Proteins
| English | Chinese | Pinyin |
|---|---|---|
| amino acid | 氨基酸 | ān jī suān |
| peptide bond | 肽键 | tài jiàn |
| polypeptide | 多肽 | duō tài |
| disulfide bond | 二硫键 | èr liú jiàn |
| globular | 球状 | qiú zhuàng |
| fibrous | 纤维状 | xiān wéi zhuàng |
| haemoglobin | 血红蛋白 | xuè hóng dàn bái |
| collagen | 胶原蛋白 | jiāo yuán dàn bái |
The body's master molecules
- Proteins do almost every job in the body — enzymes, oxygen carriers, structure.
- They are all built from the same 20 small units, amino acids 氨基酸.
- Their precise 3-D shape is what lets them work.
Amino acids and the peptide bond 肽键
- Every amino acid has the same core: an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen, and a variable R group (side chain) — the R group is what differs.
- Two amino acids join by condensation: a peptide bond forms and one water is removed.
- Many joined make a polypeptide 多肽; hydrolysis (adding water) breaks peptide bonds.
Two amino acids join by condensation, forming a peptide bond and releasing one water
The bond that joins two amino acids is a:
Amino acids join by peptide bonds, formed by condensation with the loss of one water molecule.
Four levels of structure
| Level | Meaning |
|---|---|
| primary | the order of amino acids in the chain |
| secondary | local shapes — α-helix and β-pleated sheet — held by hydrogen bonds |
| tertiary | the whole chain folded into a precise 3-D shape |
| quaternary | two or more polypeptide chains joined into one protein |
- Tertiary shape is held by R-group interactions: hydrophobic interactions, hydrogen bonds, ionic bonds, and strong disulfide bonds 二硫键.
The four levels of protein structure
The four levels of protein structure
Build a protein up one level at a time: sequence → local shapes → a folded 3-D shape → several chains joined.
Match each level of protein structure to its meaning.
Primary = sequence; secondary = local helix/sheet; tertiary = overall 3-D fold; quaternary = multiple chains.
Put the levels of protein structure in order, simplest first.
Each level builds on the one before: sequence → local shape → 3-D fold → several chains.
A protein made of two or more polypeptide chains joined together has a ______ structure.
Quaternary structure is two or more chains in one protein — e.g. haemoglobin (four chains).
Strong disulfide bonds are among the R-group bonds that hold a protein's tertiary 3-D shape.
Tertiary shape is held by hydrophobic interactions, hydrogen bonds, ionic bonds and strong disulfide bonds between R groups.
Globular 球状 vs fibrous 纤维状
- Globular proteins fold into a rounded, usually soluble shape and do active jobs — e.g. haemoglobin 血红蛋白, which has a quaternary structure (4 chains, each with a haem group holding an iron atom) and carries 4 oxygen molecules.
- Fibrous proteins form long, insoluble strands for support — e.g. collagen 胶原蛋白, three chains wound into a triple strand, cross-linked into strong fibres.
Which correctly pairs a protein with its type?
Haemoglobin is a soluble globular protein; collagen is an insoluble fibrous structural protein.
Haemoglobin can carry four oxygen molecules because it has:
Four polypeptide chains, each holding a haem group with an iron atom, give four oxygen-binding sites.
You've got it
- amino acid = –NH₂ + –COOH + H + R group; join by peptide bond (condensation)
- primary (order) → secondary (helix/sheet) → tertiary (3-D fold) → quaternary (≥2 chains)
- globular = rounded, soluble, functional (haemoglobin, 4 chains + haem/iron, carries O₂)
- fibrous = long, insoluble, structural (collagen, triple strand, cross-linked)