1.7 Proteins

Summary

Key Definition Proteins are polymers of amino acids joined by covalent peptide bonds, which form through dehydration synthesis between the carboxyl group of one amino acid and the amino group of the next.

Each amino acid differs only in its R group, which can be hydrophobic, hydrophilic, or ionic. The chemical properties of these R groups drive the folding interactions that give each protein its unique shape — and therefore its unique function.

Key Definition The specific linear sequence of amino acids constitutes the primary structure, which serves as the blueprint for all higher-order folding.

Secondary Structure

• Arises from hydrogen bonds between atoms of the polypeptide backbone
• Produces alpha-helices and beta-pleated sheets

Tertiary Structure

• Results from interactions between R (side) groups that fold the polypeptide into a precise three-dimensional shape, stabilised by:
• Hydrogen bonds
• Hydrophobic interactions
• Ionic bonds between oppositely charged R groups
• Disulfide bridges (covalent bonds between cysteine residues)

Quaternary Structure

• Forms when two or more polypeptide subunits associate into a single functional complex through the same types of interactions (for example, the four subunits of hemoglobin)

All four structural levels work together to determine protein function, and disruption at any level can impair or destroy biological activity.

At a Glance