The sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) is an electrophoretic technique used in biotechnology to separate proteins based on their molecular weight. Generally, proteins are amphoteric molecules that possess both positive as well as negative charges within the same molecule. Therefore, a uniform negative charge is given to protein molecules in order to move them in a single direction during electrophoresis. The negative charge is given by the sodium dodecyl sulfate (SDS), an anionic detergent. The native proteins are denatured by the SDS as it disturbs the non-covalent forces of proteins.
Key Areas Covered
1. What is SDS
– Definition, Structure
2. How Does SDS Denature Proteins
– Interaction Between Protein and SDS
3. What is the Role of SDS
– SDS in PAGE
Key Terms: Charge/Mass Ratio, Molecular Weight, Net Negative Charge, Proteins, Sodium Dodecyl Sulfate (SDS), SDS-PAGE
What is SDS
SDS (Sodium Dodecyl Sulfate) refers to an anionic detergent, consisting of a hydrophilic head group and a hydrophobic tail. Hence, when dissolved, its molecules form a net negative charge within a wide pH range. The structure of SDS is shown in figure 1.
Figure 1: SDS
How Does SDS Denature Proteins
As SDS is a detergent, the tertiary structure of proteins is disrupted by SDS, bringing the folded protein down into a linear molecule. Moreover, SDS binds to the linear protein in a uniform manner. Around 1.4 g SDS binds to 1 g of protein. Hence, SDS coats the protein in a net negative charge uniformly. This negative charge masks the intrinsic charges on various types of R-groups of the amino acids of the protein. In addition, the charge of the protein becomes proportional to the molecular weight. The linearized protein molecule by SDS is 18 Angstroms wide and the length of the protein is proportional to the molecular weight. The interaction between a protein and SDS is shown in figure 2.
Figure 2: SDS and Protein Interaction
What is the Role of SDS
The R-groups of amino acids in a particular protein may bear either positive or negative charge, making the protein an amphoteric molecule. Therefore, in the native state, different proteins with the same molecular weight migrate at different speeds on the gel. This makes the separation of proteins in the polyacrylamide gel difficult. The addition of SDS to the protein denatures the proteins and covers them in a uniformly-distributed, net negative charge. This allows the migration of proteins towards the positive electrode during electrophoresis. In other words, SDS linearizes the protein molecules and masks the various types of charges on R-groups. In conclusion, the charge to mass ratio in SDS-coated proteins is same; hence, there will be no differential migration based on the charge of the native protein. A SDS-PAGE of red blood cell membrane proteins is shown in figure 3.
Figure 3: SDS-PAGE
In addition to SDS-PAGE, SDS is used as a detergent in nucleic acid extractions for the disruption of the cell membrane and dissociation of nucleic acid: protein complexes.
Conclusion
SDS is an anionic detergent used as a detergent in various types of biotechnological techniques. It denatures the tertiary structure of a protein to produce a linear protein molecule. Furthermore, it binds to the denatured protein in a uniform manner, providing a uniform charge to mass ratio to all types of proteins. A net negative charge is given to the protein molecule by SDS by masking the charges on R-groups of amino acids of the protein. Hence, SDS allows the separation of proteins based on their molecular weight on a PAGE as the charge is proportional to the molecular weight of the denatured proteins by SDS.
Reference:
1. “How SDS-PAGE Works.” Bitesize Bio, 16 Feb. 2018, Available here.
Image Courtesy:
1. “SDS with structure description” By CindyLi2016 – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Protein-SDS interaction” By Fdardel – Own work (CC BY-SA 4.0) via Commons Wikimedia
3. “RBC Membrane Proteins SDS-PAGE gel” By Ernst Hempelmann – Ernst Hempelmann (Public Domain) via Commons Wikimedia
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