Proteins

Summary: Proteins are natural condensation polymers (polyamides) formed from amino acid monomers. Amino acids link via peptide bonds with elimination of water. Proteins have essential roles as enzymes, structural components, hormones, and antibodies. Tags: igcse chemistry organic-chem polymers biochemistry Created: 2026-07-14 Last Updated: 2026-07-23

What Are Proteins?

Proteins are natural condensation polymers — specifically, they are polyamides formed from amino acid monomers.

Each amino acid contains two functional groups:

• —NH₂ (amine group, basic)
• —COOH (carboxylic acid group, acidic)

Because amino acids have two different functional groups, they can link together via condensation polymerisation, forming peptide bonds (amide linkages, —CONH—) and eliminating water with each bond formed.

Amino Acids — The Monomers

General structure of an amino acid:

    H   O
    |   ‖
H—N—C—C—O—H
    |   \
    H   O—H
    |
    R   ← Variable side chain (20 different R groups exist in nature)

At IGCSE, the simplest amino acid glycine (R = H) is the key example:

    H   O
    |   ‖
H—N—C—C—O—H
    |   \
    H   O—H

Formation of Proteins — The Peptide Bond

When two amino acids join:

1. The —COOH group of one amino acid reacts with the —NH₂ group of another
2. A peptide bond (amide linkage, —CONH—) forms
3. A molecule of water (H₂O) is eliminated

General reaction:

n H₂N—CHR—COOH → —[HN—CHR—CO]—ₙ + n H₂O

(The —COOH of one amino acid loses —OH; the —NH₂ of another loses —H → H₂O eliminated)

Peptide Bond Formation (Glycine + Glycine → Dipeptide)

Glycine + Glycine → Glycyl-glycine (a dipeptide) + H₂O

H₂N—CH₂—COOH  +  H₂N—CH₂—COOH
         ↓
H₂N—CH₂—CO—NH—CH₂—COOH  +  H₂O
          ↑
     peptide bond

• Dipeptide: 2 amino acids joined
• Polypeptide: many amino acids joined (a protein is a long polypeptide chain)

Protein Structure (Simplified for IGCSE)

1. Primary structure: The sequence/order of amino acids in the chain
2. Secondary structure: The chain folds into α-helices or β-pleated sheets (held by hydrogen bonds)
3. Tertiary structure: Further folding into a specific 3D shape (held by ionic bonds, hydrogen bonds, and disulfide bridges between R groups)

The 3D shape determines the protein’s function. If the shape is destroyed (denaturation), the protein stops working.

Types and Functions of Proteins

Denaturation of Proteins

Denaturation is the irreversible loss of a protein’s 3D shape, caused by:

• High temperature (breaks hydrogen and ionic bonds)
• Extreme pH (disrupts ionic bonds between R groups)
• Heavy metal ions (e.g. Pb²⁺, Hg²⁺) — bind to and disrupt protein structure

When a protein is denatured:

• The primary structure (amino acid sequence) is unchanged
• The secondary and tertiary structure is destroyed
• The protein loses its function permanently
• Cooking an egg (albumin denatures) is a common example

Proteins as Condensation Polymers

Proteins vs Nylon

Both are polyamides with —CONH— linkages:

Key Facts

• Proteins = natural condensation polymers (polyamides)
• Monomers = amino acids (contain —NH₂ and —COOH)
• Amino acids join via peptide bonds (−CONH−) with elimination of water
• A dipeptide = 2 amino acids; a polypeptide = many amino acids
• Denaturation = irreversible loss of 3D shape → loss of function
• Caused by: high temperature, extreme pH, heavy metal ions
• Cooking an egg denatures the protein albumin

Related Notes

• Condensation Polymerisation
• Polymers
• Nylon
• Enzyme
• IGCSE-Chem-Index