SDS-PAGE: Principles and Applications

How does SDS-PAGE separate proteins?

SDS-PAGE separates proteins primarily by molecular weight. Sodium dodecyl sulphate (SDS), a strong anionic detergent, binds uniformly to proteins, imparting a negative charge proportional to their mass. This masks the intrinsic charge of the proteins, ensuring that separation is based solely on size. The proteins are then denatured using a reducing agent such as DTT or TCEP, which breaks disulphide bonds, ensuring complete unfolding. When an electric field is applied, the negatively charged proteins migrate through the polyacrylamide gel matrix, with smaller proteins moving faster than larger ones.

What are the key components and setup of an SDS-PAGE system?

The system consists of a polyacrylamide gel, typically made from acrylamide and bis-acrylamide, which forms a molecular sieve. The gel is cast in a vertical apparatus with stacking and resolving gel layers. The stacking gel (lower acrylamide concentration) concentrates proteins into a sharp band before they enter the resolving gel (higher concentration), improving resolution. Proteins are loaded with a sample buffer containing SDS, a reducing agent, and a tracking dye (e.g., bromophenol blue). After electrophoresis, proteins are visualised using stains such as Coomassie Brilliant Blue, silver stain, or fluorescent dyes.

What are the limitations and considerations?

While SDS-PAGE is robust and reproducible, it does not preserve native protein structure or function. It also cannot resolve proteins with very similar molecular weights or those that are highly charged or hydrophobic. Additionally, the method requires careful handling of acrylamide, a neurotoxin, and proper disposal of waste. It is commonly used in conjunction with Western blotting (immunoblotting) for protein identification.

Related concepts

SDS-PAGE is often used alongside techniques such as Western blotting, mass spectrometry, and protein quantification assays. It is a standard method in protein purification, quality control, and research in biotechnology, pharmaceuticals, and life sciences.