Protein usually exists in tissues or cells in the form of complex mixtures, and each type of cells contains thousands of different protein. The separation and purification of protein is an arduous and arduous task. Up to now, there is no one or a set of ready-made methods to extract any kind of protein from complex mixtures, but it is possible to choose appropriate separation and purification procedures for any kind of protein to obtain high-purity products.
The general goal of protein purification is to try to improve the purity or specific activity of the product. The requirement of purification is to separate the desired protein, especially the unwanted exogenous protein, from all other components of the cell with reasonable efficiency, speed, yield and purity, while still retaining the biological activity and chemical integrity of the polypeptide.
Protein can be purified from thousands of protein mixtures because different protein have great differences in many physical, chemical, physicochemical and biological properties, which are caused by the different amino acid sequences and quantities in protein. Amino acid residues attached to the polypeptide backbone can be positively charged, negatively charged, polar or nonpolar, hydrophilic or hydrophobic. In addition, peptides can be folded into well-defined secondary structure (α helix, β folding and various corners), tertiary structure and quaternary structure, forming unique residues with unique size, shape and distribution on the surface of protein. Based on the differences between protein to be separated and other protein, a set of reasonable classification steps can be designed.