It is difficult to describe in a simple sentence the role of proteins. Lets say:
when there is something to do, it is a protein that does it.Therefore proteins are
Some examples of proteins
The role (or function) of a protein depends on its shape, and chemical formula. Therefore, the structure of proteins will be explained first.
Proteins are synthesised primarily during a process called translation. The building blocks of the proteins are amino acids . Proteins are made of a long chain of amino acids, sometimes modified by the addition of heme, sugars, or phosphates.
This is the sequence of amino acids, which form a chain connected by peptide bonds. The amino acid sequence of a protein determines the higher levels of structure of the molecule. A single change in the primary structure (the amino acid sequence) can have a profound biological change in the overall structure and function (e.g. Please refer to the chapter about genes where examples are given).
Note: If there are some cysteines in the amino-acid sequence, they often react two by two to form disulphide bridges. Disulphide bridges are part of the primary structure.
The primary structure of a protein is its amino acid sequence and the disulphide bridges, i.e. all covalent connections in a protein.
The secondary structure is the way a small part, saptially near in the linear sequence of a protein folds up into:
The tertiary structure (or conformation)
is the way random coils, alpha -helixes and beta
-pleated sheets fold in respect to each other, ie it refers to the
protein as a whole.
The quaternary structure is the arrangement
of polypeptide subunits within complex proteins made up of two or more
subunits, sometimes associated with non proteic groups.
This image shows a dimer (two monomers) of a simple protein.
A good example is the quaternary structure of haemoglobin, made up of two alpha and two beta polypeptide chains.
When you have loaded the molecule into RasMol, try to type the following commands in the Command window:
Now the quaternary structure of the protein should be easy to spot.