Levels Of Organisation In Proteins

Introduction:

Proteins are a class of bio-molecules essential for living organisms. They are high molecular mass complex biopolymers of amino acids joined together through peptide bonds having three-dimensional structures. They are vital chemical substances essential for growth and maintenance of life.

The structures of proteins are quite complex. Their complete structures are usually discussed at four different levels: Primary, Secondary, Tertiary, Quaternary and each level being more complex than the previous one.

Primary structure:

This structure refers to the covalent structure including disulphide bridges of each polypeptide chain. It simply refers to the sequence in which the various amino acids present in a protein are linked to one another.

The primary structure is usually determined by its successive hydrolysis with either enzymes or mineral acids into various products having decreasing molecular weight.

proteins----- proteoses------peptones----polypeptides----simple peptides----- α-amino acids.

This structure was first determined by Sanger. The different chemical and biological properties of various proteins are primarily due to the differences in their primary structures.

Secondary Structure:

The conformation which the polypeptide chains assume as a result of hydrogen-bonding is called the secondary structure of protein. Depending upon the size of the R groups, the following two different secondary structures are possible.

• α- Helix structure: This model was first postulated by Linus Pauling in 1951 purely on theoretical grounds and it was only later that it was verified through experiments. If the size of the R groups is large, the H-bonds (intermolecular) are formed between the C=O of one amino acid residue in the chain. This causes the polypeptide chain to coil up into a spiral structure called Right handed -helix structure.
• β- Flat sheet or β-pleated sheet structure: This structure was also proposed by Pauling. In this conformation, all polypeptide chains are stretched out to nearly maximum extension and then held side by side by inter-molecular H-bonds. This structure resembles pleated folds so called as or β-pleated sheet structure.

Tertiary Structure:

This structure of protein refers to its complete three-dimensional structure. Tertiary structure refers to the manner in which the entire protein molecule folds up in the three-dimensional space to produce a specific shape to produce a specific shape (or compact form). At normal pH and temperature, each protein will take up a shape and a tertiary structure that is most stable in terms of energy. The primary structure of protein dictates its secondary structure.

Quaternary structure:

Many proteins exist as a single polypeptide chains, there are certain proteins which exist as assemblies of two or more polypeptide chains called subunits or protomers. The quaternary structure refers to the determination of the number of subunits and their arrangement in an aggregate protein molecule.

Example: Haemoglobin, it is an aggregate of four polypeptide chains or subunits, two identical alpha chains and two identical beta chains. These four subunits lie more or less at the vertices of a regular tetrahedron and each chain carries a heme group at its end.