Proteins
Introduced| English | Meaning |
|---|---|
| amino acid/əˈmiːnəʊ ˈæsɪd/ | An organic molecule containing amino and carboxyl groups; these molecules join to form proteins. |
| peptide bond/ˈpeptaɪd bɒnd/ | A covalent bond joining amino acids, formed between an amino group and a carboxyl group with water removed. |
| polypeptide/ˌpɒlɪˈpeptaɪd/ | A chain of amino acids joined by peptide bonds. |
| disulfide bond/ˈdaɪsəlfaɪd bɒnd/ | A covalent bond between two sulfur atoms, often linking cysteine residues within or between protein chains. |
| globular/ˈɡlɒbjʊlə/ | Rounded, soluble, functional (haemoglobin, 4 chains + haem/iron, carries O₂). |
| fibrous/ˈfɪbrəs/ | Long, insoluble, structural (collagen, triple strand, cross-linked). |
| haemoglobin/ˌhiːməˈɡləʊbɪn/ | The iron-containing protein in red blood cells that binds and transports oxygen. |
| collagen/ˈkɒlədʒn/ | A fibrous structural protein that provides tensile strength in connective tissues. |
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:
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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.
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Primary = sequence; secondary = local helix/sheet; tertiary = overall 3-D fold; quaternary = multiple chains.
Put the levels of protein structure in order, simplest first.
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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.
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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.
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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?
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Haemoglobin is a soluble globular protein; collagen is an insoluble fibrous structural protein.
Haemoglobin can carry four oxygen molecules because it has:
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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)