📚 Appendix 1: Amino Acid R Groups | 附录1：氨基酸R基团
Proteins are polymers of amino acids, and the unique properties of each amino acid are determined by its R group (side chain). In CAIE A-Level Biology, understanding the diversity and chemical nature of these R groups is essential for grasping protein structure and function. This appendix provides a comprehensive overview of the 20 standard amino acids, classified by their R group characteristics.
蛋白质是氨基酸的多聚体，每种氨基酸的独特性质由其R基团（侧链）决定。在CAIE A-Level生物学中，理解这些R基团的多样性和化学本质对于掌握蛋白质的结构与功能至关重要。本附录对20种标准氨基酸按照其R基团特性进行分类，并提供一个全面的概述。
1. General Structure of an Amino Acid | 氨基酸的通用结构
All standard amino acids share a common structure: a central α-carbon atom bonded to an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom, and a variable R group. At physiological pH, the amino group is usually protonated (-NH₃⁺) and the carboxyl group is deprotonated (-COO⁻), forming a zwitterion. This core structure allows amino acids to be linked via peptide bonds.
所有标准氨基酸都有一个共同结构：一个中心α-碳原子连接着一个氨基(-NH₂)、一个羧基(-COOH)、一个氢原子和一个可变的R基团。在生理pH条件下，氨基通常质子化(-NH₃⁺)，羧基去质子化(-COO⁻)，形成两性离子。这一核心结构使氨基酸能够通过肽键连接。
2. The R Group Defines Each Amino Acid | R基团决定每种氨基酸
The R group, also called the side chain, is the variable part that distinguishes one amino acid from another. It can be as simple as a single hydrogen atom (glycine) or as complex as a bicyclic ring (tryptophan). The chemical nature of the R group dictates the amino acid's polarity, charge, hydrophobicity, and reactivity, which directly influence how the amino acid behaves in an aqueous environment and how it interacts with other molecules.
R基团，也称侧链，是区分不同氨基酸的可变部分。它可以简单如一个氢原子（甘氨酸），也可以复杂如一个双环结构（色氨酸）。R基团的化学性质决定了氨基酸的极性、电荷、疏水性及反应性，这直接影响氨基酸在水溶液中的行为以及它与其他分子的相互作用。
3. Classification Based on R Group Properties | 基于R基团性质的分类
Amino acids are commonly grouped into four main categories according to the properties of their R groups: non-polar (hydrophobic), polar uncharged, acidic (negatively charged), and basic (positively charged). This classification is fundamental to understanding protein folding, as hydrophobic residues tend to cluster in the protein interior, while charged and polar residues are often found on the surface, interacting with water and ligands.
氨基酸通常根据R基团的性质分为四大类：非极性（疏水性）、极性不带电、酸性（带负电）和碱性（带正电）。这一分类对于理解蛋白质折叠至关重要，因为疏水性残基倾向于聚集在蛋白质内部，而带电荷和极性残基则常位于表面，与水及配体相互作用。
4. Non-polar, Hydrophobic R Groups | 非极性疏水R基团
Non-polar R groups consist mainly of hydrocarbon chains or rings, and they do not form hydrogen bonds with water. These amino acids are hydrophobic and tend to be buried in the core of globular proteins, stabilising the structure through hydrophobic interactions. They commonly contain alkyl or aromatic groups.
非极性R基团主要由烃链或环组成，它们不与水形成氢键。这些氨基酸是疏水的，倾向于埋藏在球状蛋白质的核心，通过疏水相互作用稳定结构。它们通常含有烷基或芳香族基团。
5. Examples of Non-polar Amino Acids | 非极性氨基酸举例
Glycine (Gly) has an R group of -H, the simplest side chain. Alanine (Ala) has a methyl group (-CH₃). Valine (Val), leucine (Leu), and isoleucine (Ile) have branched hydrocarbon chains. Phenylalanine (Phe) contains a benzyl ring, and tryptophan (Trp) has an indole ring. Proline (Pro) is unusual because its side chain forms a cyclic structure by bonding back to the amino nitrogen. Methionine (Met) contains a thioether group.
甘氨酸(Gly)的R基团为-H，是最简单的侧链。丙氨酸(Ala)有一个甲基(-CH₃)。缬氨酸(Val)、亮氨酸(Leu)和异亮氨酸(Ile)具有分支烃链。苯丙氨酸(Phe)含有一个苄基环，色氨酸(Trp)含有一个吲哚环。脯氨酸(Pro)比较特殊，其侧链与氨基氮原子键合形成环状结构。甲硫氨酸(Met)含有一个硫醚基团。
6. Polar, Uncharged R Groups | 极性不带电R基团
Polar uncharged amino acids have R groups that can form hydrogen bonds with water, making them hydrophilic. Although they possess electronegative atoms such as oxygen or nitrogen, they do not carry a net charge at physiological pH. These side chains are often found on protein surfaces or in active sites.
极性不带电氨基酸的R基团能与水形成氢键，因此是亲水的。尽管它们含有电负性原子如氧或氮，但在生理pH下不带净电荷。这些侧链常出现在蛋白质表面或活性位点。
7. Examples of Polar Amino Acids | 极性氨基酸举例
Serine (Ser) has a hydroxylmethyl group (-CH₂OH). Threonine (Thr) has a hydroxyl-containing side chain with a chiral centre. Cysteine (Cys) contains a sulfhydryl group (-SH), which can form disulfide bonds (-S-S-) with another cysteine, stabilising protein tertiary structure. Asparagine (Asn) and glutamine (Gln) contain amide groups (-CONH₂). Tyrosine (Tyr) has a phenol ring with an -OH group.
丝氨酸(Ser)有一个羟甲基(-CH₂OH)。苏氨酸(Thr)有一个含羟基且带有手性中心的侧链。半胱氨酸(Cys)含有巯基(-SH)，能与另一个半胱氨酸形成二硫键(-S-S-)，稳定蛋白质三级结构。天冬酰胺(Asn)和谷氨酰胺(Gln)含有酰胺基团(-CONH₂)。酪氨酸(Tyr)有一个带有-OH基团的苯酚环。
8. Acidic R Groups (Negatively Charged) | 酸性R基团（带负电）
Acidic amino acids have R groups that contain a carboxyl group (-COOH) which ionises to -COO⁻ at physiological pH, giving a net negative charge. They are highly hydrophilic and often participate in ionic interactions and metal ion binding.
酸性氨基酸的R基团含有一个羧基(-COOH)，在生理pH下电离为-COO⁻，带净负电荷。它们高度亲水，常参与离子相互作用和金属离子结合。
9. Examples of Acidic Amino Acids | 酸性氨基酸举例
Aspartic acid (Asp) has a -CH₂COOH side chain. Glutamic acid (Glu) has a -CH₂CH₂COOH side chain. The carboxylate groups of Asp and Glu are essential in enzyme catalysis, such as in the active site of lysozyme or ribonuclease, where they act as nucleophiles or general acid-base catalysts.
天冬氨酸(Asp)的侧链为-CH₂COOH。谷氨酸(Glu)的侧链为-CH₂CH₂COOH。天冬氨酸和谷氨酸的羧酸根在酶催化中至关重要，例如在溶菌酶或核糖核酸酶的活性位点，它们充当亲核剂或广义酸碱催化剂。
10. Basic R Groups (Positively Charged) | 碱性R基团（带正电）
Basic amino acids possess R groups that contain amino groups which become protonated (-NH₃⁺) at physiological pH, conferring a positive charge. They are hydrophilic and often interact with negatively charged substrates, such as DNA phosphate backbones.
碱性氨基酸的R基团含有氨基，在生理pH下质子化(-NH₃⁺)，带正电荷。它们是亲水的，常与带负电荷的底物（如DNA磷酸骨架）相互作用。
11. Examples of Basic Amino Acids | 碱性氨基酸举例
Lysine (Lys) has a long side chain ending in -NH₂. Arginine (Arg) contains a guanidino group that is permanently protonated. Histidine (His) has an imidazole ring with a pKa near 6, meaning it can exist in both protonated and deprotonated forms at physiological pH; this property makes histidine a key player in enzyme active sites, such as the catalytic triad in serine proteases.
赖氨酸(Lys)有一条长侧链，末端为-NH₂。精氨酸(Arg)含有一个胍基，在生理条件下总是质子化的。组氨酸(His)有一个咪唑环，其pKa接近6，意味着在生理pH下可同时存在质子化和去质子化形式；这一特性使组氨酸在酶活性位点中起关键作用，例如丝氨酸蛋白酶中的催化三联体。
12. Impact on Protein Folding and Function | 对蛋白质折叠和功能的影响
The precise sequence of R groups in a polypeptide determines how it folds into its specific three-dimensional conformation. Hydrophobic collapse drives the initial folding, while hydrogen bonds, ionic interactions, van der Waals forces, and disulfide bridges between R groups stabilise the structure. Changes in R group properties due to mutation (e.g., in sickle-cell anaemia, glutamic acid is replaced by valine in haemoglobin) can drastically alter protein function and cause disease.
多肽中R基团的精确序列决定了它如何折叠成特定的三维构象。疏水塌缩驱动初期折叠，而R基团之间的氢键、离子相互作用、范德华力和二硫键则稳定结构。由于突变导致R基团性质改变（例如镰刀型细胞贫血症中，血红蛋白的谷氨酸被缬氨酸取代）可显著改变蛋白质功能并引发疾病。
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