The proteins are irregular or mixed polymers like the nucleic acids and unlike cellulose. The four basic aspects of protein structure are primary, secondary, tertiary and quartenary.
The primary structure of a protein is the linear order or unique sequence of amino acid monomers held together by peptide bonds to form the polypeptide chain. The primary structure controls the final three-dimensional structure of protein molecules and hence, also the physiological role it plays.
The secondary structure of protein refers to its specific spatial (in space) arrangement (organisation) of the polypeptide chain as a result of (or stabilised by) hydrogen bond formation between closely placed amino acid residues. It is a direct consequence of the sequential arrangement of the monomers.
The tertiary structure of protein refers to the complete three dimensional structure of the protein as a whole. Bending and folding of polypeptide in various ways to form spherical, rodlike or fibrilar structures. The folded polypeptide is stabilised by hydrogen bonds, ionic bonds, Vander Waals’ forces, besides the covalent bonds and hydrophobic interactions, to form the tertiary structure.
In proteins the covalent bonds (peptide, bonds and disulphide bonds) are the strongest. The disulphide (-S-S-) bonds are the most frequently encountered covalent bonds between the side chains of protein molecules. It is formed when two -SH groups of two cysteinyl residues lie close to each other. Besides, the protein conformation seem to depend on a large number non-covalent interactions. Though weak individuals, these interactions contribute enormously to the three-dimensional configuration.
The quaternary structure of protein results when it is composed of two or more polypeptide chains. Such proteins are often called multimeric proteins or oligomeric proteins. Each polypeptide develops its own tertiary structure and functions as subunit of the protein. The chains of subunits are packed together to give the conformation of a particular protein (Ex., Haemoglobin, four polypeptides). The allostenic proteins are often multimeric.