Proteins
Proteins
- Proteins do almost every job in the body — enzymes, oxygen carriers, structure.
- They are all built from the same 20 small units, amino acids.
- Their precise 3-D shape is what lets them work.
Amino acids and the peptide bond
- 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; hydrolysis (adding water) breaks peptide bonds.
Practice
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.
Practice
Match each level of protein structure to its meaning.
Primary = sequence; secondary = local helix/sheet; tertiary = overall 3-D fold; quaternary = multiple chains.
Practice
The primary structure of a protein is:
Primary structure is simply the order of amino acids; everything else follows from it.
Globular vs fibrous
- Globular proteins fold into a rounded, usually soluble shape and do active jobs — e.g. haemoglobin, 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, three chains wound into a triple strand, cross-linked into strong fibres.
Practice
Which correctly pairs a protein with its type?
Haemoglobin is a soluble globular protein; collagen is an insoluble fibrous structural protein.
Practice
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
Key idea
- 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)