How pH Affects Enzyme Activity | pH 对酶活性的影响

📚 How pH Affects Enzyme Activity | pH 对酶活性的影响

Enzymes are biological catalysts that speed up chemical reactions in living organisms. Their activity depends on several factors, and pH is one of the most important. This article explores how pH changes affect the shape and function of enzymes, and why each enzyme has an optimum pH.

酶是生物体内加速化学反应的生物催化剂。它们的活性受多种因素影响，而 pH 是最重要的因素之一。本文将探讨 pH 变化如何影响酶的形状和功能，以及为什么每种酶都有其最适 pH。

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1. What Are Enzymes? | 什么是酶？

Enzymes are proteins made of long chains of amino acids folded into a specific 3D shape. This shape includes a region called the active site, where substrate molecules bind and react. The unique fit between enzyme and substrate is often described as a ‘lock and key’ mechanism.

酶是由氨基酸长链折叠成特定三维形状的蛋白质。这个形状包括一个称为活性位点的区域，底物分子在此结合并发生反应。酶与底物之间独特的匹配关系常被描述为“锁钥”机制。

• Enzymes remain unchanged after the reaction, so they can be reused.

  酶在反应后保持不变，因此可以重复使用。

• Each enzyme catalyses only one type of reaction; this is called specificity.

  每种酶只催化一种类型的反应，这称为专一性。

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2. The Active Site and pH | 活性位点与 pH

The active site is a small pocket or groove on the enzyme surface. Its shape is maintained by forces such as hydrogen bonds and ionic bonds between amino acid side chains. These forces are sensitive to pH because pH changes the concentration of hydrogen ions (H⁺) and hydroxide ions (OH⁻).

活性位点是酶表面的一个小凹陷或沟槽。其形状依靠氨基酸侧链之间的氢键和离子键等作用力维持。这些作用力对 pH 敏感，因为 pH 改变氢离子（H⁺）和氢氧根离子（OH⁻）的浓度。

When pH falls or rises, the charges on amino acid side chains change. For example, a carboxyl group may lose H⁺ at high pH, becoming negatively charged. This alters the internal attractions that stabilise the enzyme’s 3D shape.

当 pH 降低或升高时，氨基酸侧链上的电荷会发生变化。例如，羧基在高 pH 下可能失去 H⁺，变成带负电。这会改变稳定酶三维形状的内部吸引力。

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3. How pH Changes Affect the Active Site | pH 改变如何影响活性位点

As the pH moves away from the optimum, the shape of the active site begins to change. The substrate can no longer bind as easily, so the rate of reaction decreases. This is not an all-or-nothing effect; even small pH shifts can reduce enzyme activity noticeably.

当 pH 偏离最适值时，活性位点的形状开始改变。底物不再那么容易结合，因此反应速率下降。这不是全有或全无的效果；即使很小的 pH 偏移也能显著降低酶活性。

• At slightly different pH, the enzyme-substrate complex forms less efficiently.

  在稍有不同的 pH 下，酶-底物复合物的形成效率降低。

• The binding strength between enzyme and substrate is weakened.

  酶与底物之间的结合力被削弱。

• Reaction rate falls progressively as pH moves further from the optimum.

  随着 pH 越来越偏离最适值，反应速率逐渐下降。

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4. The Optimum pH | 最适 pH

Every enzyme has an optimum pH – the pH at which its activity is highest. At this pH, the active site has the ideal shape to bind the substrate. For most human enzymes, the optimum is close to neutral pH (around 7), but this depends on the enzyme’s location and function.

每种酶都有一个最适 pH——即其活性最高时的 pH。在此 pH 下，活性位点具有理想的底物结合形状。对于大多数人体酶，最适 pH 接近中性（约 7），但这取决于酶的位置和功能。

Optimum pH = maximum rate of reaction

最适 pH = 最大反应速率

The graph of reaction rate against pH is typically bell-shaped, with a clear peak at the optimum.

反应速率对 pH 的曲线通常呈钟形，在最适 pH 处有明显峰值。

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5. Denaturation at Extreme pH | 极端 pH 下的变性

If the pH becomes too high or too low, the forces holding the enzyme’s 3D structure are completely broken. This process is called denaturation. The active site is permanently destroyed, and the enzyme loses its catalytic ability.

如果 pH 过高或过低，维持酶三维结构的作用力被完全破坏。这个过程称为变性。活性位点被永久破坏，酶失去催化能力。

• Denaturation is usually irreversible: the enzyme cannot regain its original shape.

  变性通常不可逆：酶无法恢复其原始形状。

• Extreme pH also affects the substrate, but the main effect on enzymes is their loss of structure.

  极端 pH 也影响底物，但对酶的主要影响是其结构丧失。

You should be careful to distinguish ‘reduced activity’ from ‘denaturation’. Reduced activity can be reversed by returning to the optimum pH, but denaturation cannot.

你要注意区分“活性降低”和“变性”。活性降低可以通过回到最适 pH 来恢复，但变性不能。

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6. Experimental Investigation: Amylase and pH | 实验探究：淀粉酶与 pH

A common experiment uses amylase to break down starch. A solution of starch is mixed with amylase at different pH buffers, and samples are tested with iodine solution at regular intervals. Iodine turns blue-black in the presence of starch, so the time taken for the colour to disappear indicates the reaction rate.

一个常见的实验使用淀粉酶分解淀粉。将淀粉溶液与不同 pH 缓冲液中的淀粉酶混合，并定期用碘液测试样品。碘在淀粉存在时变蓝黑色，因此颜色消失所需时间可指示反应速率。

• Use a range of pH values, e.g. pH 2, 4, 6, 7, 8, 10.

  使用一系列 pH 值，例如 pH 2、4、6、7、8、10。

• Keep temperature constant to ensure only pH is changed.

  保持温度恒定，以确保只改变 pH。

• The shortest time to lose the blue-black colour indicates the fastest reaction and thus the optimum pH.

  失去蓝黑色所需时间最短表明反应最快，也就是最适 pH。

Rate = 1 ÷ time taken for starch to disappear

反应速率 = 1 ÷ 淀粉消失所需时间

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7. Interpreting pH–Activity Graphs | 解读 pH-活性曲线

When analysing a graph of enzyme activity against pH, always identify the peak. The pH at the peak is the optimum. Also note how the curve falls on both sides – this tells you that activity decreases as pH moves away from the optimum, even before denaturation occurs.

在分析酶活性对 pH 的曲线时，一定要找到峰值。峰值对应的 pH 就是最适 pH。同时注意曲线两侧如何下降——这告诉你当 pH 偏离最适值时活性下降，甚至在变性发生之前就会下降。

Some graphs show activity falling to zero at extreme pH values. This indicates denaturation. If the graph only shows a small decrease, it likely represents a reversible change in activity.

有些曲线显示在极端 pH 下活性降为零。这表明变性。如果曲线只是略有下降，那很可能代表可逆的活性变化。

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8. Biological Examples of Different Optimum pH | 不同最适 pH 的生物学实例

Different environments in the body have different pH values, so enzymes have evolved to work best in their particular location.

体内不同环境具有不同 pH 值，因此酶已进化为在其特定位置最有效。

Enzyme | 酶	Location | 位置	Optimum pH | 最适 pH
Pepsin | 胃蛋白酶	Stomach | 胃	1.5 – 2.0
Salivary amylase | 唾液淀粉酶	Mouth | 口腔	6.5 – 7.5
Pancreatic lipase | 胰脂肪酶	Small intestine | 小肠	8.0 – 9.0

Pepsin works in the strongly acidic stomach, while intestinal enzymes work best in slightly alkaline conditions. This shows that optimum pH is not fixed but depends on the enzyme’s environment.

胃蛋白酶在强酸性胃中起作用，而肠道酶在微碱性条件下最佳。这表明最适 pH 不是固定的，而是取决于酶所处的环境。

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9. Exam Tips for pH and Enzyme Questions | 考试技巧：pH 与酶题目

In IGCSE exams, you may be asked to describe, explain, or predict the effect of pH on enzyme activity. Use precise scientific language and refer to the active site and denaturation.

在 IGCSE 考试中，你可能会被要求描述、解释或预测 pH 对酶活性的影响。请使用准确的科学语言，并提及活性位点和变性。

• Remember: pH affects the shape of the active site, not the substrate.

  记住：pH 影响的是活性位点的形状，而不是底物。

• Use the term ‘denatured’ only for permanent loss of enzyme function.

  只有在酶功能永久丧失时才使用“变性”一词。

• When drawing a graph, label axes with ‘Rate of reaction’ and ‘pH’, and plot a smooth bell-shaped curve.

  画图时，轴需标注“反应速率”和“pH”，并绘制平滑的钟形曲线。

• For experimental questions, state that temperature must be controlled as a variable.

  对于实验题，应说明温度必须作为控制变量。

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Published by TutorHao | Science Revision Series | aleveler.com

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