Proteins

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

There are four levels of protein structure:

Primary Structure: The primary structure of a protein is its amino acid sequence. This amino acid sequence is determined by the base sequence of the gene which codes for the protein. 

Secondary Structure: Secondary structures have α-helices and β-pleated sheets. These form as a result of hydrogen bonds between the peptide groups of the main chain. Therefore, proteins that contain secondary structures will have regions that are cylindrical (α-helices) and/or regions that are planar (β-pleated sheets).

Tertiary Structure: The tertiary structure of a protein is its three-dimensional conformation which occurs as a result of the protein folding. This folding is stabilised by hydrogen bonds, hydrophobic interactions, ionic bonds and disulphide bridges.  These intramolecular bonds form between the R groups of different amino acids. 

Quaternary Structure: A quaternary structure is formed when two or more polypeptide chains associate to form a single protein. An example is haemoglobin which consists of four polypeptide chains. In some cases, some proteins can have a non-polypeptide structure called a prosthetic group. These proteins are called conjugated proteins. The haem group in haemoglobin is a prosthetic group.

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

Protein shape can be categorised as either fibrous or globular. Fibrous proteins tend to be elongated, physically tough and insoluble in water. Collagen found in the skin and keratin found in hair are examples of fibrous proteins. Globular proteins tend to be compact, rounded and water soluble. Haemoglobin and enzymes are examples of globular proteins.

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

Amino acids have different R groups. Some of these R groups will be hydrophilic, making the amino acid polar, while others will be hydrophobic, making the amino acid non-polar. The distribution of the polar and non-polar amino acids in a protein influences the function and location of the protein within the body. Non-polar amino acids are found in the centre of water soluble proteins while the polar amino acids are found at the surface. 

Examples of how the distribution of non-polar and polar amino acids affect protein function and location:

Controlling the position of proteins in membranes: The non-polar amino acids cause proteins to be embedded in membranes while polar amino acids cause portions of the proteins to protrude from the membrane. 

Creating hydrophilic channels through membranes: Polar amino acids are found inside membrane proteins and create a channel through which hydrophilic molecules can pass through. 

Specificity of active site in enzymes: If the amino acids in the active site of an enzyme are non-polar then it makes this active site specific to a non-polar substance. On the other hand, if the active site is made up of polar amino acids then the active site is specific to a polar substance.

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