An example of these structures is shown below
A tertiary structure is another stage up from the secondary structure and consists of a complicated three-dimensional shape made up when the polypeptide chain coils around itself. The tertiary structure is also different in the way it contains four different types of bonding, compared to the normal one. Each bond is between adjacent parts of the coil. The four different types of bond are:
Hydrogen – They normally occur between two hydrogen atoms but can exist with some oxygen and nitrogen atoms
The hydrogen atoms contain a small positive charge and the oxygen/nitrogen small negative charges. These opposite charges attract to form the bonds.
Ionic – these bonds occur between any charged groups that are not joined by a peptide bond. This type of bonding is stronger than the hydrogen but can be broken by temperature of a change in pH levels.
Disulphide – Some amino acids like methionine and cysteine contain sulphur within them, and this is where the disulphide bonds occur. Disulphide bonds are exceptionally strong and they contribute to most of the structural strength of the protein.
Hydrophobic effect – this helps only some proteins maintain their structure by when globular proteins are placed in a solution, their hydrophobic groups point inwards away from the water.
Tertiary proteins can also be split into two categories: Globular and Fibrous Proteins.
Fibrous proteins are constructed of long polypeptide chains laid down in parallel lines along each other, connected by hydrogen bonds. Examples of these fibrous proteins are: Fibrin, Collagen, and Keratin.
Globular proteins are more of a spherical shape and most enzymes, antibodies and some hormones are globular proteins. These compact molecules are highly twisted polypeptide chains that form into a ball shape, with the hydrophobic amino acids in the middle and the hydrophilic amino acids on the outside. This make them water soluble and less stable, being suitable for metabolic reactions. The picture below shows an example of a fibrous and globular protein.
A Quaternary structure is when a number of tertiary structures are linked together thus creating a complex, biologically active molecule. Also, a quaternary structure may also contain non-protein molecules inside it. These molecules are referred to as prosthetic groups. This diagram below provides an example of a quaternary structure.
Section B
A Haemoglobin molecule consists of four polypeptide chains. These chains are two identical alpha-glucose chains and two beta-glucose chains, all held together by a series of disulphide bonds. Within each chain there is an iron-containing group called Haem which allows it to pick up 4 oxygen molecules (one for each Haem group).
The haemoglobin molecule is an example of a prosthetic group, which when joined to another protein creates a conjugated protein.
Collagen is a Fibrous tertiary structure which contains three polypeptide chains. Each chain is coiled to form a helix and is then coiled round one another to form a rope like structure (as shown below). This structure is reasonably tough considering it is held together by hydrogen bonds. Collagen is a component of the blood, tendons and bones.
