Protein Purification by Affinity Chromatography

Medicilon’s protein scientists have been working on protein expression and purification for many years. We can start your project even you have nothing in hand but the name of your protein. In Medicilon’s laboratories, protein purification is performed in scales from micrograms and milligrams. All Protein Purification Services start with the analysis of physico-chemical and biological properties of a target protein resulting in the development of tailored procedures for its extraction, purification and characterization. Email: marketing@medicilon.com.cn web:www.medicilon.com

1. Introduction Affinity chromatography is a method which depends essentially on the interaction between the molecule to be purified and a solid phase that will allow the separation of contaminants. Lectins are carbohydrate-binding proteins which can be purified by affinity chromatography; also, the presence of multiple molecular forms of lectins in a preparation can be separated. Immobilized lectins have been useful to affinity protein purification. In immunoaffinity chromatography an antibody or an antigen is immobilized on a support so as to purify the protein against which the antibody was developed. Monoclonal antibodies are extremely useful as immunosorbents for purification of antigen. Immobilization of monoclonal antibody on a suitable material to the column produces a support that will bind with high selectivity to protein against which the antibody was developed. Affinity chromatography containing DNA is a highly specific and important technique for the purification of DNA-binding proteins involved in the transcription, replication and recombination. The success of affinity chromatography depends on the conditions used in each chromatographic step. So, the optimization of protocol is essential to achieve optimal protein purification with maximum recovery. 2. Nomenclature and basic concepts The term "affinity chromatography", first used by Cuatrecasas et al. (1968), refers to a purification technique which depends essentially on the highly specific interaction between the molecule to be purified and the solid phase that will allow the separation of contaminants. This method has several other terms such as "bioselective adsorption", which was appropriately used to denominate an adsorption chromatography that uses a very special kind of affinity between the desired biological product and a biomolecule (Porath, 1973). For example, the biological affinity between an enzyme (protein with catalytic activity) and its substrate and/or other small ligand – usually in the active or allosteric site of the enzyme – results from a selective interaction. The adsorption corresponds to the fixation of the molecules of a substance (the adsorbate) on the surface of another substance (the adsorbent), which may be immobilized to an insoluble support. The adsorbate and the adsorbent can be referred as bioligands. The bioligands may be specific or may not have absolute specificity of interaction. Many bioligands (e.g. NAD+, ATP, coenzyme A) may bind different enzymes, being then called group-specific ligands. In the same manner, chitin (a polysaccharide composed by N-acetylD-glucosamine units) may be an adsorbent for several different molecules if they possess a group (e.g. a binding or catalytic site) able to interact with chitin. Affinity chromatography is a powerful tool for the purification of substances in a complex biological mixture. It can also serve to separate denatured and native forms of the same substance. Thus, biomolecules which are difficult to purify have been obtained using bioselective adsorbents, e.g. immobilized metal ions (Ni2+ and Zn2+) used to purify proteins containing zinc finger domains with natural affinity to divalent ions (Voráčková et al., 2011). The relative specificity degree of the affinity chromatography is due to the exploitation of biochemical properties inherent in certain molecules, instead of using small differences in physicochemical properties (such as size, form and ionic charge, which are employed by other chromatographic methods). Affinity chromatography may be used with different final objectives. If the aim is a rapid purification of a macromolecule with high yield, many controls and careful attention are necessary to establish the best conditions for a high bioselectivity of the system; the researcher must be prepared to adjust the chromatographic conditions and to circumvent possible absence of bioselectivity or low yields. If the objective is to first demonstrate a bioselectivity for further purification, the choice of the bioselective adsorbent is dependent on the physiological interaction between the bioselective component and the macromolecule to be purified. In this case, the researcher must spend a lot of time establishing the bioselectivity before starting the isolation experiments. A good bioselectivity means that the affinity of the molecule by the ligand exceeds all factors of non-specific adsorption that are present in the system. Also, the affinity should not be so strong, since the biomolecule must be removed from the column. A well-designed affinity method should consider the selection of the ligand molecule or the insoluble support to be used; they must have specific and reversible binding affinity for the molecule being purified. After defining the protocol, purification by affinity chromatography is a rapid method, compared with others less specific. The technique also enables the concentration of the molecule of interest resulting in a small volume of a concentrated product. Standard procedures of protein purification result in obtainment of homogeneous protein. However, a considerable cost of supplies and hours of work is often required and a low yield is obtained after several steps. The power of affinity chromatography is often larger than other chromatographic techniques, resulting in several hundred or thousand-fold purification factors in a single step. 3. Supports for affinity chromatography A good support for affinity chromatography should be chemically inert or have minimal interaction with other molecules, having high porosity and large number of functional groups capable of forming covalent bonds with the molecule to be immobilized. Many materials are available (Table 1). A variety of supports with immobilized ligands, or stable media for the immobilization of ligands through different functional groups are commercially available. The ligand molecule to be used should contain a group capable of being chemically modified, often an amino group, which will allow connection with the matrix without destroying its capacity to bind to the molecule of interest.