📚 1.4.1 Enzymes: Biological Catalysts | 1.4.1 酶：生物催化剂
Enzymes are globular proteins that act as biological catalysts, dramatically speeding up biochemical reactions without being consumed in the process. Understanding how enzymes work, the factors that affect their activity, and how they can be inhibited is central to many areas of biology and medicine.
酶是球状蛋白质，作为生物催化剂，能极大加速生化反应而自身不被消耗。理解酶的工作方式、影响其活性的因素以及如何被抑制，是生物学和医学许多领域的核心内容。
1. What Are Enzymes? | 什么是酶？
Enzymes are proteins with a specific three‑dimensional shape, which includes a region called the active site. They lower the activation energy of a reaction, enabling metabolic processes to occur at the relatively low temperatures found inside living cells. Without enzymes, most cellular reactions would be too slow to sustain life.
酶是具有特定三维形状的蛋白质，其中包含一个叫做活性位点的区域。它们降低反应的活化能，使代谢过程能够在细胞内相对较低的温度下进行。没有酶，大多数细胞反应会太慢而无法维持生命。
Enzymes are highly specific; each enzyme usually catalyses only one reaction or a group of closely related reactions. This specificity is due to the precise shape and chemical properties of the active site.
酶具有高度专一性；每种酶通常只催化一种反应或一组密切相关的反应。这种专一性源于活性位点精确的形状和化学性质。
2. Active Site and Specificity | 活性位点与专一性
The active site is a groove or pocket on the enzyme surface, formed by a few amino acid residues. Its shape is complementary to the shape of the substrate molecule. The interaction between enzyme and substrate can be compared to a key fitting into a lock.
活性位点是酶表面的凹槽或口袋，由几个氨基酸残基形成。它的形状与底物分子的形状互补。酶与底物之间的相互作用可以类比为钥匙插入锁中。
Enzyme–substrate specificity depends on hydrogen bonds, ionic interactions, and hydrophobic forces between the R‑groups of the active site and the substrate. Any change in the shape of the active site – for example, due to mutation or denaturation – can destroy catalytic activity.
酶与底物的专一性取决于活性位点的 R 基团与底物之间形成的氢键、离子相互作用和疏水力。活性位点形状的任何改变——例如由于突变或变性——都会破坏催化活性。
3. Lock‑and‑Key Model | 锁钥模型
The lock‑and‑key model proposes that the active site has a rigid shape that is exactly complementary to the substrate, like a lock that only accepts a specific key. Once the substrate binds, an enzyme–substrate complex is formed, the reaction takes place, and the products are released.
锁钥模型提出，活性位点具有刚性的形状，与底物完全互补，就像只有特定钥匙才能打开的锁。底物一旦结合，形成酶‑底物复合物，反应随即发生，产物被释放。
While this model explains enzyme specificity well, it fails to account for the ability of enzymes to catalyse the reverse reaction or the way the active site can slightly change shape upon substrate binding.
虽然这个模型很好地解释了酶的专一性，但它无法解释酶催化逆反应的能力，也无法说明活性位点在底物结合时如何发生轻微的形状变化。
4. Induced‑Fit Model | 诱导契合模型
The induced‑fit model is a refinement: the active site is not perfectly complementary to the substrate initially, but when the substrate approaches the enzyme, the shape of the active site alters slightly to mould around the substrate. This conformational change puts strain on bonds within the substrate, lowering the activation energy further.
诱导契合模型是一个更完善的模型：活性位点最初与底物并非完全互补，但当底物靠近酶时，活性位点的形状发生轻微改变，包裹住底物。这种构象变化对底物中的化学键施加压力，从而进一步降低活化能。
Induced fit also explains why many enzymes can work in both directions; the active site can adapt to accommodate the transition state of the forward or reverse reaction.
诱导契合也解释了为什么许多酶能双向工作；活性位点能够调整以适应正向或反向反应的过渡态。
5. Effect of Temperature | 温度的影响
As temperature increases, enzyme‑catalysed reactions initially speed up because molecules have more kinetic energy and collide more frequently. The rate approximately doubles for every 10 °C rise in temperature, up to an optimum point.
随着温度升高，酶促反应起初加速，因为分子具有更大的动能并且碰撞更频繁。在达到最适温度前，温度每升高 10 °C，反应速率大约翻倍。
Above the optimum temperature, the increased kinetic energy breaks the hydrogen bonds and hydrophobic interactions that maintain the enzyme’s tertiary structure. The active site becomes distorted – the enzyme denatures – and activity falls sharply. Most human enzymes have an optimum around 37–40 °C.
超过最适温度后，过高的动能会破坏维持酶三级结构的氢键和疏水相互作用。活性位点变形——酶变性——活性急剧下降。人类大多数酶的最适温度在 37–40 °C 左右。
6. Effect of pH | pH 的影响
pH measures the concentration of hydrogen ions (H⁺). Changes in pH alter the charge of the R‑groups in the active site. This affects ionic bonding and hydrogen bonding, which are crucial for maintaining the precise shape of the active site.
pH 值衡量氢离子 (H⁺) 的浓度。pH 的变化会改变活性位点内 R 基团的电荷，从而影响离子键和氢键，而这些键对维持活性位点的精确形状至关重要。
Each enzyme has an optimum pH at which its activity is maximal. For example, pepsin in the stomach works best at pH 2, while trypsin in the small intestine has an optimum around pH 8. Deviating far from the optimum pH causes denaturation and irreversible loss of activity.
每种酶都有一个使其活性最大的最适 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最适 pH 约为 8。偏离最适 pH 过远会引起变性，导致不可逆的活性丧失。
7. Effect of Substrate Concentration | 底物浓度的影响
At low substrate concentrations, many active sites are empty, and the rate of reaction increases almost linearly as substrate concentration rises. This is because more enzyme–substrate complexes can form per unit time.
在底物浓度较低时，许多活性位点空置，反应速率随底物浓度升高几乎线性增加。这是因为单位时间内可以形成更多的酶‑底物复合物。
As substrate concentration increases further, the rate of reaction eventually levels off. All active sites become occupied, and the enzyme is said to be saturated. At this point, adding more substrate does not increase the rate; the maximum velocity (Vₘₐₓ) has been reached.
当底物浓度继续增加，反应速率最终趋于平稳。所有活性位点均被占据，酶达到饱和。此时增加底物不再提高速率；已达到最大速率 (Vₘₐₓ)。
8. Enzyme Concentration | 酶浓度的影响
Provided there is an excess of substrate, the initial rate of an enzyme‑catalysed reaction is directly proportional to enzyme concentration. Doubling the enzyme concentration doubles the rate, because more active sites are available to bind substrate.
在底物过量的情况下，酶促反应的初速率与酶浓度成正比。酶浓度加倍，速率加倍，因为有更多的活性位点可结合底物。
If substrate is limiting, increasing enzyme concentration will have little effect once all substrate molecules are already being processed. This linear relationship is used to design experiments and to determine enzyme activity in samples.
如果底物有限，一旦所有的底物分子都已在处理中，再增加酶浓度也几乎没有效果。这种线性关系常被用于设计实验和测定样品中的酶活性。
9. Inhibitors – Competitive and Non‑competitive | 抑制剂：竞争性与非竞争性
Inhibitors are molecules that reduce enzyme activity. Competitive inhibitors have a shape similar to the substrate and bind to the active site, blocking the substrate. Their effect can be overcome by increasing substrate concentration.
抑制剂是降低酶活性的分子。竞争性抑制剂的形状与底物相似，与活性位点结合，从而阻挡底物。增加底物浓度可以克服其抑制作用。
Non‑competitive inhibitors bind to an allosteric site, not the active site. This changes the shape of the active site so the substrate can no longer bind effectively. Because they do not compete for the active site, increasing substrate concentration does not reverse their effect.
非竞争性抑制剂结合于别构位点而非活性位点，从而改变活性位点的形状，使底物无法有效结合。由于它们不与底物竞争活性位点，增加底物浓度无法逆转其作用。
| Feature | Competitive Inhibitor | Non‑competitive Inhibitor |
|---|---|---|
| Binding site | Active site | Allosteric site |
| Effect on Vₘₐₓ | Unchanged (can be reached at high [S]) | Lowered |
| Effect on Kₘ | Increased | Unchanged |
| Reversed by excess substrate? | Yes | No |
Table: Comparison of competitive and non‑competitive inhibitors | 表：竞争性与非竞争性抑制剂对比
10. Cofactors and Coenzymes | 辅因子与辅酶
Many enzymes require non‑protein helpers to function. Cofactors are inorganic ions such as Zn²⁺, Mg²⁺ or Fe²⁺ that bind to the enzyme and help to stabilise the enzyme–substrate complex or participate directly in the catalytic reaction.
许多酶需要非蛋白质辅助因子才能发挥作用。辅因子是无机离子，如 Zn²⁺、Mg²⁺ 或 Fe²⁺，它们与酶结合，帮助稳定酶‑底物复合物或直接参与催化反应。
Coenzymes are organic molecules, often derived from vitamins, that transport chemical groups between enzymes. For example, NAD⁺ carries hydrogen atoms in respiration, and coenzyme A carries acetyl groups.
辅酶是有机分子，通常来自维生素，在酶之间转运化学基团。例如，NAD⁺ 在呼吸作用中携带氢原子，辅酶 A 携带乙酰基。
11. Measuring the Rate of Reaction | 测量反应速率
The rate of an enzyme‑controlled reaction can be measured by monitoring either the disappearance of substrate or the appearance of product over time. Common methods include measuring the volume of gas produced, change in colour intensity using a colorimeter, or change in mass.
酶控反应速率可以通过监测底物的消失或产物随时间的生成来测量。常用方法包括测量产生气体的体积、使用比色计测量颜色强度变化，或测量质量变化。
It is important to measure the initial rate – the rate in the first few seconds of the reaction – because substrate concentration is still high and product inhibition has not yet occurred. The initial rate is found by drawing a tangent to the progress curve at time zero.
测量初速率——即反应最初几秒内的速率——非常重要，因为此时底物浓度仍然很高，且产物抑制尚未发生。初速率通过在反应进程曲线上于时间为零处绘制切线求得。
12. Calculating Initial Rate and Q₁₀ | 计算初速率与 Q₁₀
ope of the tangent at t = 0 gives the initial rate in units such as cm³ s⁻¹ or μmol min⁻¹.
计算初速率时，以产物生成量（或底物消耗量）对时间作图。t = 0 时刻切线的斜率即为初速率，单位如 cm³ s⁻¹ 或 μmol min⁻¹。
The temperature coefficient Q₁₀ shows how much the rate increases with a 10 °C temperature rise:
Q₁₀ = rate at (T + 10 °C) ÷ rate at T
For many enzyme reactions, Q₁₀ is about 2 below the optimum temperature. Above the optimum, Q₁₀ becomes less meaningful because denaturation dominates.
对于许多酶反应，在最适温度以下 Q₁₀ 约为 2。高于最适温度，Q₁₀ 意义不大，因为变性占主导。
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