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

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

Enzymes are biological catalysts that speed up chemical reactions in living organisms. They remain unchanged after the reaction, so they can be used again and again. Nearly every metabolic process in a cell requires enzymes.

酶是生物催化剂，能够加速生物体内的化学反应。它们在反应后保持不变，因此可以反复使用。细胞中几乎每一个代谢过程都需要酶。

1. What Are Enzymes? | 什么是酶？

Enzymes are globular proteins made from long chains of amino acids folded into a specific three-dimensional shape. This shape is crucial to the enzyme’s function because it contains a special region called the active site.

酶是由长链氨基酸折叠成特定三维形状的球状蛋白质。这种形状对酶的功能至关重要，因为它包含一个称为活性位点的特殊区域。

The active site is where the substrate binds. The substrate is the reactant that the enzyme acts on. When the substrate is attached, the enzyme catalyses the reaction and releases the products. For example, catalase breaks down hydrogen peroxide into water and oxygen.

活性位点是底物结合的区域。底物是酶作用的反应物。当底物结合后，酶催化反应并释放产物。例如，过氧化氢酶将过氧化氢分解为水和氧气。

Enzymes are highly specific. This means that each enzyme only catalyses one reaction or a small group of reactions, because only certain substrates can fit into the active site.

酶具有高度专一性。这意味着每种酶只催化一种反应或一小类反应，因为只有特定的底物能够进入活性位点。

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2. Lock and Key Model | 锁钥模型

Scientists use the lock and key model to explain how enzymes work. The enzyme is the lock and the substrate is the key. The active site has a fixed shape that is exactly complementary to the shape of the substrate.

科学家用锁钥模型来解释酶的工作原理。酶是锁，底物是钥匙。活性位点具有固定的形状，与底物的形状完全互补。

When the substrate enters the active site, it forms an enzyme-substrate complex. The reaction occurs quickly, and then the products leave the enzyme, which is ready to accept another substrate molecule.

当底物进入活性位点时，形成酶-底物复合物。反应快速发生，然后产物离开酶，此时酶可以接受另一个底物分子。

Some textbooks also mention an induced fit model, in which the active site changes shape slightly to improve the fit. For IGCSE Biology, the lock and key model is the one most commonly required.

一些教科书还提到诱导契合模型，即活性位点会稍微改变形状以更好地结合。对于IGCSE生物，锁钥模型是最常要求的。

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3. Factors Affecting Enzyme Activity | 影响酶活性的因素

The rate of an enzyme-controlled reaction depends on several conditions. The main factors are temperature, pH, substrate concentration, and enzyme concentration.

酶促反应速率取决于几个条件。主要因素是温度、pH、底物浓度和酶浓度。

• Temperature: higher temperature increases molecular movement, leading to more collisions, but too much heat destroys the enzyme.
• 温度：温度升高会增加分子运动，导致更多碰撞，但过多热量会破坏酶。
• pH: optimum pH gives the fastest reaction; extremes denature the enzyme.
• pH：最适pH时反应最快；极端pH会使酶变性。
• Substrate concentration: as substrate increases, rate increases up to saturation point.
• 底物浓度：底物增加，速率提高直到饱和点。
• Enzyme concentration: more enzyme molecules provide more active sites, increasing the rate.
• 酶浓度：更多的酶分子提供更多的活性位点，提高速率。

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

In a cold environment, molecules have low kinetic energy, so enzymes and substrates meet less frequently. As temperature increases, kinetic energy rises and the rate of reaction increases. For most human enzymes, the rate doubles for every 10 °C rise until the optimum of about 37 °C.

在寒冷环境中，分子动能低，因此酶与底物相遇的频率较低。随着温度升高，动能增加，反应速率加快。对大多数人体酶而言，每升高10 °C速率翻倍，直到约37 °C的最适温度。

Above the optimum temperature, the heat energy causes vibrations that break the hydrogen bonds and ionic bonds holding the enzyme in shape. The active site loses its precise shape and becomes denatured. The enzyme can no longer catalyse the reaction, and the rate drops sharply to zero.

超过最适温度后，热能引起的震动会破坏维持酶形状的氢键和离子键。活性位点失去精确形状，酶变性。酶不能再催化反应，速率急剧下降至零。

Optimum temperature is the temperature at which the enzyme works fastest.

最适温度是酶工作最快的温度。

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5. pH and Enzyme Activity | pH与酶活性

pH measures the acidity or alkalinity of a solution. Changes in pH alter the charges on amino acids and can break the bonds that maintain the enzyme’s structure. Each enzyme has an optimum pH where the reaction rate is highest.

pH衡量溶液的酸碱度。pH的变化会改变氨基酸上的电荷，并可能破坏维持酶结构的键。每种酶都有一个最适pH，此时反应速率最高。

For example, pepsin in the stomach works best at pH 2, where the acidic conditions help it to digest proteins. Trypsin and other enzymes in the small intestine work best in slightly alkaline conditions, around pH 8.

例如，胃中的胃蛋白酶在pH 2时活性最高，酸性条件有助于其消化蛋白质。小肠中的胰蛋白酶和其他酶在微碱性环境（约pH 8）下活性最高。

Extreme pH values denature the enzyme permanently by changing the shape of the active site. Unlike temperature, pH changes can be reversible if the pH is restored quickly, unless the enzyme has been severely damaged.

极端pH通过改变活性位点的形状使酶永久变性。与温度不同，如果pH迅速恢复，pH变化可能是可逆的，除非酶已严重受损。

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

When enzyme concentration is kept constant, increasing substrate concentration increases the rate of reaction. This is because more substrate molecules are present to fill the active sites.

当酶浓度保持不变时，增加底物浓度会

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