- organic molecules of **carbon, hydrogen, oxygen** and **nitrogen**
- may contain sulfur if cysteine or methionine is present
- Each protein molecule is made up of smaller units called amino acids
- Amino acids are made up of:
- an **amino group** ($\ce{NH2}$)
- an **acid group** ($\ce{COOH}$)
- O-H bond is polar, causing the shared electron to be closer to O
- allows $\ce{H+}$ ion to escape easily
- a **side chain** ($\ce{R}$)
- Two amino acid molecules are **combined** by condensation reaction to form a **dipeptide** and **broken down** via hydrolysis into its constituent amino acids
- **Peptide** bonds link up with **many amino acids** to form a **polypeptide** via condensation reaction (Remember polysaccharides?)
- One or more polypeptide chains fold to become a **3D molecule**, called a **protein**

# Levels of Organisation
- Primary level: the sequence of amino acids in the polypeptide chain
- Secondary level: local folding of polypeptide chain causes formation of helices and sheets
- Tertiary level: Further folding of polypeptide chain to form 3D shape
- Quaternary level: association of more than one polypeptide to form final functional protein

# Function
- used in the synthesis of new protoplasm for growth & repair of worn-out cells
- Can function as:
- Enzymes (biological catalysts)
- Hormones (chemical messengers in the blood)
- Transport proteins (e.g. Haemoglobin)
- Structural proteins (e.g. Collagen (in bones, skin, tendons), keratin (in hair, nails))
- Antibodies (specialised proteins that fight bacteria and viruses)

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