Protteins are needed for growth and repair, they are formed from the monomer - amino acids and are broken down by protease.
Amino acids, Dipeptides and Polypeptides
There are around 20 amino acids in the body, and these join together in different combinations to form proteins. This is done through condensation reactions, resulting in a peptide bond and a loss of water.
When two amino acids join together in a condensation reaction, a dipeptide is formed - held together with a peptide bond.
These dipeptides can bond together in condensation reactions to form a polypeptide chain, and water is lost.
Proteins are polypeptides held together by peptide bonds, these are formed during condensation reactions.
Primary structure
The primary structure is the sequence of amino acids. This is not a protein, as it is not processed (folded), instead it is known as a polypeptide.
The primary structure is held by peptide bonds between the amino acids.
Secondary Structure
The secondary structure is where alpha-helices and beta-pleated sheets are formed. These form due to hydrogen bonding between different amino acids in the polypeptide chain.
Hydrogen bonds can form between:
- The -CO (carboxyl group) of one amino acid and the -NH (amine group) of another amino acid
- The -CO (carboxyl group) of one amino acid and the -OH (hydroxyl group) of another amino acid
Tertiary Structure
The tertiary structure is formed when the secondary structures (alpha helices and beta pleated sheets) fold up to form a very precise three-dimensional structure.
The forces which hold the tertiary structure are:
- Hydrogen bonds
- Ionic bonds
- Disulphide bonds
- Van der Waals forces
Hydrogen bonds
These bonds form between O- and H+
Where the shared electrons spend longer, there is a slight negative charge.
These bonds can be split using high temperatures and altered pH.
Ionic bonds
These bonds form between acids groups and basic groups.
They are also stronger than hydrogen bonds.
Ionic bonds are split by changing the pH.
Disulphide bonds
Disulphide bonds are covalent bonds which form between sulphur atoms
These bonds are stronger than hydrogen and ionic bonds
Reducing agents are used to split these bonds
Van der Waals forces
These are weak forces of attraction between non-polar groups
The water excluded from these hydrophobic side chains, helps to keep the side chains together
Van der Waals forces can be split by a rise in temperature
NOTE: anything with rings, -OH and NH3 is polar
Quaternary Structure
This is where proteins have more than one polypeptide chain held together in a precise 3D structure. They are held together with the same forces responsible for the formation of tertiary structures, these are: hydrogen, ionic, disulphide and van der waals forces.
They can also involve the addition of non-amino acid derived groups known as prosthetic groups. These can be formed from: metal ions, sugars, vitamins, methyl groups, phosphate groups ect.
An example of a quaternary protein is haemoglobin