Gel electrophoresis of proteins
Protein electrophoresis is a method for analysing the proteins in a fluid or an extract. The electrophoresis may be performed with a small volume of sample in a number of alternative ways with or without a supporting medium: SDS polyacrylamide gel electrophoresis (in short gel electrophoresis, PAGE, or SDS-electrophoresis, free flow electrophoresis, electrofocusing, isotachophoresis, affinity electrophoresis, immunoelectrophoresis, counterelectrophoresis, and capillary electrophoresis). Each method has many variations with individual advantages and limitations. Gel electrophoresis is often performed in combination with electroblotting immunoblotting to give additional information about a specific protein. Because of practical limitations, protein electrophoresis is generally not suited as a preparative method.
Denaturing gel methods 
SDS-PAGE, sodium dodecyl sulfate polyacrylamide gel electrophoresis, describes a collection of related techniques to separate proteins according to their electrophoretic mobility (a function of the length of a polypeptide chain and its charge) while in the denatured (unfolded) state. In most proteins, the binding of SDS to the polypeptide chain imparts an even distribution of charge per unit mass, thereby resulting in a fractionation by approximate size during electrophoresis.
SDS is a strong detergent agent used to denature native proteins to unfolded, individual polypeptides. When a protein mixture is heated to 100 °C in presence of SDS, the detergent wraps around the polypeptide backbone. In this process, the intrinsic charges of polypeptides becomes negligible when compared to the negative charges contributed by SDS. Thus polypeptides after treatment become rod-like structures possessing a uniform charge density, that is same net negative charge per unit length. The electrophoretic mobilities of these proteins will be a linear function of the logarithms of their molecular weights.
Native gel methods 
Native gels, also known as non-denaturing gels, analyze proteins that are still in their folded state. Thus, the electrophoretic mobility depends not only on the charge-to-mass ratio, but also to the physical shape and size of the protein.
Blue Native PAGE 
BN-PAGE is a native PAGE technique, where the Coomassie Brilliant Blue dye provides the necessary charges to the protein complexes for the electrophoretic separation. The disadvantage of Coomassie is that in binding to proteins it can act like a detergent causing complexes to dissociate. Another drawback is the potential quenching of chemoluminescence (e.g. in subsequent western blot detection or activity assays) or fluorescence of proteins with prosthetic groups (e.g. heme or chlorophyll) or labelled with fluorescent dyes.
Clear Native PAGE 
CN-PAGE (commonly referred to as Native PAGE) separates acidic water-soluble and membrane proteins in a polyacrylamide gradient gel. It uses no charged dye so the electrophoretic mobility of proteins in CN-PAGE (in contrast to the charge shift technique BN-PAGE) is related to the intrinsic charge of the proteins. The migration distance depends on the protein charge, its size and the pore size of the gel. In many cases this method has lower resolution than BN-PAGE, but CN-PAGE offers advantages whenever Coomassie dye would interfere with further analytical techniques, for example it has been described as a very efficient microscale separation technique for FRET analyses. Also CN-PAGE is milder than BN-PAGE so it can retain labile supramolecular assemblies of membrane protein complexes that are dissociated under the conditions of BN-PAGE.
Quantitative Preparative Native Continuous PAGE 
In contrast to CN- and BN-PAGE the folded protein complexes of interest may separate cleanly and predictably in fractions, since they move through the polyacrylamide gel as quickly as individual, denatured proteins due to pH 10.00 of the electrophoresis buffer and non-restrictive conditions. The separated proteins are continuously eluted into a physiological eluent (pH 8.00) and transported to a fraction collector. In specific PAGE fractions the metal cofactors can be identified and quantified. The structure of isolated metalloproteins can be elucidated by non-denaturing methods.
Protein electrophoresis in medicine 
In medicine, protein electrophoresis is a method of analysing the proteins mainly in blood serum (blood plasma is not suitable). Before the widespread use of gel electrophoresis, protein electrophoresis was performed as free flow electrophoresis, on paper, or as immunoelectrophoresis.
Traditionally, two classes of blood proteins are considered: serum albumin and globulin. They are generally equal in proportion, but albumin as a molecule is much smaller and lightly negatively charged, leading to an accumulation of albumin on the electrophoretic gel. A small band before albumin represents transthyretin (also named prealbumin). Some forms of medication or body chemicals can cause their own band, usually small. Abnormal bands (spikes) are seen in monoclonal gammopathy of undetermined significance and multiple myeloma, and are useful in the diagnosis of these conditions.
The globulins are classified by their banding pattern (with their main representatives):
- The alpha (α) band consists of two parts, 1 and 2:
- The beta (β) band - transferrin, LDL, complement
- The gamma (γ) band - immunoglobulin (IgA, IgD, IgE, IgG and IgM). Paraproteins (in multiple myeloma) usually appear in this band.
Normal present medical procedure involves determination of numerous proteins in plasma including hormones and enzymes, some of them also determined by electrophoresis. However, gel electrophoresis is mainly a research tool, also when the subject is blood proteins.
See also 
- Affinity electrophoresis
- Polyacrylamide gel electrophoresis, PAGE, or gel electrophoresis
- Native gel electrophoresis
- Fast parallel proteolysis (FASTpp)
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- Wittig, I.; Braun, H.P.; Schägger, H. (2006). "Blue native PAGE". Nat. Protoc. 1 (1): 418–428. doi:10.1038/nprot.2006.62. PMID 17406264
- Wittig, I.; Schägger, H. (Nov 2005). "Advantages and limitations of clear-native PAGE". Proteomics 5 (17): 4338–46. doi:10.1002/pmic.200500081. PMID 16220535.
- Gavin P.D., Devenish R.J., Prescott M. (2003). "FRET reveals changes in the F1–stator stalk interaction during activity of F1F0-ATP synthase". Biochim Biophys Acta 1607 (2-3): 167–79. doi:10.1016/j.bbabio.2003.09.013.
- Kastenholz, B. (2006). "Important contributions of a new quantitative preparative native continuous polyacrylamide gel electrophoresis (QPNC-PAGE) procedure for elucidating metal cofactor metabolisms in protein-misfolding diseases – a theory". Protein Pept Lett 13 (5): 503–8. doi:10.2174/092986606776819637. PMID 16800806.
- Minde DP (2012). "Determining biophysical protein stability in lysates by a fast proteolysis assay, FASTpp". PLOS ONE 7 (10): e46147. doi:10.1371/journal.pone.0046147. PMC 3463568. PMID 23056252.
- The comprehensive text edited by Niels H. Axelsen in Scandinavian Journal of Immunology, 1975 Volume 4 Supplement. This is the text of choice for immunoelectrophoresis.
- Discontinuous native protein gel electrophoresis
- Protein purification facility
- Educational resource for protein electrophoresis