Protein Structure and Function

This lesson covers the four levels of protein structure — primary, secondary, tertiary and quaternary — and how structure determines function. This is required by the Edexcel A-Level Biology B specification (9BI0), Topic 1: Biological Molecules. You need to understand the bonds that maintain each level of structure and be able to relate the overall shape of a protein to its biological role.

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The Four Levels of Protein Structure

Proteins have up to four levels of structural organisation, each building upon the previous one.

The following diagram summarises how each level of protein structure builds upon the previous one:

graph TD
    A["Primary Structure<br/>Sequence of amino acids"] --> B["Secondary Structure<br/>α-helix and β-pleated sheet"]
    B --> C["Tertiary Structure<br/>3D folding<br/>(H bonds, ionic, disulfide, hydrophobic)"]
    C --> D["Quaternary Structure<br/>Multiple polypeptide chains"]

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Primary Structure

The primary structure of a protein is the specific sequence of amino acids in the polypeptide chain, held together by peptide bonds.

Key Points