A level-readyAll subjects · Biology · WBI11 (Unit 1)
Proteins: structure and function
Proteins do most jobs in cells. Their function depends on their 3D shape, which comes from the sequence of amino acids.
Amino acids and peptide bonds
Every amino acid has a central carbon bonded to an amine group (-NH2), a carboxyl group (-COOH), a hydrogen atom and a variable R group. There are 20 different R groups.
Two amino acids join in a condensation reaction between the -COOH of one and the -NH2 of the other, forming a peptide bond and releasing water. Many joined together make a polypeptide.
Levels of structure
- Primary: the sequence of amino acids (held by peptide bonds).
- Secondary: the chain coils into an α-helix or folds into a β-pleated sheet, held by hydrogen bonds.
- Tertiary: further folding into a specific 3D shape, held by hydrogen bonds, ionic bonds, disulfide bridges (between cysteine R groups) and hydrophobic interactions between R groups.
- Quaternary: two or more polypeptides joined (haemoglobin has four).
Globular and fibrous proteins
- Globular proteins (such as haemoglobin and enzymes) are compact and roughly spherical, with hydrophilic R groups on the outside, so they are soluble. Haemoglobin's shape lets it bind and release oxygen.
- Fibrous proteins (such as collagen) are long and insoluble. Collagen is three polypeptide chains wound into a triple helix, with many cross-links between molecules, giving high tensile strength for tendons, skin and artery walls.
Changing even one amino acid can change the tertiary structure and the function.
Worked example
How many water molecules are released when a polypeptide of 150 amino acids is made?
- Each peptide bond releases one water molecule.
- A chain of n amino acids has n - 1 peptide bonds.
- 150 - 1 = 149
Answer: 149 water molecules.
Check yourself
- How many peptide bonds are in a single polypeptide of 300 amino acids?
Show answer
299. n - 1 = 299.
- Which group varies between different amino acids?
- The amine group
- The carboxyl group
- The central carbon
- The R group
Show answer
The R group. The R group differs in each of the 20 amino acids.
- Which bonds hold the α-helix in shape?
- Peptide bonds only
- Hydrogen bonds
- Disulfide bridges only
- Ester bonds
Show answer
Hydrogen bonds. Secondary structure is held by hydrogen bonds.
- Disulfide bridges form between which amino acids?
- Two glycines
- Any two amino acids
- Two cysteines
- Two alanines
Show answer
Two cysteines. Cysteine R groups contain sulfur.
- Which feature gives collagen its high tensile strength?
- Three polypeptide chains in a triple helix with cross-links between molecules
- A compact spherical shape
- Hydrophilic R groups on the outside
- A haem group
Show answer
Three polypeptide chains in a triple helix with cross-links between molecules. The wound chains and cross-links resist pulling forces.
- Why are globular proteins such as haemoglobin soluble in water?
- They have no R groups
- They are long fibres
- Hydrophilic R groups are on the outside of the molecule
- They are made of lipids
Show answer
Hydrophilic R groups are on the outside of the molecule. Polar R groups facing outwards interact with water.
- Haemoglobin has quaternary structure because it
- is made of more than one polypeptide chain
- has an α-helix
- has a sequence of amino acids
- contains peptide bonds
Show answer
is made of more than one polypeptide chain. Quaternary structure = more than one polypeptide.
Sources
Last updated 7 November 2026. Spotted a mistake? Tell us.