Short Essay on the 3D Shape of Proteins

All the function of organisms at sub-cellular, cellular, tissue and organ levels are performed by a variety of proteins. Shape of cells also depends on proteins. Do you know the properties of protein function largely depends on its three dimensional (3D) structures which it attains. The 3D structure of those proteins is determined which are in pure form.

Therefore, for determination of 3D shape, proteins are isolated from the cells and purified. The pure form of protein is crystallised, passed through X ray crystallography and its 3D shape is deduced.

Purification and characterisation have been discussed in the preceding section. Protein structure refers to two important points (i) the building block of protein i.e. amino acid sequence, and (ii) shape of protein that arises due to folding.

In 1953, for the first time Frederick Sanger (twice Nobel prize winner) developed the method of protein sequencing. For this purpose he developed the reagent 1- fluoro-2,4-dinitrobenzene (FDNB). He demonstrated that a protein is made up of linear polymer of amino acids.

The alpha amino (NH₂~) and alpha carboxyl (COO”) group of two adjacent amino acids are linked together by a peptide bond. In 1950, Pehr Edman developed ‘phenyl isothioc anate’ reagent which sequentially removes amino acids from protein. Protein sequencing has been Frederick Sanger, described in detail in A Textbook of Biotechnology for Class XI by R.C. Dubey.

Sequential removal of amino acids from proteins can be done only when protein is in pure form.

During 1940s, E. Lilly (USA) provided pure insulin from which Sanger used 300 mg to sequence protein and deduced structural composition. But at present sequencing can be done by using 1-2 mg of protein in Edman’s sequenator.

Purified crystals of protein helped in studying the three dimensional shapes through X ray crystallography and modelling. The study was done by several scientists such as Linus Pauling, G.N. Ramachandran, Max Perutz, John Kendrew, etc.

In providing 3D shape they followed the fundamental rules and forces of protein folding. This idea gave birth to the concept of ‘planarity’ of peptide bond, alpha helix beta pleats, etc. (see A Textbook of Biotechnology for Class XI by R.C. Dubey.