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Describe the composition of proteins and their role in the body
Proteins consist of many amino acids bonded together (peptide bond)

Amino acids are the building blocks of life
What is the structure of a general amino acid
Amino acids contain a central alpha carbon with an amino acid group (H2N), and hydrogen group (H), carboxylic acid group (COOH) and a side chain (R)
What causes amino acids to be polar, non polar or charged
In solution, amino acids will exist in an ionised or charged form (zwitterion form).
The classification of amino acids as polar, non-polar, polar charged is determined by their side chain
  • If the side chain (R) is a hydrocarbon = non-polar
  • Polar (neutral) amino acids are likely to have hydroxyl or sulfydyl groups in their side chain
  • if charge is shown, the molecule is a charged polar molecure. Polar charged can be acidid (carboxylic acid side chain) or basic (amine side chain)
Describe the nature of the peptide bond
A peptide bond is formed when the carboxyl group of one amino acid, joins to the amino group of another amino acid. A water molecule is lost when a peptide bond is formed between amino acids

Side chains are not involved in the peptide bond. A chain of any length can be formed.
Describe the levels of protein structure:
Primary
The order of the amino acids in the chain (i.e. the sequence).
This gives the peptide, or protein, its uniqueness (e.g. insulin has two polypeptide chains linked by disulfide bonds)
Describe the levels of protein structure:
Secondary
Alpha helix; a 3D spatial arrangement of amino acids in a peptide chain, coiled. Hald by H bonds.
A triple helix is three alpha helix polypeptide chains woven together 'rope-like', providing strength (e.g. collagen)
Beta pleated sheet, consists of polypeptide chain arranged side-by-side. H bonds between different parts provide extra strength. Sheets not as stong as helix structure. The R groups are above and below the sheet (e.g. keratin)
Describe the levels of protein structure:
Tertiary
a complex 3D shape. The shape is determined by attractions and repulsions between side chains of amino acids in a single polypeptide chain. This is a typica structure of a globular protein, such as myoglobin.
Bonds/Forces that stabilise tertiary structures include:
  • Hydrophobic interactions, attractions between non-polar groups
  • Hydrophillic interactions, attractions between polar groups and water
  • Salt bridges, ionic interactions between acid and basic amino acids
  • Hydrogen bonds, occur between H and O or N
  • Disulfide bonds, strong covalent links between sulphur atoms of cysteine
Describe the levels of protein structure:
Quarternery
Is the combination of two or more protein units. It is stabilised by the same interactions foind in tertiary structures (e.g. four polypeptides join together in quaternary structure to form haemoglobin)
Describe how protein shape is maintained and various bonds that are involved
The unique conformation (shape) of a protein, gives the protein its biological activity.
Loss of conformation usually means loss of biological activity (denaturation)
Intermolecular bonds provide a proteins shape
Explain what is meant by denaturation of a protein
Denaturation is the disruption of bonds in secondary, tertiary and quaternary protein structures, causing them to lose/change their conformation
How can denaturation occur
Denaturation can occur through:
  • heat and chemicals - break in hydrogen bonds and disrupt hydrphobic interactions
  • Change in pH (acids and bases) - disrupts ionic bonds and breaks salt bridges
  • Heavy metal ions - can disrupt disulfide bonds
  • Agitation - breaks hydrogen bonds between peptide chains
Differentiate between globular and non-globular proteins in terms of structure and function
Globular proteins aquire a compact shape from attraction between side chains of the amino acid in the protein
Globular proteins carry out the work of the cells: functions such as synthesis, transport and metabolism
Myogflobin is a globular protein which stotres oxygen in skeletal muscle. Quaternary structure also binds iron and is known as haemoglobin
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