📚 Proteins Revision for IGCSE WJEC Biology | IGCSE WJEC 生物：蛋白质 考点精讲
Proteins are some of the most versatile and essential macromolecules found in all living organisms. Made up of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur, they act as enzymes, structural components, transport molecules, hormones, and antibodies. A thorough understanding of protein structure and function is key for the IGCSE WJEC Biology exam, as questions often link the shape of a protein to its role, and explain how denaturation leads to loss of function.
蛋白质是所有生命体中最具多样性和必需性的大分子之一。它们由碳、氢、氧、氮，有时还有硫组成，充当酶、结构组分、运输分子、激素和抗体。透彻理解蛋白质的结构和功能是 IGCSE WJEC 生物考试的关键，因为题目经常将蛋白质的形状与其作用联系起来，并解释变性如何导致功能丧失。
1. Introduction to Proteins | 蛋白质简介
Proteins are polymers made from monomers called amino acids. They are present in every cell and perform a dazzling array of tasks, from speeding up chemical reactions to providing mechanical support. The word 'protein' comes from the Greek word 'proteios', meaning 'primary' or 'first', reflecting their fundamental importance.
蛋白质是由称为氨基酸的单体组成的聚合物。它们存在于每个细胞中，执行着从加速化学反应到提供机械支撑等令人眼花缭乱的任务。“蛋白质”一词源自希腊语“proteios”，意为“首要的”，反映了它们的基本重要性。
In the human body alone, there are an estimated 100,000 different proteins, each with a unique three-dimensional shape that determines its specific function. The relationship between structure and function is a central theme in WJEC assessment: if the shape is altered, the protein stops working properly.
仅在人体内，估计就有 100,000 种不同的蛋白质，每种都具有独特的三维形状，决定其特定功能。结构与功能之间的关系是 WJEC 评估的核心主题：如果形状改变，蛋白质就会停止正常工作。
2. Amino Acids: The Building Blocks | 氨基酸：构建单元
All proteins are built from a set of 20 standard amino acids. Despite their variety, every amino acid shares a common structure: a central carbon atom (the alpha carbon) bonded to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a variable 'R' group. The R group, also called the side chain, is different in each amino acid and gives it unique chemical properties.
所有蛋白质都由 20 种标准氨基酸构成。尽管种类繁多，但每种氨基酸都有一个共同结构：一个中心碳原子（α碳）连接一个氨基（–NH₂）、一个羧基（–COOH）、一个氢原子和一个可变的“R”基团。R 基团也称为侧链，在每个氨基酸中各不相同，并赋予其独特的化学性质。
NH₂ – CH(R) – COOH
氨基酸通式
For example, in the simplest amino acid, glycine, the R group is just a single hydrogen atom. In alanine, the R group is –CH₃. Some amino acids have hydrophobic R groups, while others are hydrophilic or carry a charge, influencing how the final protein folds.
例如，在最简单的氨基酸甘氨酸中，R 基团只是一个氢原子。在丙氨酸中，R 基团是 –CH₃。有些氨基酸具有疏水性 R 基团，而另一些则是亲水性的或带有电荷，这会影响最终蛋白质的折叠方式。
3. Peptide Bond Formation | 肽键形成
Amino acids link together through a condensation reaction. The –OH from the carboxyl group of one amino acid and a hydrogen atom from the amino group of another amino acid are removed, forming a water molecule. The resulting covalent bond between the carbon and nitrogen atoms is called a peptide bond (–CO–NH–). When only two amino acids join, the product is a dipeptide; longer chains are called polypeptides.
氨基酸通过缩合反应连接在一起。一个氨基酸羧基中的 –OH 和另一个氨基酸氨基中的一个氢原子被移除，形成一分子水。碳原子和氮原子之间产生的共价键称为肽键（–CO–NH–）。当只有两个氨基酸连接时，产物称为二肽；更长的链称为多肽。
Amino acid 1 + Amino acid 2 → Dipeptide + H₂O
氨基酸 1 + 氨基酸 2 → 二肽 + 水
This process is repeated hundreds or thousands of times, always at the ribosomes inside living cells. The sequence of amino acids is determined by the genetic code carried by mRNA, ensuring that each protein is built precisely according to the instructions in DNA.
这一过程在活细胞内的核糖体上重复成百上千次。氨基酸序列由 mRNA 携带的遗传密码决定，确保每种蛋白质严格按照 DNA 中的指令精准构建。
4. Levels of Protein Structure | 蛋白质的结构层次
Proteins are not simply straight chains of amino acids. They fold into complex, three-dimensional shapes organised into four recognised levels: primary, secondary, tertiary, and sometimes quaternary structure. Understanding these levels helps explain why a protein's shape is so easily damaged by heat or extreme pH.
蛋白质不仅仅是氨基酸的直线链。它们折叠成复杂的三维形状，可分为四个公认的结构层次：一级、二级、三级，有时还有四级结构。理解这些层次有助于解释为什么蛋白质的形状很容易被高温或极端 pH 破坏。
Each level of structure is stabilised by different types of chemical bonds. The primary structure relies on peptide bonds, while the higher levels involve hydrogen bonds, ionic interactions, hydrophobic forces, and disulfide bridges. Disruption of any of these bonds can unravel the protein.
每个结构层次由不同类型的化学键维持稳定。一级结构依靠肽键，而更高级层次涉及氢键、离子相互作用、疏水力以及二硫桥。这些键中任何一个被破坏都可能导致蛋白质解折叠。
5. Primary Structure | 一级结构
The primary structure is simply the linear sequence of amino acids in a polypeptide chain, held together by strong covalent peptide bonds. This sequence is encoded by a gene and is unique to each protein. Even a single amino acid substitution can have a dramatic effect – a famous example is sickle-cell disease, where one amino acid change in haemoglobin causes the protein to aggregate incorrectly.
一级结构仅仅是多肽链中氨基酸的线性序列，由牢固的共价肽键连接在一起。这个序列由基因编码，每种蛋白质都独一无二。即使单一氨基酸的替换也可能产生剧烈影响——一个著名例子是镰刀形细胞病，血红蛋白中一个氨基酸的改变导致蛋白质异常聚集。
Importantly, denaturation does not break peptide bonds, so the primary sequence remains intact when a protein is unfolded. This is why the Biuret test, which detects peptide bonds, still gives a positive result after cooking an egg.
重要的是，变性不会破坏肽键，因此当蛋白质解折叠时，一级序列保持完整。这就是为什么双缩脲试验（检测肽键）在鸡蛋煮熟后仍能给出阳性结果。
6. Secondary Structure | 二级结构
ctures are the alpha-helix and the beta-pleated sheet. These structures are held in place by hydrogen bonds between the –NH group of one peptide bond and the –C=O group of another.
二级结构指多肽骨架局部折叠成规则的重复模式。两种最常见的二级结构是 α-螺旋和 β-折叠片。这些结构由一条肽键的 –NH 基团与另一条肽键的 –C=O 基团之间的氢键维持。
- Alpha-helix: A right-handed coil where hydrogen bonds form between every fourth amino acid, making the structure elastic and strong. Found in keratin, the protein of hair and nails.
- α-螺旋：一种右手螺旋，每第四个氨基酸之间形成氢键，使结构具有弹性和强度。存在于头发和指甲的角蛋白中。
- Beta-pleated sheet: Polypeptide chains lie side by side, linked by hydrogen bonds. The resulting material is flexible but resistant to stretching. Silk fibroin is a classic example.
- β-折叠片：多肽链并排排列，由氢键连接。所得材料柔韧但抗拉伸。丝心蛋白就是一个经典例子。
7. Tertiary Structure | 三级结构
The tertiary structure is the overall three-dimensional conformation of a single polypeptide chain. It is formed when the secondary structural elements fold further, driven by interactions among the R groups. This level of folding is critical because it creates the specific shape required for a protein's function, such as the active site of an enzyme or the binding pocket of a hormone receptor.
三级结构是单条多肽链的整体三维构象。当二级结构元件在 R 基团间相互作用的驱动下进一步折叠时形成。这一折叠层次至关重要，因为它创造了蛋白质功能所需的特定形状，例如酶的活性位点或激素受体的结合口袋。
Several types of bonds and forces maintain the tertiary structure:
- Hydrogen bonds – between polar R groups.
- 氢键 – 位于极性 R 基团之间。
- Ionic bonds – between positively and negatively charged R groups (e.g., –NH₃⁺ and –COO⁻).
- 离子键 – 位于带正电和带负电的 R 基团之间（如 –NH₃⁺ 和 –COO⁻）。
- Hydrophobic interactions – non-polar R groups cluster away from water, inside the protein.
- 疏水相互作用 – 非极性 R 基团远离水，聚集在蛋白质内部。
- Disulfide bridges (–S–S–) – strong covalent bonds between the sulfur atoms of two cysteine amino acids, which lock the folding in place.
- 二硫桥 (–S–S–) – 两个半胱氨酸氨基酸的硫原子之间的强共价键，锁定折叠位置。
8. Fibrous vs. Globular Proteins | 纤维蛋白与球状蛋白
Proteins can be broadly classified into two structural types: fibrous and globular. Fibrous proteins have long, thin, thread-like shapes, are insoluble in water, and primarily serve structural roles. Globular proteins have roughly spherical shapes, are usually soluble, and are involved in metabolic processes.
蛋白质可根据结构大致分为两类：纤维蛋白和球状蛋白。纤维蛋白形状细长如线索，不溶于水，主要起结构作用。球状蛋白大致呈球形，通常可溶，参与代谢过程。
The key differences are summarised in the table below and are important to recall in exam questions that ask you to explain why haemoglobin is soluble but collagen is not.
下表总结了关键区别，在要求解释为什么血红蛋白可溶而胶原蛋白不溶的考试题目中，务必牢记。
| Feature | 特征 | Fibrous | 纤维状 | Globular | 球状 |
|---|---|---|
| Shape | 形状 | Long, narrow strands | Compact, roughly spherical |
| Solubility | 溶解性 | Insoluble in water | Soluble in water |
| Function | 功能 | Structural (e.g., collagen in bones, keratin in hair) | Metabolic (e.g., enzymes, haemoglobin, insulin) |
| Examples | 例子 | Collagen, keratin, elastin | Amylase, haemoglobin, antibodies |
Both types are made of the same amino acids, but the sequence and folding differ dramatically. Fibrous proteins often have repetitive amino acid sequences that promote extensive secondary structure, whereas globular proteins have a more irregular tertiary structure that combines both secondary elements and loops to create a compact, water-soluble surface.
两种类型均由相同的氨基酸构成，但序列和折叠方式差异显著。纤维蛋白通常具有重复的氨基酸序列，促进广泛的二级结构；而球状蛋白的三级结构更不规则，结合二级结构元件和环状区域，形成致密的水溶性表面。
9. Functions of Proteins | 蛋白质的功能
Proteins are the 'do-ers' of the cell. Their incredibly diverse shapes allow them to carry out a wide range of functions. In the WJEC specification, you should be able to link specific named proteins to their roles.
蛋白质是细胞内的“实干家”。它们极其多样的形状使其能够执行广泛的功能。在 WJEC 大纲中，你应该能够将具体命名的蛋白质与其作用联系起来。
Enzymes are biological catalysts that speed up metabolic reactions without being used up. Examples include amylase, which breaks down starch in the mouth and small intestine, and catalase, which splits hydrogen peroxide in liver cells.
酶是生物催化剂，能加速代谢反应而本身不被消耗。例子包括唾液和小肠中分解淀粉的淀粉酶，以及肝细胞中分解过氧化氢的过氧化氢酶。
Structural proteins provide support. Collagen, the most abundant protein in mammals, forms strong fibres in tendons, ligaments, and skin. Keratin makes up hair, nails, and the outer layer of skin, providing toughness and waterproofing.
结构蛋白提供支撑。胶原蛋白是哺乳动物中最丰富的蛋白质，在肌腱、韧带和皮肤中形成强韧的纤维。角蛋白构成头发、指甲和皮肤外层，提供韧性和防水性。
Transport proteins carry substances around the body. Haemoglobin in red blood cells binds oxygen in the lungs and releases it at respiring tissues. Carrier proteins in cell membranes transport molecules such as glucose into cells via facilitated diffusion or active transport.
运输蛋白在体内运输物质。红细胞中的血红蛋白在肺中结合氧气，并在呼吸组织中释放。细胞膜中的载体蛋白通过协助扩散或主动运输将葡萄糖等分子运入细胞。
Hormones and receptors trigger responses. Insulin is a small protein hormone that lowers blood glucose by promoting the uptake of glucose into cells. Receptor proteins embedded in the cell membrane recognise and bind specific signalling molecules, initiating a cellular response.
激素和受体触发反应。胰岛素是一种小蛋白质激素，通过促进细胞摄取葡萄糖来降低血糖。嵌入细胞膜的受体蛋白识别并结合特定信号分子，启动细胞反应。
Defence proteins protect the body. Antibodies (immunoglobulins) are Y-shaped proteins produced by white blood cells that bind specifically to antigens on pathogens, marking them for destruction. Fibrinogen is converted to fibrin during blood clotting, creating a mesh that traps platelets and red blood cells.
防御蛋白保护身体。抗体（免疫球蛋白）是由白细胞产生的 Y 形蛋白质，能特异性结合病原体上的抗原，标记它们以便摧毁。纤维蛋白原在凝血时转化为纤维蛋白，形成捕捉血小板和红细胞的网状结构。
10. Enzymes as Proteins | 酶作为蛋白质
All enzymes are globular proteins, and their catalytic ability depends entirely on their precise three-dimensional tertiary structure. Each enzyme has an active site – a pocket or groove shaped only to accept a specific substrate, like a lock and key. If the active site loses its shape, the enzyme can no longer fit its substrate and becomes inactive.
所有酶都是球状蛋白，其催化能力完全取决于精确的三维三级结构。每种酶都有一个活性位点——一个口袋或沟槽，其形状专门用来接纳特定的底物，如同锁和钥匙。如果活性位点失去形状，酶就无法再与底物匹配，从而失活。
The lock-and-key model (and the induced-fit model in more advanced courses) emphasises that enzyme specificity arises from the complementary shape between the active site amino acids and the substrate. Changes in temperature or pH that disrupt the weak bonds maintaining the active site will denature the enzyme, irreversibly damaging its shape and stopping the reaction.
锁钥模型（以及更高级课程中的诱导契合模型）强调，酶的特异性来源于活性位点氨基酸与底物之间的互补形状。破坏维持活性位点弱键的温度或 pH 变化会使酶变性，不可逆地破坏其形状并终止反应。
For most human enzymes, the optimum temperature is around 37 °C, and at very high temperatures (above about 50 °C) the protein unfolds rapidly. Similarly, extreme pH values alter the charges on R groups, breaking ionic bonds and hydrogen bonds, which causes denaturation.
对大多数人体酶而言，最适温度约为 37 °C，在极高温度（约 50 °C 以上）下蛋白质会迅速解折叠。同样，极端 pH 值会改变 R 基团的电荷，破坏离子键和氢键，从而导致变性。
11. Denaturation of Proteins | 蛋白质变性
Denaturation is the permanent loss of a protein's specific three-dimensional shape without breaking its peptide bonds. When a protein denatures, the weak interactions – hydrogen bonds, ionic bonds, and hydrophobic forces – are disrupted, so the tertiary and secondary structures unravel. The protein becomes non-functional because its shape no longer matches its target.
变性是蛋白质特定三维形状的永久丧失，且不破坏其肽键。当蛋白质变性时，弱相互作用——氢键、离子键和疏水力——被破坏，导致三级和二级结构解折叠。蛋白质因形状不再与其靶标匹配而丧失功能。
Two main agents cause denaturation: heat and extremes of pH. Heating provides enough kinetic energy to break the weak bonds that maintain folding. An egg white, which is a solution of protein albumen, turns from transparent and liquid to white and solid when boiled – this is irreversible denaturation. Similarly, adding acid or alkali changes the electrical charges on R groups, so ionic attractions are lost and the protein unfolds.
两种主要因素导致变性：高温和极端 pH。加热提供足够的动能来打破维持折叠的弱键。蛋清是蛋白质卵清蛋白的溶液，煮沸时从透明液体变成白色固体——这是不可逆的变性。同样，加入酸或碱会改变 R 基团上的电荷，导致离子吸引丧失，蛋白质解折叠。
Some denaturation is reversible if the denaturing factor is removed quickly and the polypeptide chain can refold correctly, but in most cases, particularly in exam contexts, denaturation is regarded as permanent. Cooked egg white or meat cannot be 'uncooked'.
如果快速移除变性因素并且多肽链能正确重新折叠，某些变性是可逆的；但在大多数情况下，尤其在考试场景中，变性被视为永久性的。煮熟的蛋清或肉类无法“复原”。
12. Testing for Proteins (Biuret Test) | 蛋白质检测（双缩脲试验）
The Biuret test is a simple biochemical assay used to detect the presence of peptide bonds, and therefore proteins. It does not test for a specific protein but gives a positive result with any substance containing two or more peptide linkages. This includes short peptides as well as large proteins.
双缩脲试验是一种简单的生化检测法，用于检测肽键的存在，从而检测蛋白质。它不检测特定蛋白质，但对任何含有两个或以上肽键的物质均产生阳性结果。这包括短肽和大型蛋白质。
The test procedure is as follows:
- Add an equal volume of sodium hydroxide solution (NaOH) to the food sample solution.
- 向待测样品溶液中加入等体积的氢氧化钠溶液 (NaOH)。
- Then add a few drops of dilute copper(II) sulfate solution (CuSO₄, usually 0.1% to 1%) and mix gently.
- 然后加入几滴稀硫酸铜溶液 (CuSO₄，通常 0.1% 至 1%)，轻轻混合。
- A purple or mauve colour indicates the presence of peptide bonds and thus protein. If no protein is present, the solution remains blue.
- 紫色或淡紫色表示存在肽键，从而说明有蛋白质。如果没有蛋白质，溶液保持蓝色。
The colour change happens because copper(II) ions coordinate with the nitrogen atoms in the peptide bonds, forming a violet-coloured complex in the alkaline conditions provided by NaOH. This reaction is specific to the peptide bond and is not given by individual amino acids.
Published by TutorHao | IGCSE Biology Revision Series | aleveler.com更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图