Amino acids can form bond with each other through a process of condensation reactions (loss of water). An OH- is lost from the carboxyl group and an H+ from the anime group, these form water. The bond is known as a peptide bond. This can also continue with more than one bond being formed, a bond of two amino acids is called a dipeptide and if more join forming a chain it is know as a polypeptide. This is very similar to polymers and monomers. These condensation reactions are undergone at the ribosomes inside cells and are controlled by enzymes. This reaction can also be reversed which is called hydrolysis (gain of water), this reaction is usually done in the stomach and small intestine but at ribosome as well.
There are three different ways of representing the structure of proteins:
- Primary structure
- Secondary structure
- Tertiary Structure
- Quaternary structure
The primary structure shows the basic form of the polypeptide. It shows the amino acids involved and in which sequence they are joined. This form is mainly used to work out the chemistry of the protein since it shows all of the chemicals used.
The secondary structure shows the effect the amino acids have on each other. The structures given are simplified down to, in a chain of polypeptide the –CO group is attracted to the –NH group ahead of it and so move towards each other, forming an α-helix. The hydrogen bond formed by this helix is strong enough to hold the structure together but is easily broken by interferences such as temperature and pH changes. This would have an important effect on a living organism. Not all proteins coil into an α-helix some make flat but slightly bent shape known as β-pleated sheet. Others have no arrangement at all. The R group affects what the secondary structure of the chemical is like.
The Tertiary Structure is an advancement on the secondary structure and shows how more complex molecules fold in a 3D environment, since they cannot be simplified to diagrams like the α-helix. The picture shown has very accurate and precise dimensions and measurements. There are four different types of bonds which can form between the amino acids:
Hydrogen bonds form between a variety of R groups,
Disulphide bonds form between cysteine molecules,
Ionic bonds form between R groups containing amine and carboxyl groups,
Hydrophilic bonds interactions occur between R groups which are non-polar or hydrophobic.
Quaternary Structure is the association of different polypeptides chains in protein. How different proteins are joined together.
Globular and fibrous proteins have a special structure where the molecules curl up into a “ball” e.g. myoglobin, haemoglobin. The reason for the molecule folding is to place the hydrophobic ends in the center and the hydrophilic ends pointing outwards. This will allow it to dissolve. Globular proteins have a lot of roles in metabolic reactions, enzymes being globular proteins. The opposite of globular proteins are long strands known as fibrous proteins. These have more structural roles such as keratin in hair and collagen in skin.
