📚 IB & Edexcel Biology: Proteins - Key Points Review | IB & Edexcel 生物：蛋白质 考点精讲
Proteins are fundamental biomolecules that play a crucial role in virtually every biological process. For IB and Edexcel Biology, understanding protein structure, function, and analysis is essential for exam success. This guide consolidates all key concepts you need to master.
蛋白质是几乎所有生物学过程都不可或缺的基础生物分子。无论对于 IB 还是 Edexcel 生物学考试，掌握蛋白质的结构、功能与分析都是获得高分的核心。本文整合了所有你必须掌握的考点。
1. Introduction to Proteins | 蛋白质简介
Proteins are large, complex polymers made from amino acid monomers linked by peptide bonds. They account for more than 50% of the dry mass of most cells and are involved in structure, catalysis, transport, signalling, movement, and immune defence. The diversity of protein functions arises from their specific three-dimensional shapes, which are determined by the sequence of amino acids.
蛋白质是由氨基酸单体通过肽键连接而成的大型复杂聚合物。它们在大多数细胞干重中占一半以上，参与结构支持、催化、运输、信号转导、运动和免疫防御等多种活动。蛋白质功能的多样性源于其特定的三维形状，而三维形状又由其氨基酸序列所决定。
Enzymes are proteins that act as biological catalysts, and structural proteins like collagen and keratin provide mechanical support. Transport proteins, such as haemoglobin, carry oxygen, while antibodies protect against pathogens. The central dogma of molecular biology explains how DNA codes for the primary structure of every protein.
酶是作为生物催化剂的蛋白质，胶原蛋白和角蛋白等结构蛋白提供机械支撑。血红蛋白等运输蛋白负责携带氧气，而抗体则保护机体免受病原体侵害。分子生物学的中心法则解释了DNA如何编码每种蛋白质的一级结构。
2. Amino Acids: The Building Blocks | 氨基酸：组成单元
All amino acids share a common structure: a central (alpha) carbon atom bonded to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a variable R group (side chain). The general formula is H₂N–CH(R)–COOH. There are 20 standard amino acids used in protein synthesis, each differing only in the chemical nature of the R group.
所有氨基酸都具有共同的结构：一个中心（α）碳原子分别与一个氨基（–NH₂）、一个羧基（–COOH）、一个氢原子和一个可变的 R 基团（侧链）相连。其通式为 H₂N–CH(R)–COOH。在蛋白质合成中，共有 20 种标准氨基酸，它们的唯一区别在于 R 基团的化学性质。
R groups can be non-polar (hydrophobic), polar (hydrophilic), positively charged (basic), or negatively charged (acidic). Cysteine contains a sulfhydryl (–SH) group that can form disulfide bridges, and glycine has the simplest R group (a single hydrogen). The precise sequence of these amino acids determines the protein's ultimate structure and function.
R 基团可以是非极性的（疏水）、极性的（亲水）、带正电荷的（碱性）或带负电荷的（酸性）。半胱氨酸含有巯基（–SH），能形成二硫桥，而甘氨酸则具有最简单的 R 基团（仅一个氢原子）。这些氨基酸的精确序列决定了蛋白质的最终结构和功能。
3. Peptide Bond Formation | 肽键的形成
When two amino acids join, a condensation reaction (dehydration synthesis) occurs: the hydroxyl (–OH) from the carboxyl group of one amino acid and a hydrogen (–H) from the amino group of another are removed, releasing a water molecule. The resulting covalent link is a peptide bond (–CO–NH–). A dipeptide is formed from two amino acids, and a polypeptide is a long chain of many amino acids.
当两个氨基酸连接时，发生缩合反应（脱水合成）：一个氨基酸羧基中的羟基（–OH）与另一个氨基酸氨基中的一个氢（–H）被脱去，释放出一分子水。形成的共价键即为肽键（–CO–NH–）。两个氨基酸形成二肽，许多氨基酸相连则形成多肽长链。
Peptide bonds are rigid and planar due to resonance, restricting rotation around the C–N bond. This has important implications for protein folding. The reverse reaction, hydrolysis, breaks peptide bonds by the addition of water, a process that occurs during digestion catalyzed by proteases.
由于共振作用，肽键具有刚性和平面性，限制 C–N 键的旋转，这对蛋白质折叠有重要影响。逆反应即水解，通过加入水来断裂肽键，这一过程在消化过程中由蛋白酶催化完成。
4. Levels of Protein Structure (1) – Primary Structure | 蛋白质结构层次(一) – 一级结构
The primary structure is the unique sequence of amino acids in a polypeptide chain, held together by peptide bonds. This sequence is determined by the gene coding for the protein and is written from the N-terminus (free –NH₂) to the C-terminus (free –COOH). Even a single amino acid substitution can drastically alter protein function, as seen in sickle cell anaemia.
一级结构是多肽链中氨基酸的独特排列顺序，靠肽键维系。该序列由编码该蛋白质的基因决定，书写方向从 N 端（游离 –NH₂）到 C 端（游离 –COOH）。即使单个氨基酸发生替换，也可能剧烈改变蛋白质功能，镰刀型细胞贫血症就是典型例子。
In sickle cell disease, the amino acid valine replaces glutamic acid at position 6 in the β-chain of haemoglobin. This change causes haemoglobin molecules to aggregate under low oxygen, distorting red blood cells into a sickle shape and impairing oxygen transport.
在镰刀型细胞病中，血红蛋白 β 链第 6 位的谷氨酸被缬氨酸取代。这一改变导致血红蛋白分子在低氧条件下聚集，使红细胞扭曲成镰刀状，损害氧气运输功能。
5. Secondary Structure: α-Helix and β-Pleated Sheet | 二级结构：α-螺旋与β-折叠
a polypeptide chain, stabilized primarily by hydrogen bonds between the carbonyl oxygen (–C=O) of one amino acid and the amide hydrogen (– ther down the backbone. The two most common types are the α-helix and the β-pleated sheet.
二级结构指多肽链内部的局部折叠模式，主要通过一条骨架上氨基酸的羰基氧（–C=O）与更下游氨基酸的酰胺氢（–NH）之间形成的氢键来维持。最常见的两种类型是 α-螺旋和 β-折叠片。
In an α-helix, the polypeptide chain coils into a right-handed spiral, with hydrogen bonds parallel to the axis, bridging every fourth peptide unit. Fibrous proteins like keratin contain extensive α-helical regions. In a β-pleated sheet, segments of the chain lie parallel or antiparallel to each other, forming a pleated, sheet-like structure with inter-strand hydrogen bonds. Spider silk is rich in β-sheets.
在 α-螺旋中，多肽链盘绕成右手螺旋，氢键平行于螺旋轴，每隔四个肽单位形成桥联。角蛋白等纤维状蛋白质含有大量 α-螺旋区域。在 β-折叠片中，多肽链片段平行或反平行排列，通过链间氢键形成褶皱的片状结构。蜘蛛丝富含 β-折叠片。
6. Tertiary Structure: 3D Folding | 三级结构：三维折叠
Tertiary structure is the overall three-dimensional shape of a single polypeptide chain, resulting from interactions between the R groups of amino acids. These interactions include disulfide bridges, ionic bonds, hydrogen bonds, and hydrophobic/hydrophilic interactions. The tertiary structure determines the protein's specific biological activity.
三级结构是指单条多肽链的整体三维形状，由氨基酸 R 基团之间的相互作用决定。这些相互作用包括二硫桥、离子键、氢键以及疏水/亲水相互作用。三级结构决定了蛋白质特定的生物活性。
- Disulfide bridges: Strong covalent bonds (–S–S–) formed between the sulfhydryl groups of two cysteine residues. They lock the structure in place. – 二硫桥：两个半胱氨酸残基的巯基之间形成的强共价键（–S–S–），能将结构锁定在固定位置。
- Ionic bonds: Electrostatic attractions between oppositely charged R groups (e.g., –NH₃⁺ and –COO⁻). – 离子键：带相反电荷的 R 基团之间的静电吸引（如 –NH₃⁺ 与 –COO⁻）。
- Hydrogen bonds: Weak bonds formed between polar R groups. – 氢键：极性 R 基团间形成的弱键。
- Hydrophobic interactions: Non-polar R groups cluster together in the protein's interior, away from water, stabilized by van der Waals forces. – 疏水作用：非极性 R 基团在蛋白质内部聚集成团，远离水环境，由范德华力稳定。
7. Quaternary Structure and Examples (Haemoglobin, Collagen) | 四级结构及实例（血红蛋白、胶原蛋白）
Quaternary structure exists when two or more polypeptide chains (subunits) associate to form a functional protein. The same types of bonds that stabilize tertiary structure also hold subunits together. Not all proteins have quaternary structure; many are functional as a single folded chain.
当两条或多条多肽链（亚基）组合形成一个功能性蛋白质时，就存在四级结构。维持三级结构的各类键同样负责亚基之间的连接。并非所有蛋白质都具有四级结构，许多蛋白质仅由一条折叠的链即可行使功能。
Haemoglobin is a classic example: it consists of four polypeptide subunits – two α-globin and two β-globin chains. Each subunit contains a haem group with an iron ion (Fe²⁺) that binds one O₂ molecule. The quaternary structure allows cooperative binding, where binding of oxygen to one subunit facilitates binding to the others, increasing efficiency.
血红蛋白是一个经典实例：它由四个多肽亚基组成，即两条 α-珠蛋白链和两条 β-珠蛋白链。每个亚基都含有一个血红素辅基，其中的铁离子（Fe²⁺）可结合一分子 O₂。四级结构实现了协同结合，即氧气与一个亚基的结合会促进其他亚基的结合，从而提高运输效率。
Collagen is another key example: it is composed of three polypeptide chains wound together into a triple helix. Abundant in skin, tendons, and bones, collagen provides tensile strength. Its structure relies on repeating Gly-X-Y sequences (where X is often proline and Y is hydroxyproline) and requires vitamin C for proper cross-linking.
胶原蛋白是另一个重要实例：由三条多肽链相互缠绕形成三重螺旋。它大量存在于皮肤、肌腱和骨骼中，提供抗拉强度。其结构依赖于重复的 Gly-X-Y 序列（X 常为脯氨酸，Y 为羟脯氨酸），并且需要维生素 C 参与正确的交联。
8. Protein Functions | 蛋白质的功能
Proteins are extraordinarily versatile. Common functional categories tested in exams include:
蛋白质的功能极为广泛。考试中常见的功能类别包括：
- Enzymes: catalyse biochemical reactions (e.g., amylase, catalase) – 酶：催化生化反应（如淀粉酶、过氧化氢酶）
- Structural: provide support (e.g., collagen, keratin) – 结构蛋白：提供支撑（如胶原蛋白、角蛋白）
- Transport: carry molecules (e.g., haemoglobin for O₂, channel proteins) – 运输：携带分子（如血红蛋白运 O₂、通道蛋白）
- Hormonal: signalling molecules (e.g., insulin, glucagon) – 激素：信号分子（如胰岛素、胰高血糖素）
- Defence: antibodies (immunoglobulins) recognise and neutralise pathogens – 防御：抗体（免疫球蛋白）识别并中和病原体
- Contractile: enable movement (e.g., actin, myosin in muscles) – 收缩蛋白：实现运动（如肌肉中的肌动蛋白、肌球蛋白）
- Storage: store amino acids and ions (e.g., casein in milk, ferritin) – 储存：储存氨基酸和离子（如乳汁中的酪蛋白、铁蛋白）
A protein's shape is directly linked to its function. For instance, the active site of an enzyme has a specific conformation complementary to its substrate, while the fibrous structure of collagen gives it high tensile strength suitable for tendons.
蛋白质的形状与其功能直接相关。例如，酶的活性位点具有与其底物互补的特定构象，而胶原蛋白的纤维状结构赋予了它适合肌腱的高抗拉强度。
9. Denaturation of Proteins | 蛋白质的变性
Denaturation is the loss of the specific three-dimensional conformation of a protein, leading to loss of function. It occurs when the non-covalent bonds (hydrogen bonds, ionic bonds, hydrophobic interactions) and sometimes disulfide bridges that maintain the tertiary and secondary structures are disrupted. The peptide bonds of the primary structure remain intact.
变性是指蛋白质失去特定的三维构象，从而导致功能丧失。当维持三级和二级结构的非共价键（氢键、离子键、疏水作用）以及有时二硫桥被破坏时，就会发生变性，但一级结构中的肽键保持完整。
Common denaturing agents include:
常见的变性因素包括：
- High temperature: disrupts hydrogen bonds and increases kinetic energy, causing the protein to unfold and coagulate (e.g., egg white albumin). – 高温：破坏氢键并增加动能，使蛋白质去折叠和凝固（例如蛋清中的清蛋白）。
- Extreme pH: alters the ionisation of R groups, affecting ionic bonds and overall folding. – 极端 pH：改变 R 基团的离子化状态，影响离子键和整体折叠。
- Heavy metal ions (e.g., Pb²⁺, Hg²⁺): can bind to –SH groups and disrupt disulfide bridges. – 重金属离子（如 Pb²⁺、Hg²⁺）：可与 –SH 基团结合，破坏二硫桥。
Denaturation is often irreversible, especially when disulfide bonds are broken or the protein aggregates. However, some proteins can renature if the denaturing agent is removed quickly, demonstrating that the primary sequence contains all the information needed for correct folding.
变性通常是不可逆的，尤其是当二硫键断裂或蛋白质发生聚集时。然而，有些蛋白质在迅速去除变性因素后可复性，这表明一级序列含有正确折叠所需的全部信息。
10. Testing for Proteins: Biuret Test | 蛋白质检测：缩二脲试验
The Biuret test is a colorimetric assay for peptide bonds. It does not detect free amino acids or dipeptides because at least two peptide bonds are needed for the colour change. The reagent contains sodium hydroxide (NaOH) and copper(II) sulfate (CuSO₄).
缩二脲试验是一种检测肽键的比色方法。它不能检测游离氨基酸或二肽，因为显色反应至少需要两个肽键。试剂包含氢氧化钠（NaOH）和硫酸铜（CuSO₄）。
Procedure: To a sample solution, add an equal volume of sodium hydroxide solution, mix, then add a few drops of dilute copper sulfate solution. Mix gently. If protein is present, the solution turns from blue to purple/violet. The intensity of the colour can be used for semi-quantitative analysis. A negative result remains blue.
实验步骤：向待测样本中加入等体积的氢氧化钠溶液，混匀，然后滴加几滴稀硫酸铜溶液，轻轻混合。如果存在蛋白质，溶液从蓝色变为紫色/紫罗兰色。颜色深浅可用于半定量分析。阴性结果仍保持蓝色。
The colour change occurs because Cu²⁺ ions form a coordination complex with the nitrogen atoms in peptide bonds, absorbing light at a different wavelength from hydrated Cu²⁺ ions. This test is reliable for detecting all proteins regardless of their amino acid composition.
颜色变化的原因是 Cu²⁺ 离子与肽键中的氮原子形成配位络合物，其吸收波长不同于水合 Cu²⁺ 离子。该试验能可靠地检测所有蛋白质，无论其氨基酸组成如何。
11. Key Exam Tips | 考试要点提示
Explain 'structure determines function' with specific examples. When asked how a protein's shape is suited to its role, refer to levels of structure and the bonds that maintain them. For haemoglobin, mention quaternary structure and cooperative binding; for collagen, mention the triple helix and high proportion of glycine.
回答“结构决定功能”时，务必引用具体例子。当被问及蛋白质形状如何适应其作用时，应提及结构层次以及维系它们的化学键。对于血红蛋白，要提到四级结构和协同结合；对于胶原蛋白，则要提到三重螺旋和高比例的甘氨酸。
Be able to describe how a single gene mutation can alter primary structure and affect the entire protein. Sickle cell anaemia is the classic example: a substitution of valine for glutamic acid introduces a hydrophobic patch, causing haemoglobin to polymerise and red cells to sickle. This demonstrates the link between DNA, protein
Published by TutorHao | IB 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