Structure of molecules means different things in different contexts.
In inorganic chemistry, the structure invariably refers to the molecular formulae (e.g., NaCl, `MgCl_2`, etc.).
Organic chemistry always write a two dimensional view of the molecules while representing the structure of the molecules (e.g., benzene, naphthalene, etc.).
Physicists conjure up the three dimensional views of molecular structures while biologists describe the protein structure at four levels.
(a) Primary Structure : The sequence of amino acids i.e., the positional information in a protein - which is the first amino acid, which is second, and so on - is called the primary structure.
(b) Secondary Structure : A protein is imagined as a line, the left end is represented by the first amino acid and the right end represented by the last amino acid.
The first amino acid is called as N-terminal amino acid. The last amino acid is called the C-terminal amino acid.
Protein thread is folded in the form of a helix, (similar to a revolving staircase)
In proteins, only right handed helices are observed. Other regions of the protein thread are folded into other forms in what is called the secondary structure.
( c) Tertiary Structure : The long protein chain is also folded upon itself like a hollow woolen ball, giving rise to tertiary structure. It gives us a 3-dimensional view of a protein.
Tertiary structure is absolutely necessary for the many biological activities of proteins.
( d) Quaternary Structure : Some proteins are an assembly of more than one polypeptide or subunits. These individual folded polypeptides are arranged with respect to each other (e.g. linear string of spheres, cubes etc.) it is the architecture of a protein called quaternary structure of a protein.
Haemoglobin : Human haemoglobin consists of 4 subunits. Two are identical to each other. Hence, two subunits are a type and two subunits are (3) type.
