📚 Enzymes | 酶
Enzymes are biological catalysts that speed up chemical reactions in living organisms without being used up themselves. They are essential for life, controlling everything from digestion to DNA replication.
酶是生物催化剂，能在生物体内加速化学反应，而自身不会被消耗。它们对生命至关重要，控制着从消化到 DNA 复制的一切过程。
1. What are Enzymes? | 什么是酶？
Enzymes are proteins made up of long chains of amino acids folded into specific three-dimensional shapes. Each enzyme has an active site, a region with a unique shape that allows specific substrate molecules to bind.
酶是由氨基酸长链折叠成特定三维结构的蛋白质。每种酶都有一个活性位点，该区域具有独特的形状，使得特定的底物分子能够与之结合。
The molecule that an enzyme acts upon is called the substrate. During a reaction, the substrate binds to the active site, forming an enzyme-substrate complex. The enzyme then converts the substrate into product(s) and releases them, ready to catalyse another reaction.
酶作用的分子称为底物。在反应中，底物与活性位点结合，形成酶-底物复合物。随后酶将底物转化为产物并释放它们，为下一次反应做好准备。
Since enzymes are not changed by the reaction, they can be used repeatedly. This makes them highly efficient — a single enzyme molecule can catalyse thousands of reactions per second.
由于酶在反应中不发生变化，因此可以重复使用。这使得酶非常高效——一个酶分子每秒可催化数千次反应。
2. The Lock-and-Key Model | 锁钥模型
The lock-and-key model explains how enzymes work. The active site of the enzyme is the 'lock', and the substrate is the 'key'. Only the correct key fits into the lock, and once it fits, the reaction occurs.
锁钥模型解释了酶的工作方式。酶的活性位点好比“锁”，底物好比“钥匙”。只有正确的钥匙才能插入锁中，一旦匹配，反应便会发生。
This model emphasises the specificity of enzymes: each enzyme only catalyses one type of reaction or acts on one type of substrate. For example, amylase only breaks down starch, not proteins or lipids.
该模型强调了酶的专一性：每种酶只催化一种类型的反应或只作用于一种类型的底物。例如，淀粉酶只分解淀粉，而不分解蛋白质或脂质。
An updated model, the induced-fit model, suggests that the active site is slightly flexible and changes shape to fit the substrate more perfectly after binding. This is more accurate than the rigid lock-and-key model, but the lock-and-key model is still a useful simplification.
一种更新的模型——诱导契合模型——认为活性位点具有一定柔性，在底物结合后会发生形变以实现更完美的匹配。这比刚性的锁钥模型更为准确，但锁钥模型仍然是一个有用的简化。
3. Enzyme Specificity | 酶的专一性
Enzyme specificity arises from the unique shape and chemical properties of the active site. The amino acid residues in the active site create a particular environment that only complementary substrates can enter.
酶的专一性源于活性位点独特的形状和化学性质。活性位点中的氨基酸残基营造出特定的环境，只有互补的底物才能进入。
Absolute specificity: an enzyme acts on only one substrate (e.g. catalase acts on hydrogen peroxide).
绝对专一性：酶只作用于一种底物（例如过氧化氢酶作用于过氧化氢）。
Group specificity: an enzyme acts on a group of similar substrates (e.g. peptidase acts on multiple peptides).
基团专一性：酶作用于一组类似的底物（例如肽酶作用于多种肽）。
Stereochemical specificity: an enzyme can distinguish between isomers (e.g. some enzymes only act on L-amino acids).
立体化学专一性：酶能够区分异构体（例如某些酶只作用于 L-氨基酸）。
This specificity is crucial for metabolic pathways, ensuring that each step is carried out by the correct enzyme and that unwanted side reactions do not occur.
这种专一性对代谢途径至关重要，确保每一步都由正确的酶催化，并且不会发生不需要的副反应。
4. Temperature and Enzyme Activity | 温度与酶活性
Temperature has a significant effect on the rate of enzyme-catalysed reactions. As temperature increases, the kinetic energy of molecules increases, so more enzyme-substrate collisions occur and the rate of reaction rises.
温度对酶催化反应的速率有显著影响。随着温度升高，分子动能增大，酶与底物碰撞机会增多，反应速率因此上升。
However, enzymes are proteins and can be denatured by high temperatures. Beyond a certain point, the vibrations of the protein structure break the bonds that maintain the active site's shape. The active site changes shape, and the enzyme can no longer bind to its substrate — it is denatured.
然而，酶是蛋白质，高温可使其变性。超过某一温度点，蛋白质结构的振动会破坏维持活性位点形状的键。活性位点形状改变，酶无法再与底物结合——即酶变性了。
For most human enzymes, the optimum temperature is around 37°C (body temperature). The graph below shows the typical curve:
对于大多数人体酶，最适温度约为 37°C（体温）。下图显示了典型的曲线：
of reaction → Temperature / °C: rises, peaks at optimum, falls rapidly after denaturation
反应速率 → 温度 / °C： 先上升，在最适温度达到峰值，变性后急剧下降
Low temperatures do not denature enzymes, but they slow down molecular movement, reducing the rate of reaction. This is why refrigerating food slows spoilage — enzymes in bacteria and fungi work more slowly.
低温不会使酶变性，但会减慢分子运动，从而降低反应速率。这就是冷藏食物能延缓变质的原因——细菌和真菌中的酶活性减弱。
5. pH and Enzyme Activity | pH 与酶活性
Each enzyme has an optimum pH at which its activity is highest. Changes in pH affect the ionic bonds and hydrogen bonds that hold the enzyme's structure together. Extreme pH values can denature the enzyme permanently.
每种酶都有其活性最高的最适 pH。pH 的改变会影响维持酶结构的离子键和氢键。极端 pH 值会使酶永久变性。
For example, pepsin in the stomach works best at pH 2, while trypsin in the small intestine works best at pH 8. The graph for pH shows a bell-shaped curve, with the maximum at the optimum pH.
例如，胃中的胃蛋白酶在 pH 2 时活性最高，而小肠中的胰蛋白酶在 pH 8 时活性最高。pH 曲线呈钟形，峰值出现在最适 pH 处。
It is important to note that a small change near the optimum pH may be reversible, but large deviations usually cause irreversible denaturation because the active site loses its shape.
值得注意的是，最适 pH 附近的小幅变化可能是可逆的，但较大的偏差通常导致不可逆的变性，因为活性位点失去其形状。
6. Substrate Concentration and Enzyme Concentration | 底物浓度与酶浓度
As substrate concentration increases, the rate of reaction also increases, because there are more substrate molecules to collide with enzyme active sites. However, this only happens up to a point.
随着底物浓度的增加，反应速率也随之增加，因为有更多的底物分子与酶活性位点碰撞。然而，这只会增加到某个限度。
When all enzyme active sites are occupied, the enzyme is said to be saturated. Further increases in substrate concentration will no longer increase the rate of reaction — the rate reaches a plateau (Vmax).
当所有酶活性位点都被占据时，酶即达到饱和。进一步增加底物浓度不会再提高反应速率——速率达到平台期（Vmax）。
Rate of reaction → Substrate concentration: linear increase, then plateau
反应速率 → 底物浓度： 先线性上升，后趋于平台
If the enzyme concentration is increased while substrate is in excess, the rate of reaction increases proportionally. More active sites are available for the substrate, so more reactions can occur per unit time.
如果底物过量而酶浓度增加，反应速率将成比例增加。更多活性位点可供底物使用，因此单位时间内可发生更多反应。
In laboratory experiments, it is common to keep enzyme concentration constant and vary substrate concentration, or vice versa, to study these relationships clearly.
在实验室实验中，通常保持酶浓度不变而改变底物浓度，或反之，以便清晰地研究这些关系。
7. Inhibitors | 抑制剂
Inhibitors are substances that reduce the activity of an enzyme. They can be competitive or non-competitive.
抑制剂是降低酶活性的物质。它们可分为竞争性抑制剂和非竞争性抑制剂。
Competitive inhibitors have a similar shape to the substrate and compete with the substrate for the active site. Increasing substrate concentration can overcome their effect, as more substrate molecules out-compete the inhibitor.
竞争性抑制剂与底物形状相似，与底物竞争活性位点。增加底物浓度可以克服其影响，因为更多底物分子会在竞争占据优势。
Non-competitive inhibitors bind to the enzyme at a site other than the active site (an allosteric site). This changes the shape of the active site, so the substrate cannot bind. Increasing substrate concentration cannot reverse this effect.
非竞争性抑制剂结合在酶的非活性位点（别构位点）上，改变活性位点的形状，使底物无法结合。增加底物浓度无法逆转这种效应。
Inhibitors are important in medicine — many drugs are enzyme inhibitors. For example, aspirin inhibits the enzyme cyclooxygenase (COX), reducing inflammation and pain.
抑制剂在医学中非常重要——许多药物都是酶抑制剂。例如，阿司匹林抑制环氧化酶（COX），从而减轻炎症和疼痛。
8. Enzymes in Industry and Daily Life | 酶在工业和日常生活中的应用
Enzymes are widely used in biotechnology and everyday products due to their specificity and efficiency.
由于酶具有专一性和高效性，它们在生物技术和日常产品中广泛应用。
Biological washing powders contain proteases and lipases to break down protein and fat stains. They work best at low temperatures, saving energy.
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。它们在低温下效果最佳，节省能源。
In food production, invertase is used to make artificial honey, and glucose isomerase is used to convert glucose into fructose, which is sweeter.
在食品生产中，蔗糖酶用于制作人造蜂蜜，葡萄糖异构酶用于将葡萄糖转化为更甜的果糖。
Lactase is added to milk to break down lactose for people who are lactose intolerant.
乳糖酶被添加到牛奶中，为乳糖不耐受人群分解乳糖。
In medicine, enzymes such as streptokinase are used to dissolve blood clots, and enzymes in diagnostic tests (e.g. glucose biosensors) help monitor blood sugar levels.
在医学上，链激酶等酶用于溶解血栓，诊断测试中的酶（如葡萄糖生物传感器）有助于监测血糖水平。
These industrial uses rely on the fact that enzymes are biodegradable, work under mild conditions, and are highly specific, producing fewer side products.
这些工业应用依赖于酶可生物降解、在温和条件下工作且高度专一，产生较少的副产物。
9. Summary | 总结
Enzymes are specific biological catalysts that speed up reactions by lowering the activation energy. Their activity is affected by temperature, pH, substrate concentration, enzyme concentration, and inhibitors.
酶是专一性的生物催化剂，通过降低活化能来加速反应。其活性受温度、pH、底物浓度、酶浓度和抑制剂的影响。
Understanding enzyme function is fundamental to biology — from cellular respiration to digestion, and from industrial biotechnology to medical treatments. In IGCSE Biology, you should be able to describe the lock-and-key model, interpret graphs of enzyme activity, and explain the effects of denaturation and inhibition.
理解酶的功能是生物学的基础——从细胞呼吸到消化，从工业生物技术到医学治疗。在 IGCSE 生物考试中，你应该能够描述锁钥模型、解读酶活性图表，并解释变性和抑制的影响。
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