1.3.3 Protein Structure & Function

The structure of proteins is defined by the sequence and arrangement of amino acids, the types of bonding between them, and the resulting shape.

Levels of Protein Structure:

Protein structure determines its function and can be broken down into four levels:

1. Primary Structure:

• This is the sequence of amino acids in a protein chain. The specific order of amino acids is crucial, as it dictates the protein's ultimate shape and function.

2. Secondary Structure:

• The protein chain folds into specific shapes due to hydrogen bonding between nearby amino acids.
• This folding creates two common shapes:
• Alpha-helix: A coiled structure.
• Beta-pleated sheet: A folded, sheet-like structure.
• The slight positive charge of hydrogen in the NH group and the slight negative charge of oxygen in the C=O group create weak hydrogen bonds, stabilising these structures.

3. Tertiary Structure:

• The 3D structure of the protein, is formed by further twisting and folding of the secondary structure.
• Various bonds and interactions stabilise the tertiary structure:
• Disulfide Bridges: Strong covalent bonds between sulfur atoms in the amino acid cysteine, resistant to breaking.
• Ionic Bonds: Form between charged amino acid side chains. These are weaker than disulfide bridges and can be disrupted by changes in pH.
• Hydrogen Bonds: Abundant but weaker bonds that contribute to stability.

4. Quaternary Structure (when applicable):

• In some proteins, multiple polypeptide chains combine to form a functional unit. This level of structure includes the interactions between these individual chains.