形状的氢键和离子键。如果 pH 值偏离最适值较远，活性位点的形状就会改变，无法与底物结合。

When investigating pH, you should not mix up pH with substrate or enzyme concentration. The graph of rate against pH usually looks like a bell-shaped curve, with zero activity at very low and very high pH.

在探究 pH 的影响时，不要将 pH 与底物浓度或酶浓度混淆。反应速率随 pH 变化的曲线通常呈钟形，在 pH 过低或过高时活性为零。

7. Effect of Substrate and Enzyme Concentration | 底物浓度与酶浓度的影响

When the enzyme concentration is fixed, increasing the substrate concentration increases the rate of reaction, but only up to a limit. As more substrate is added, more active sites are being used. Once every active site is occupied, the enzyme is working at its maximum rate, and adding more substrate has no further effect. This point is called the maximum rate or Vmax.

当酶浓度固定时，增加底物浓度可以提高反应速率，但只能提高到一个上限。随着底物增多，越来越多的活性位点被占用。当每个活性位点都被占据时，酶已经达到最大反应速率，此时再增加底物也不会提高速率。这个点称为最大速率 Vmax。

When the substrate concentration is in excess, the limiting factor is the amount of enzyme available. Increasing the enzyme concentration will increase the rate of reaction, because there are more active sites available for the substrate to collide with.

当底物过量时，限制因素是酶的数量。增加酶浓度可以提高反应速率，因为底物能碰撞的活性位点更多。

In an exam, you may be asked to explain why the graph of substrate concentration versus rate levels off. You should say that the enzyme’s active sites are saturated — all of them are occupied, so the enzyme cannot work any faster.

在考试中，你可能会被要求解释底物浓度曲线为何最终变平。你应该说这是因为酶的活性位点已经饱和——所有位点都被占用，所以酶无法再更快地工作。

8. Denaturation | 酶的变性

Denaturation is the permanent change in the three-dimensional shape of an enzyme, caused by high temperature or extreme pH. When an enzyme denatures, the bonds that hold its protein structure together break, so the active site changes shape. The substrate can no longer fit into the active site, and the enzyme can no longer catalyse the reaction.

变性是指酶的三维形状发生永久性改变，通常由高温或极端 pH 引起。当酶变性时，维持蛋白质结构的化学键断裂，活性位点改变形状。底物无法再进入活性位点，酶也就不能再催化反应。

It is vital to understand that denaturation is not the same as slowly decreasing activity. For example, at very high temperature, the rate of reaction does not simply decrease gradually; it drops quickly because the enzyme’s shape is destroyed. The enzyme cannot recover even if the temperature is lowered again.

必须理解，变性并不仅仅意味着活性缓慢下降。例如，在极高温度下，反应速率不是逐渐降低，而是迅速下降，因为酶的形状被破坏了。即使再降温，酶也无法恢复活性。

However, cooling an enzyme below its optimum temperature does not denature it. It only slows the reaction down. If the enzyme is warmed again, its activity returns. This is why enzymes can be stored in a refrigerator before use.

然而，将酶冷却到最适温度以下并不会使它变性，只会降低反应速率。如果重新升温，酶活性就会恢复。这就是酶在使用前可以存放在冰箱里的原因。

9. Enzyme Inhibition | 酶的抑制作用

Some chemicals can reduce the rate of an enzyme-catalysed reaction; these are called inhibitors. Inhibitors may be competitive or non-competitive.

有些化学物质能降低酶催化反应的速率，这些物质叫做抑制剂。抑制剂分为竞争性抑制剂和非竞争性抑制剂。

A competitive inhibitor has a similar shape to the substrate. It competes with the substrate for the active site. If a competitive inhibitor is occupying the active site, the substrate cannot bind. The effect of a competitive inhibitor can be reduced by increasing the substrate concentration, because more substrate molecules will out-compete the inhibitor for the active sites.

竞争性抑制剂的形状与底物相似，它与底物竞争活性位点。如果竞争性抑制剂占据活性位点，底物就无法结合。增加底物浓度可以削弱竞争性抑制剂的作用，因为更多底物分子会在竞争中获得优势。

A non-competitive inhibitor binds to an enzyme at a site other than the active site, called the allosteric site. This binding changes the shape of the enzyme, including the active site, so the substrate can no longer bind. Increasing the substrate concentration does not reverse this type of inhibition.

非竞争性抑制剂则结合在酶上活性位点以外的位置，称为别构位点。这种结合会改变酶的整体形状，包括活性位点，使底物无法再结合。增加底物浓度不能解除这种抑制。

In IGCSE Biology, a common example of inhibitors is the use of certain metal ions or drugs to stop bacterial enzymes. You do not need to memorise many examples, but you should be able to interpret graphs showing inhibitor effects.

在 IGCSE 生物学中，常见的抑制剂例子包括某些金属离子或药物用来阻止细菌酶工作。你不需要记住太多例子，但应能解释显示抑制剂效应的曲线图。

10. Practical Investigations | 实践探究：测量酶活性

A common practical for this topic uses the enzyme catalase. Catalase is found in potatoes or liver, and it catalyses the breakdown of hydrogen peroxide solution. The oxygen gas produced can be collected in a gas syringe, and the volume of gas can be measured every few seconds.

常见的实验使用过氧化氢酶。过氧化氢酶存在于土豆或肝脏中，能催化过氧化氢溶液分解。反应产生的氧气可以用气体注射器收集，并每隔几秒记录气体体积。

To investigate the effect of temperature, the enzyme (potato or liver) and the hydrogen peroxide should first be placed in a water bath at the chosen temperature until they reach that temperature. Then they are mixed, and the volume of oxygen is measured over time. The steeper the initial slope of the graph, the faster the initial rate of reaction.

为了探究温度的影响，应将酶（土豆或肝脏）和过氧化氢首先放入设定温度的水浴中，使其达到该温度。然后混合二者，并定时测量氧气体积。曲线起始部分越陡，说明初始反应速率越快。

For a fair experiment, the variables that must be kept constant include the mass of the enzyme, the volume and concentration of hydrogen peroxide, and the surface area of the solid enzyme if used.

为了保证实验公平，必须保持不变的变量包括酶的质量、过氧化氢的体积和浓度，以及如果使用固体酶时酶的表面积。

Graph work in this topic is very important. A rate can be calculated from a graph using the formula:

本主题中的图表题非常重要。速率可以根据图表用以下公式计算：

rate = change in product or substrate ÷ time taken

For example, if 24 cm³ of oxygen is collected in 2 minutes, the average rate is 12 cm³ per minute.

例如，如果在 2 分钟内收集到 24 cm³ 氧气，则平均速率为每分钟 12 cm³。

11. Common Mistakes and Exam Tips | 常见错误与考试提示

A frequent error is to say that enzymes are “killed” or “die” at high temperatures. Enzymes are not alive, so they cannot die. They denature, meaning their shape changes permanently. You should use the word “denature” exactly.

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Enzymes: Nature’s Biological Catalysts | 酶：天然生物催化剂