📚 Enzymes and Factors Affecting Enzyme Activity | 酶及影响酶活性的因素
Enzymes are biological catalysts that dramatically speed up chemical reactions in living organisms. Without enzymes, most metabolic reactions would occur far too slowly to sustain life. In IGCSE Edexcel Science, it is essential to understand how enzymes work, what factors affect their activity, and why these factors matter in both the human body and practical experiments.
酶是生物催化剂，能极大加速生物体内的化学反应。没有酶，大多数代谢反应会过于缓慢而无法维持生命。在IGCSE Edexcel科学课程中，理解酶的工作原理、影响其活性的因素以及这些因素在人体和实际实验中的重要性是必不可少的。
1. What Are Enzymes? | 什么是酶？
Enzymes are globular proteins that act as biological catalysts. A catalyst is a substance that speeds up a chemical reaction without being used up or permanently changed itself. Enzymes are highly specific, meaning each enzyme usually catalyses only one particular reaction or a group of very similar reactions.
酶是球状蛋白质，充当生物催化剂。催化剂是一种能加速化学反应而自身不被消耗或永久改变的物质。酶具有高度特异性，意味着每种酶通常只催化一种特定的反应或一组非常类似的反应。
The molecules that enzymes act upon are called substrates. The enzyme binds to its substrate at a special region known as the active site. The shape of the active site is complementary to the shape of the substrate, just like a lock and key. This ensures specificity.
酶作用的分子称为底物。酶在称为活性位点的特殊区域与底物结合。活性位点的形状与底物的形状互补，就像锁和钥匙一样。这保证了特异性。
2. Enzyme Structure and the Active Site | 酶的结构与活性位点
All enzymes have a complex three-dimensional shape held together by hydrogen bonds, ionic bonds, and disulfide bridges. This shape is crucial because the active site is a pocket or cleft on the enzyme surface formed by the folding of the polypeptide chain. The amino acid side chains inside the active site create a unique chemical environment that facilitates the binding of the substrate.
所有的酶都具有复杂的三维结构，由氢键、离子键和二硫桥维持。这种形状至关重要，因为活性位点是酶表面上的一个口袋或裂缝，由多肽链折叠形成。活性位点内的氨基酸侧链创造了独特的化学环境，有利于底物的结合。
If the enzyme’s shape is altered – for example, by high temperature or extreme pH – the active site can lose its precise shape. This process is called denaturation. Once denatured, the substrate can no longer fit, and the enzyme stops working. Denaturation is often irreversible.
如果酶的形状发生改变——例如高温或极端pH——活性位点会失去其精确的形状。这个过程称为变性。一旦变性，底物就无法再契合，酶就停止工作。变性通常不可逆。
3. Lock and Key Model vs Induced Fit Model | 锁钥模型与诱导契合模型
The original model of enzyme action is the lock and key model. In this explanation, the substrate fits exactly into the rigid active site, like a key into a lock. This model highlights the specificity of enzymes but does not explain how the enzyme stabilises the transition state.
最初的酶作用模型是锁钥模型。在这种解释中，底物精确地嵌入刚性的活性位点，就像钥匙插入锁中。该模型强调了酶的特异性，但没有解释酶如何稳定过渡态。
A more modern understanding is the induced fit model. Here, the active site is not completely rigid; it changes shape slightly as the substrate binds. The active site moulds around the substrate, putting stress on chemical bonds and lowering the activation energy even more effectively. This model is widely accepted today.
更现代的理解是诱导契合模型。在这种模型中，活性位点并非完全刚性；当底物结合时，它的形状会略微改变。活性位点围绕底物成型，对化学键施加压力，更有效地降低活化能。该模型如今被广泛接受。
4. Effect of Temperature on Enzyme Activity | 温度对酶活性的影响
Increasing the temperature generally increases the rate of an enzyme‑catalysed reaction, up to a certain point. This is because higher temperatures give the substrate and enzyme molecules more kinetic energy, so they collide more frequently and with greater energy. This increases the chance of forming enzyme‑substrate complexes.
升高温度通常会提高酶催化反应的速率，但只到某个点为止。这是因为较高的温度赋予底物和酶分子更多的动能，因此它们碰撞更频繁、更有力。这增加了形成酶‑底物复合物的机会。
However, if the temperature exceeds the enzyme’s optimum (often around 37–40 °C for human enzymes), the increased vibration breaks the hydrogen and ionic bonds maintaining the enzyme’s tertiary structure. The active site becomes distorted, the enzyme denatures, and the reaction rate falls sharply. Most enzymes denature irreversibly above about 60 °C.
然而，如果温度超过酶的最适温度（人类酶通常约为37–40 °C），增加的振动会破坏维持酶三级结构的氢键和离子键。活性位点变形，酶变性，反应速率急剧下降。大多数酶在约60 °C以上不可逆变性。
A graph of reaction rate against temperature shows a steep rise to a peak (the optimum temperature) followed by a rapid decline. The Q₁₀ temperature coefficient is often about 2, meaning the rate doubles for a 10 °C rise, but only before denaturation begins.
反应速率对温度的关系曲线显示，先急剧上升到峰值（最适温度），然后迅速下降。Q₁₀温度系数通常约为2，意味着温度每升高10 °C，速率加倍，但这仅发生在变性开始之前。
5. Effect of pH on Enzyme Activity | pH对酶活性的影响
Every enzyme has an optimum pH at which it works best. For example, pepsin, a stomach enzyme, works best at pH 2, whereas trypsin, found in the small intestine, has an optimum around pH 8. Most intracellular enzymes have optimum pH values near neutral (pH 7).
每种酶都有一个最佳工作的最适pH值。例如，胃蛋白酶在pH 2时活性最高，而小肠中的胰蛋白酶最适pH约为8。大多数胞内酶的最适pH接近中性（pH 7）。
If the pH moves too far from the optimum, the charges on the amino acid side chains in the active site are altered. This disrupts the ionic and hydrogen bonds that maintain the precise shape. The enzyme denatures and the reaction rate decreases sharply. The graph of rate against pH is a symmetrical bell‑shaped curve.
如果pH偏离最适值太远，活性位点中氨基酸侧链上的电荷会发生改变。这会破坏维持精确形状的离子键和氢键。酶变性，反应速率急剧下降。速率对pH的曲线呈对称的钟形。
6. Substrate Concentration | 底物浓度
At low substrate concentrations, the rate of reaction increases almost linearly with substrate concentration. This is because many active sites are available, and adding more substrate increases the chance of enzyme‑substrate collisions and complex formation.
在低底物浓度时，反应速率几乎随底物浓度线性增加。这是因为有许多活性位点可用，增加底物能提高酶‑底物碰撞和复合物形成的机会。
As the substrate concentration continues to rise, the rate of increase slows down. Eventually, all enzyme active sites become saturated with substrate. At this saturation point, the reaction rate reaches a maximum (Vmax), and adding more substrate has no effect. The curve is a rectangular hyperbola.
随着底物浓度继续增加，速率增长放缓。最终，所有酶的活性位点都被底物饱和。在此饱和点，反应速率达到最大值（Vmax），再增加底物也没有效果。该曲线为矩形双曲线。
7. Enzyme Concentration | 酶浓度
When there is an excess of substrate, the rate of reaction is directly proportional to the enzyme concentration. Doubling the enzyme concentration doubles the reaction rate, because more active sites are available to bind substrate.
当底物过量时，反应速率与酶浓度成正比。酶浓度加倍，反应速率也加倍，因为有更多的活性位点可结合底物。
This relationship holds true provided that substrate molecules are not limiting. If substrate concentration is low, increasing enzyme concentration will have less effect because the active sites will not all be occupied.
只要底物分子不是限制因素，这种关系就成立。如果底物浓度低，增加酶浓度的效果会较小，因为活性位点不会全部被占据。
8. Inhibitors | 抑制剂
Inhibitors are substances that reduce or prevent enzyme activity. Competitive inhibitors have a shape similar to the substrate and compete for the active site. They can be overcome by increasing substrate concentration. Their effect depends on the relative concentrations of inhibitor and substrate.
抑制剂是能够降低或阻止酶活性的物质。竞争性抑制剂的形状与底物相似，竞争活性位点。可以通过增加底物浓度来克服它们的影响。其效果取决于抑制剂和底物的相对浓度。
Non‑competitive inhibitors bind to a different site on the enzyme (allosteric site), changing the shape of the active site so the substrate can no longer bind. Increasing substrate concentration does not overcome this type of inhibition. Metabolic poisons like cyanide are non‑competitive inhibitors of cytochrome oxidase.
非竞争性抑制剂结合到酶的不同位点（别构位点），改变活性位点形状，使底物无法结合。增加底物浓度无法克服这类抑制。氰化物等代谢毒物是细胞色素氧化酶的非竞争性抑制剂。
9. Cofactors and Coenzymes | 辅因子与辅酶
Some enzymes require additional non‑protein molecules to function. These are called cofactors. Cofactors can be metal ions, such as Mg²⁺ for DNA polymerase or Zn²⁺ for carbonic anhydrase. They often help orient the substrate or participate directly in the chemical reaction.
一些酶需要额外的非蛋白分子才能发挥作用，这些分子称为辅因子。辅因子可以是金属离子，如DNA聚合酶需要的Mg²⁺，或碳酸酐酶需要的Zn²⁺。它们通常帮助定向底物或直接参与化学反应。
Coenzymes are a type of cofactor that are organic molecules, often derived from vitamins. NAD (nicotinamide adenine dinucleotide) is a coenzyme that carries hydrogen atoms in respiration. Coenzymes bind loosely to the enzyme and can be reused.
辅酶是一类有机分子辅因子，通常来源于维生素。NAD（烟酰胺腺嘌呤二核苷酸）是一种在呼吸作用中传递氢原子的辅酶。辅酶松散地与酶结合，可以重复使用。
10. Experimental Investigation: Effect of Temperature on Amylase | 实验探究：温度对淀粉酶的影响
A common IGCSE practical involves investigating how temperature affects the activity of amylase, an enzyme that breaks down starch into maltose. The reaction can be followed using iodine solution: starch gives a blue‑black colour, but as it is digested, the colour disappears. The time taken for the colour to disappear indicates the rate.
常见的IGCSE实验涉及探究温度如何影响淀粉酶活性，淀粉酶将淀粉分解为麦芽糖。可用碘液跟踪反应：淀粉呈蓝黑色，但被消化后颜色消失。颜色消失所需的时间表示速率。
Set up water baths at a range of temperatures, such as 0, 20, 30, 40, 50, 60 °C. Add a fixed volume of 1% starch solution and 1% amylase solution, both pre‑incubated at the desired temperature. Every 30 seconds, remove a drop and add iodine. Record the time until no blue‑black colour appears. A graph of rate (1/time) against temperature shows the optimum near 37–40 °C.
设置一系列温度的水浴，如0、20、30、40、50、60 °C。加入固定体积的1%淀粉溶液和1%淀粉酶溶液，两者均已在所需温度下预温。每30秒取一滴，加入碘液。记录直到不再出现蓝黑色的时间。绘制速率（1/时间）对温度的关系图，显示最适温度约为37–40 °C。
Key control variables: pH (use a buffer), starch and enzyme concentrations, and volumes. It is crucial to equilibrate solutions before mixing to avoid immediate denaturation at high temperatures.
关键控制变量：pH（使用缓冲液）、淀粉和酶浓度以及体积。混合前必须平衡溶液温度，以免在高温下立即变性。
11. Denaturation | 变性
Denaturation is the permanent loss of the enzyme’s specific three‑dimensional shape. It occurs when the weak bonds maintaining the folded structure break. The primary structure (sequence of amino acids) remains intact, but the active site is no longer functional. Denatured enzymes cannot catalyse reactions.
变性指酶失去特定的三维形状，是永久性的。当维持折叠结构的弱键断裂时发生变性。一级结构（氨基酸序列）保持完整，但活性位点不再有功能。变性酶无法催化反应。
Causes of denaturation include extremes of temperature and pH, heavy metal ions (such as Pb²⁺, Hg²⁺), and organic solvents. Denaturation does not break peptide bonds, so the enzyme’s amino acid chain remains unchanged chemically, but its biological activity is destroyed.
变性的原因包括极端温度和pH、重金属离子（如Pb²⁺、Hg²⁺）以及有机溶剂。变性不会断裂肽键，因此酶的氨基酸链在化学上不变，但其生物活性遭到破坏。
12. Industrial Uses of Enzymes | 酶在工业中的应用
Enzymes are widely used in industry because they are specific, work at moderate temperatures, and are biodegradable. Proteases are used in biological washing powders to break down protein stains such as blood and egg. Lipases break down fat stains.
酶因其特异性、在中温下工作且可生物降解的特性，被广泛应用于工业。蛋白酶用于生物洗衣粉，分解血渍和蛋渍等蛋白质污渍。脂肪酶分解脂肪污渍。
In the food industry, pectinases are used to clarify fruit juices, and lactase converts lactose into glucose and galactose to produce lactose‑free milk. Amylases are used in baking and brewing. Immobilised enzymes, attached to an inert support, allow continuous processing and easy reuse without contamination.
在食品工业中，果胶酶用于澄清果汁，乳糖酶将乳糖转化为葡萄糖和半乳糖，以生产无乳糖牛奶。淀粉酶用于烘焙和酿造。固定化酶附着在惰性载体上，可实现连续加工，并易于重复使用而不污染产品。
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