Topic 7.5: Proteins

7.5.1 Explain the four levels of protein structure, indicating the significance of each level.

There are four levels of protein sturcture; primary, secondary, tertiary and quaternary level.

Primary:

Simple linked amino acids bonded together through peptide bonds. The amino acids are defined by their side chain (R group). Most polypeptide are 50-1000 amino acids long. The protein is read from the amino group (NH2) to the carboxyl group (COOH).


Secondary:

Alpha-helix:

The alpha helix is formed from hydrogen bonds and are often the basis of fibrous protein.

Beta-pleated sheets:

The polpeptide chain is much more stretched out in comparison to the alpha helix. This "sheet" has twists which increases the strength and rigidity of the structure.


Tertiary:

Tertiary structures contain a combination of alpha helicies and beta-pleated sheets. It is held together in that particular shape through a series of different forces of attraction, which include; Hydrogen bond, Van der Waal's forces, disulphide bond and ionic bond.


Quaternary:

The quaternary structure of protein arises when two or more proteins become held together, forming a complex, biologically active molecule. An example is haemoglobin, consisting of four polypeptide chains held around a non-protein haem group.


7.5.2 Outline the difference between fibrous and globular proteins, with references to two examples of each protein type.

7.5.3 Explain the significance of polar and non-polar amino acids.

Polar amino acid have hydrophilic R groups, whereas non-polar amino acids have hydrophobic R groups

For water-soluble proteins, non-polar amino acids tend to be found in the centre of the protein (stabilizing structure) while polar amino acids are found on the surface (capable of interacting with water molecule)

For membrane-bound proteins, non-polar amino acids tend to be localised on the surface in contact with the membrane, while polar amino acids line interior pores (to create hydrophilic channel)

For enzyme, the active site specifically depends on the location and distribution of polar and non-polar amino acids as hydrophobic and hydrophilic interaction can play a role in substrate binding to the active site.

7.5.4 State four functions of protein, giving a named example for each.

Great way to remember different functions (SHIT ME)

Structure: Support for body tissues (Collagen, Elastin or Keratin)
Hormones: Regulation of homoestatis (Insulin or Glucagon)
Immunity: Bind antigens (Antibodies or immunoglobulins)
Transport: Movement of substrates (Haemoglobin, myoglobin)

Movement: Muscle contraction (actin/myosin, troponin/tropomysin)
Enzyme: Speeding up metabolic reaction (catalase, lipase, pepsin)