Two-dimensional gel electrophoresis
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Two-dimensional gel electrophoresis, commonly abbreviated as 2-DE or 2-D electrophoresis, is a form of gel electrophoresis commonly used to analyze proteins. In 1-D electrophoresis, proteins (or other analytes) are separated in one dimension, so that all the analytes will lie along a line but be separated from each other by some property. 2-D electrophoresis begins with 1-D electrophoresis but then separates the analytes by a second property in a direction 90 degrees from the first. The result is that the analytes are spread out across a 2-D surface rather than along a line. Because it is less likely that two analytes will be the same in both properties than that they will be the same in just one property, analytes are more effectively separated in 2-D electrophoresis than in 1-D electrophoresis.
The two dimensions that proteins are separated into in this technique correspond to isoelectric point and mass.
To separate the proteins by isoelectric point, a gradient of pH is applied to a gel and an electric potential is applied across the gel, making one end more positive than the other. At all pHs other than their isoelectric point, proteins will be charged. If they are positively charged, they will be pulled towards the more negative end of the gel and if they are negatively charged they will be pulled to the more positive end of the gel. The proteins applied in the first dimension will move along the gel and will accumulate at their isolelectric point. That is, the point at which the overall charge on the protein is 0 (i.e a neutral charge).
Before separating the proteins by mass, they are treated with sodium dodecyl sulfate (SDS) along with other reagents. This denatures the proteins (that is, it unfolds them into long, straight molecules) and attaches a number of SDS molecules roughly proportional to the protein's length. Because a protein's length (when unfolded) is roughly proportional to its mass, this is equivalent to saying that it attaches a number of SDS molecules roughly proportional to the protein's mass. Since the SDS molecules are negatively charged, the result of this is that all of the proteins will have approximately the same mass-to-charge ratio as each other. In addition, proteins will not migrate when they have no charge (a result of the isoelectric focusing step) therefore the coating of the protein in SDS (negatively charged) allows migration of the proteins in the second dimension (NB SDS is not compatible for use in the first dimension as it is charged and a nonionic or zwitterionic detergent needs to be used). In the second dimension, an electric potential is again applied, but at a 90 degree angle from the first field. The proteins will be attracted to the more positive side of the gel proportionally to their mass-to-charge ratio. As previously explained, this ratio will be nearly the same for all proteins. The proteins' progress will be slowed by frictional forces. The gel therefore acts like a molecular sieve when the current is applied, separating the proteins on the basis of their molecular weight with larger proteins being retained higher in the gel and smaller proteins being able to pass through the sieve and reach lower regions of the gel.
The result of this is a gel with proteins spread out on its surface. These proteins can then be detected by a variety of means, but the most commonnly used stains are silver and coomassie staining. In this case, a silver colloid is applied to the gel. The silver binds to cysteine groups within the protein. The silver is darkened by exposure to ultra-violet light. The darkness of the silver can be related to the amount of silver and therefore the amount of protein at a given location on the gel. This measurement can only give approximate amounts, but is adequate for most purposes.
[edit] 2D Gel Analysis Software
Modern day 2-DE research often utilizes software-based image analysis tools. These tools primarily analyze bio-markers by quantifying individual proteins, and showing the separation between one or more protein "spots" on a scanned image of a 2-DE product. Additionally, these tools match spots between gels of similar samples to show, for example, proteomic differences between early and advanced stages of an illness. Software packages include Delta2D, PDQuest and Progenesis - among others. While this technology is widely utilized, the intelligence has not been perfected. For example, while PDQuest and Progenesis tend to agree on the quantification and analysis of well-defined well-separated protein spots, they deliver different results and analysis tendencies with less-defined less-separated spots.
Challenges for automatic software-based analysis include:
- incompletely separated (overlapping) spots (less-defined and/or separated)
- weak spots / noise (e.g., "ghost spots")
- running differences between gels (e.g., protein migrates to different positions on different gels)
- differences in software algorithms and therefore analysis tendencies
Modern software packages include advanced features, such as image warping, to try to compensate for running differences between gels. However, as noted above, 2-DE automated image analysis technology not been perfected - a fact which keeps manual visual analysis as the "gold standard" for validation. More advanced methods of discovery include the use of mass spectrometry.
[edit] See also
[edit] External links and references
- A 2-D electrophoresis tutorial on the web site of the Parasitology Group at Aberystwyth University
- ProteomeInformatics.net discussion forum and articles about 2D gel image analysis and related topics
- Arora, Pankaj S., et al. (2005). "Comparative evaluation of two two-dimensional gel electrophoresis image analysis software applications using synovial fluids from patients with joint disease". Journal of Orthopaedic Science 10 (2): 160-166. [1]
- JVirGel Create virtual 2-D Gels from sequence data.
- Biomolequles.com 2 Dimensional Gel Electrophoresis.
