SDS-PAGE is one of the most widely utilized techniques in many fields of science, including molecular biology, biochemistry, forensics and genetics.
SDS-PAGE is a fascinating tool as it is able to separate proteins on a gel according to their polypeptide chain length. The polypeptide chain length of a protein gives it electrophoretic mobility, which means, the ability of the protein due to its length to move through the sieves and pores of a gel. Protein folding, as well as post-translational modifications can also alter a proteins electrophoretic mobility and give readings not directly related to the proteins polypeptide chain length. A molecular marker, usually placed to the left of the tested samples, allows comparisons of marker proteins sizes with the tested proteins.
SDS-PAGE of samples with identical charge to mass ratios causes fractionation to occur by size.
A simple, effective and very high resolution method to fractionate and analyze protein mixtures, is the sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis (PAGE). Electrophoresis is a critical tool used to separate complex mixtures of proteins, to purify proteins, to elucidate homogeneity of proteins samples, but also as depicted in our investigation, to investigate the subunit compositions of proteins. SDS-PAGE separates proteins on the basis of their molecular size. This is obtainable by allowing the SDS-covered proteins to migrate through the pores of a polyacrylamide gel matrix, which consists of abundant pores (vary in size and number with % acrylamide used). (3)
SDS is an anionic detergent, which binds and reacts with the proteins in the solution, and destroys the tertiary structure of the protein, leading to partial unfolding of the polypeptide chain. In addition, SDS binds to both the hydrophilic and hydrophobic regions of the polypeptide chain, giving the protein an excessively net negative chrage, which diminuishes any intrinsic amino acid charge. The protein also adopts a cylindrical shape, which is coated along its' entire surface with negatively charged sulfonate ions. In addition, beta-mercaptoethanol may be used to reduce disulfide bonds, which forms mixed disulfides with cystein side chains. However, the SDS coated proteins are now denatured and biologically non-functionable. (3) A general rule when using SDS, is that the amount of SDS bound per gram of protein is found to be constant at a SDS:protein ratio of about 1.4 gram/SDS gram protein. Moreover, this ration is achieved under reducing conditions, but is altered with carbohydrate-containing proteins, such as immunoglobulins, which are known to bind less SDS than other similar sized proteins. Therefore, an inverse relationship develops between the mobility versus the proteins logarithm mass. A calibration curve with a set of standard proteins of known mass can be projected and then used to determine the molecular mass weights of unknown proteins through a method of comparison. (2,3,4)
Through the use of a supporting medium called polyacrylamide, with the application of an electric field through this medium, the SDS megatively charged protein complexes in a protein mixture can be separated. Polyacrylamide is a synthetic polymer, which is formed by the polymerization of acrylamide monomer with additional bifunctional crosslinking agents (aided by a catalyst). This polymerized polyacrylamide matrix is a three-dimensional network of pores whose size is determined by the percentage degree of acrylamide monomer and cross-linker concentration utilized in the mixture (pore size decreases with higher acrylamide gel concentrations) and is often referred to as a separating or running gel. The pores within the polyacrylamide gel are comparable in molecular size to the size of protein molecules. Upon electrophoresis, which applies an electric field through the pores in the gel matrix, the proteins are sieved through the pores of the gel with the larger proteins having a slower migration rate than the smaller proteins. The negative charges flow from the negative cathode terminal into the upper buffer chamber, through the gel, and into the lower buffer chamber, which is connected to the positive terminal. Therefore, the negatively charged SDS coated proteins migrate towards the anode. The combination of gel pore size and protein charge, size and shape determines the migration ability of the protein. (3)
The Laemlli gel method, dematuring SDS discontinuous gel electrophoresis. The polyacrylamide gel is topped by a stacking gel and secured in an electrophoresis apparatus. Samples are solubilized by boiling, placed in a gel lane and separated electrophoretically. The stacking gel, which the samples pass through first, has large pores. The chloride ions, leading ions, whose mobility in an electrophoretic field is greater than the protein samples. The electrophoresis buffer contains glycine ions, trailing ions, whose mobility is less than the mobility of the protein. In the sample. Thereofre, a zone of lower conductivity is found between the faster migrating ions and the migrating proteins. This high voltage gradient in the zone allows the proteins to stack in the zone between the leading and trailing ions. Following the stacking gel, the separating gel consists of a smaller pore size, a higher salt concentration and a higher pH, in comparison with the stacking gel. In the separating gel, the glycine ions pass the proteins and allow for the proteins to separate accodring to their molecular size. Subsequently, the proteins can be visualized by staining with a dye, such as Coomasie blue. (3) SDS-PAGE combined with other fractionational procedures, such as isoelectric focusing, can allow for two-dimensional electrophoresis capable of resolving similar molecular size proteins (for populations of proteins). Therefore, SDS-PAGE is capable of adding excessively high resolution and simplicity not acheived with any other electrophoretic method yet.
1. Janeway, C.A., Travers, P., Walport, M., and Capra, J.D. 1999. Immunobiology: The immune system in health and disease. Garland Publishing. 4th ed., New York, USA
2. Delves, P. and Roitt, I. 1999. Encyclopedia of Immunology: Academic Press Inc., 2nd ed., San Diego, USA
3. 1994. Current Protocols in Molecular Biology. Volume 2. John Wiley & Sons Inc., USA
4. Cruse, J. and Lewis, R. 1995. Illustrated Dictionary of Immunology. CRC Press Inc., USA
Find information on Sodium Dodecyl Sulfate Polyacrylamide Gel Electrophoresis (SDS-PAGE). Please click on the left column categories for specific information, links, and protocols.