The purification of protein is an essential first step for the study of its molecular and biological properties in order to understand its biological function. There are several properties (such as molecular weight, charge, hydrophobicity, etc.) that can be exploited to purify or single out a protein from a mixture. Based on those properties, several chromatographic and non-chromatographic (electrophoretic, precipitation, membrane-filtration) procedures have become available (see Table 4.1).
In protein purification, it is important to adopt procedures that do not cause denaturation of proteins, especially the protein of interest. The choice of purification methods is also influenced by factors such as how the purified protein is to be used in studies, the quantity of the purified protein needed, and the cost of the materials and reagents used in the purification. A purification step that may denature purified protein is not suitable for studies of its biological properties, but may be suitable for the determination of its primary structure, subunit size, etc. The purification protocols for obtaining a microgram level of purified protein (for partial peptide sequence in order to construct a gene probe) may be different from those that yield larger quantities of purified protein. The cost of ligands used for immobilization of matrix and for elution of a bound protein in affinity chromatography may be limiting factors for large-scale purification.