The Structure Of Proteins

            
            
            
             The primary structure of a protein is the type and sequence of the amino acids used in the polypeptide. It is connected by peptide bonds, and the sequence determines the structure and shape of the whole protein. A single change can make a big difference to the end result. If there are two cysteines in the sequence, as shown in the diagram, then they will react and form a disulphide bridge. All covalent links in a protein are included in the primary structure. It contains all the materials needed for the cell. Any further structure is a result of the primary structure's sequence.
             Secondary Structure.
             The secondary structure of a polypeptide is the way a small part, fairly near in the polypeptide sequence, curls up into an alpha helix or beta pleated sheets, or sometimes even no structured shape at all. The atoms attract each other across the space so that there are more links between atoms, causing the chain to curl. This creates a kinked chain. This is the alpha helix. A beta pleated sheet is a form of shape where the bonds create a kinked shape, and attract across the space, as in the diagram overleaf. This causes the structure to become more like a folded piece of paper, as the chains cannot curl round to meet the attractive atoms because there are other forces pulling it the other way. This is a much looser structure than the alpha helix. The secondary structure is still two dimensional.
             Tertiary Structure.
             The tertiary structure is that which causes the secondary structure itself to fold around, creating an even more complex structure. It creates a three dimensional shape, as it folds around in many directions following the various pulls of atoms. Various types of bonds are used in these links: hydrogen bonds between R groups, disulphide bonds between two cysteines molecules, as in the primary structure, ionic bonds between R groups containing amine and carboxyl groups, and hydrophobic reactions between R groups which are non-polar all contribute to holding the tertiary structure of a protein.