📚 Enzymes: Catalysts of Life | 酶：生命的催化剂
Enzymes are biological catalysts that speed up chemical reactions in living organisms without being used up themselves. They are essential for processes such as digestion, respiration and photosynthesis.
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。它们对消化、呼吸和光合作用等过程至关重要。
1. What Are Enzymes? | 什么是酶？
Enzymes are proteins made up of chains of amino acids. Each enzyme has a unique three-dimensional shape, which is critical to its function.
酶是由氨基酸链组成的蛋白质。每种酶都有独特的三维结构，这对它的功能至关重要。
Only a small amount of enzyme is needed to catalyse a reaction because enzymes are recycled and can be used again and again.
只需要少量酶就能催化反应，因为酶会被循环利用，可以反复使用。
- Enzymes lower the activation energy of a reaction. 酶能降低反应的活化能。
- They are specific: one enzyme usually works on one type of substrate. 它们具有特异性：一种酶通常只作用于一种类型的底物。
2. The Active Site and Specificity | 活性位点与特异性
The active site is a small region on the enzyme's surface where the substrate binds. The shape of the active site is complementary to the shape of the substrate.
活性位点是酶表面一个小的区域，底物在此处结合。活性位点的形状与底物的形状互补。
Because the active site has a specific shape, only the correct substrate can fit into it. This property is called enzyme specificity.
由于活性位点具有特定的形状，只有正确的底物才能嵌入其中。这种性质称为酶的专一性。
3. Lock and Key vs Induced Fit | 锁钥模型与诱导契合模型
The lock-and-key model suggests that the enzyme active site is rigid and perfectly matches the substrate, just like a key fits into a lock.
锁钥模型认为酶的活性位点是刚性的，与底物完美匹配，就像钥匙插入锁中一样。
However, the induced-fit model is more accurate. It proposes that the active site is flexible and changes shape slightly when the substrate binds, leading to a tighter fit.
然而，诱导契合模型更为准确。它提出活性位点是柔性的，当底物结合时形状会发生轻微改变，从而实现更紧密的结合。
The induced-fit model helps explain how enzymes can stabilise the transition state and reduce activation energy.
诱导契合模型有助于解释酶如何稳定过渡态并降低活化能。
4. How Enzymes Speed Up Reactions | 酶如何加快反应速率
Enzymes reduce the activation energy needed for a reaction to start. They provide an alternative pathway for the reaction, making it happen much faster.
酶降低反应启动所需的活化能。它们为反应提供了另一条途径，使反应发生得更快。
Enzymes do not change the overall energy change of the reaction, nor do they alter the final products. They simply lower the energy barrier.
酶不改变反应的总体能量变化，也不改变最终产物。它们只是降低了能量屏障。
Substrate + Enzyme → Enzyme-Substrate Complex → Product + Enzyme
底物 + 酶 → 酶-底物复合物 → 产物 + 酶
5. Factors Affecting Enzyme Activity: Temperature | 影响酶活性的因素：温度
As temperature increases, particles have more kinetic energy and collide more frequently, so the rate of reaction increases.
随着温度升高，粒子具有更多动能，碰撞更频繁，因此反应速率增加。
However, beyond the optimum temperature (often 37°C in humans), the enzyme's three-dimensional structure begins to denature. The active site changes shape and the enzyme can no longer bind to the substrate.
然而，超过最适温度（人类通常为37°C）后，酶的三维结构开始变性。活性位点形状改变，酶不再能结合底物。
Denaturation is permanent if the temperature is too high. The rate of reaction falls sharply.
如果温度过高，变性是不可逆的。反应速率急剧下降。
6. Factors Affecting Enzyme Activity: pH | 影响酶活性的因素：pH
Each enzyme has an optimum pH at which it works best. Most human enzymes work best at pH 7 (neutral), but pepsin in the stomach works best at pH 2.
每种酶都有其最适pH，在该pH下活性最高。大多数人体酶在pH 7（中性）时活性最高，但胃中的胃蛋白酶在pH 2时活性最高。
Extreme pH levels can break the ionic bonds and hydrogen bonds that hold the protein structure together, causing denaturation.
极端的pH值会破坏维持蛋白质结构的离子键和氢键，导致变性。
Near the optimum, small changes in pH can make the enzyme less efficient, but if it returns to the optimum pH, activity may be restored (unless denatured).
在最适pH附近，pH的微小变化会使酶的效率降低，但如果恢复到最适pH，活性可能会恢复（除非已经变性）。
7. Substrate Concentration and Enzyme Concentration | 底物浓度与酶浓度
When the enzyme concentration is constant, increasing substrate concentration increases the rate of reaction until all active sites are occupied. After that, the rate plateaus.
当酶浓度恒定时，增加底物浓度会提高反应速率，直到所有活性位点都被占据。之后速率保持不变。
When the substrate concentration is in excess, increasing enzyme concentration will increase the rate proportionally, as long as other factors are not limiting.
当底物过量时，增加酶浓度会使反应速率按比例增加，只要其他因素不受限。
The limiting factor determines how fast the reaction can go. To get a faster rate, you must remove the limiting factor.
限速因素决定反应能有多快。要使反应更快，必须解除限速因素。
8. Inhibitors: Competitive and Non-competitive | 抑制剂：竞争性抑制与非竞争性抑制
Competitive inhibitors have a similar shape to the substrate and compete for the active site. They can be overcome by increasing the substrate concentration.
竞争性抑制剂与底物形状相似，会竞争活性位点。可以通过增加底物浓度来克服其作用。
Non-competitive inhibitors bind to a different site on the enzyme (called the allosteric site) and change the shape of the active site. They cannot be overcome by adding more substrate.
非竞争性抑制剂结合在酶的不同位点（称为别构位点），改变活性位点的形状。它们不能通过增加底物浓度来克服。
Inhibitors can be reversible or irreversible. Irreversible inhibitors permanently block enzyme function.
抑制剂可以是可逆的或不可逆的。不可逆抑制剂会永久性地阻断酶的功能。
9. Enzymes in Industry and Daily Life | 酶在工业与日常生活中的应用
Biological detergents contain enzymes such as lipases and proteases to break down fat and protein stains.
生物洗衣粉含有脂肪酶和蛋白酶等酶，用于分解油脂和蛋白质污渍。
Proteases are used in meat tenderisers to break down proteins, making meat softer.
蛋白酶用于嫩肉粉中，分解蛋白质，使肉更嫩。
Isomerases are used to convert glucose into fructose for sweeter food offerings without using as much sugar.
异构酶用于将葡萄糖转化为果糖，在不使用太多糖的情况下提供更甜的食物。
Understanding enzymes also helps in medicine, for example in diagnosing conditions: high levels of certain enzymes in blood can indicate tissue damage.
了解酶也有助于医学诊断，例如血液中某些酶水平升高可能表明组织损伤。
10. Required Practical: Investigating Enzyme Activity | 必做实验：探究酶的活性
In the Edexcel IGCSE course, a common practical is to investigate the effect of pH or temperature on the rate of an enzyme-catalysed reaction, such as the breakdown of starch by amylase.
在Edexcel IGCSE课程中，常见实验是探究pH或温度对酶催化反应速率的影响，例如淀粉酶分解淀粉的反应。
Iodine solution is used to test for starch: It turns blue-black in the presence of starch. The time taken for the iodine to stay orange-brown indicates complete starch digestion.
使用碘液检测淀粉：遇淀粉变蓝黑色。碘液保持橙棕色所需的时间说明淀粉完全分解。
A buffer solution can be used to keep the pH constant. A water bath is used to control temperature.
可以使用缓冲溶液保持pH恒定，使用水浴锅控制温度。
Repeat the experiment at several pH or temperature values, and calculate the rate as 1/time.
在多个pH或温度条件下重复实验，并计算速率：1/时间。
Rate = 1 / Time taken
速率 = 1 ÷ 所用时间
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