A level Biology B · Edexcel 9BI0 · Topic 1C
Proteins
Key points
- Proteins are polymers of amino acids. Each amino acid has an amine group, a carboxyl group, a hydrogen and a variable R group.
- Amino acids join by peptide bonds (condensation) to make a polypeptide.
- Primary structure: the sequence of amino acids. Secondary: α-helix or β-pleated sheet held by hydrogen bonds. Tertiary: the overall 3D fold held by hydrogen bonds, ionic bonds and disulfide bridges. Quaternary: two or more polypeptides together, sometimes with a prosthetic group.
- Globular proteins (such as haemoglobin and enzymes) are compact and soluble with hydrophilic R groups facing outward.
- Fibrous proteins (such as collagen) are long, insoluble and strong; collagen has three chains wound into a triple helix.
- Changing one amino acid can change the 3D shape and so the function.
Topic notesPremium
Proteins do more jobs than any other kind of molecule: they catalyse, carry, signal, defend and support. Every one of these functions depends on a precise 3D shape that comes from the order of amino acids.
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