Comprehensive Study Notes on SDS-PAGE Principles and Techniques

Introduction to SDS-PAGE and Comparison with DNA Electrophoresis

• SDS-PAGE stands for Sodium dodecyl sulfate polyacrylamide gel electrophoresis.

• It is a specific technique used to separate proteins based on their molecular weights using an electric field.

• While the fundamental principle of electrophoresis remains the same, there are significant differences between separating proteins and the DNA electrophoresis performed earlier in the semester.

• Characteristics of DNA Electrophoresis:

  • There is very little variation in the structure of DNA.

  • Each base pair inserted into the DNA molecule brings a phosphate group with it.

  • The sizes of DNA bases are relatively close since they are always paired.

  • The average molecular weight of a base pair is approximately $650$ Daltons.

  • Every base pair imparts exactly two negative charges to the molecule.

  • Because the charge-to-mass ratio is consistent, DNA molecules are separated solely by length.

• Characteristics of Protein Separation:

  • There is no true molecular weight average for amino acids.

  • Amino acids exhibit massive variation in weight: glycine is very lightweight, whereas tryptophan and arginine are heavy by comparison.

  • The charge per residue varies significantly; amino acids can be negatively charged, positively charged, or neutral.

  • Due to this extreme variation in mass and charge, different techniques must be employed for protein separation compared to DNA.

The Separation Matrix: Agarose vs. Polyacrylamide

• Proteins require a different medium for separation than DNA due to their size differences.

• Agarose Gel: Used for DNA; it has a relatively low density and large pores.

• Polyacrylamide Gel: Used for proteins; it has a much higher density than agarose, which is necessary because proteins are significantly smaller than the DNA fragments typically studied.

• Example Case: Green Fluorescent Protein (GFP):

  • Protein Size: Roughly $240$ amino acids.

  • Protein Weight: Approximately $30\,kDa$.

  • Encoding Gene Size: Roughly $800$ base pairs.

  • Encoding Gene Weight: Upwards of $500\,kDa$.

• Size Disparity Rule of Thumb: As a rough estimate, taking the size of a protein and multiplying it by $20$ yields the approximate size of the gene (in Daltons) required to encode that specific protein.

• Because proteins are so much smaller than their parent genes, agarose is not dense enough for fine separation.

Chemistry and Polymerization of Polyacrylamide Gels

• Polyacrylamide is a separation matrix created by the cross-linking of acrylamide molecules.

• Key Molecules:

  • Acrylamide: Contains a single double bond that participates in the polymerization reaction.

  • Bis acrylamide (N,N'-Methylenebisacrylamide): Contains two double bonds. One polymerizes into one growing polyacrylamide chain, while the second polymerizes into a different chain, effectively linking the two strings together.

• Reagents for Polymerization:

  • Acrylamide and Bis acrylamide solution: The base materials for the matrix.

  • Ammonium persulfate (APS): An initiator that starts the free radical polymerization process.

  • Tannin: An ingredient mentioned in the transcript that assists with the polymerization process.

• Pore Size Control:

  • The concentration of acrylamide determines the density of the gel.

  • Higher concentrations of acrylamide result in a denser gel with smaller pores.

  • Larger concentrations of bis acrylamide relative to acrylamide also result in smaller pores.

Acrylamide Concentrations and Separation Utility

• Typical acrylamide concentrations range from $4\%$ to $20\%$.

• $4\%$ Gels:

  • Very difficult to handle; they tend to fall apart if touched improperly.

  • Used for specific applications, such as studying proteins that bind to DNA to observe large shifts in molecular weight.

• $20\%$ Gels:

  • Used to separate very small proteins.

  • Effective for molecules like ubiquitin, which weighs approximately $5\,kDa$.

• Stock Solutions:

  • Typically come in $30\%$ or $40\%$ weight per volume concentrations.

  • Common ratios of acrylamide to bis acrylamide include $19:1$ (smaller pores) and $37.5:1$ (larger pores).

• Performance across ranges $14\,kDa$ to $94\,kDa$:

  • $3\%$ Gel: The protein often ends up at the very bottom of the gel; not useful for this weight range.

  • $6\%$ Gel: Smaller proteins migrate too quickly to the bottom, providing poor separation.

  • $9\%$ Gel: Good for separating higher molecular weight proteins.

  • $12\%$ Gel: Excellent for separating lower molecular weight proteins in this range.

Native PAGE vs. Denaturing (SDS) PAGE

• Native PAGE:

  • Proteins are run in their original, folded state.

  • Separation is influenced by size, shape, and charge.

  • Global/globular proteins run differently than structural/filamentous proteins.

  • Internal empty space in globular proteins can lead to low density, affecting migration.

  • Charge is a massive factor; proteins with a net positive charge will not migrate correctly toward the anode.

• SDS-PAGE (Denaturing):

  • The goal is to separate proteins strictly by molecular weight.

  • SDS (Sodium Dodecyl Sulfate): A detergent that breaks hydrophobic interactions within the protein, causing it to denature.

  • SDS coats the protein uniformly, providing a consistent negative charge relative to its mass.

  • The charge-to-weight ratio becomes identical for all proteins, making migration purely a function of molecular weight.

Sample Preparation and Reducing Agents

• In addition to SDS, a sample buffer includes a reducing agent to fully denature the protein.

• Common Reducing Agents:

  • Dithiothreitol (DTT)

  • Beta-mercaptoethanol (BME or BME)

• Function: These agents reduce disulfide bonds within or between protein chains.

• Process: After adding the buffer, the sample is heated to near-boiling to ensure complete denaturation and uniform coating with SDS.

The Dual-Gel System: Stacking and Resolving Gels

• To achieve high-resolution separation, two distinct layers of gel are used.

• 1. The Stacking Gel:

  • It has a low concentration of acrylamide.

  • It allows large sample volumes to compress into a very tight band (approximately $1\,mm$ or less) before hitting the separation layer.

  • pH Layering: Uses a pH of $6.8$.

  • Glycine Chloride Gradient:

    • Chloride ions ($Cl^-$): Always negatively charged; migrate quickly toward the anode.

    • Proteins: Follow the chloride ions.

    • Glycine: At pH $6.8$ (near its isoelectric point), some glycine is neutral and some is negative. This causes glycine to move slowly and "push" the proteins into a tight band against the chloride front.

• 2. The Resolving Gel:

  • Where actual separation occurs.

  • Typically uses acrylamide concentrations of $12\%$ to $15\%$.

  • Runs at a much higher pH, where glycine becomes fully negatively charged.

  • At this higher pH, glycine becomes smaller and more mobile, passing the proteins to migrate just behind the chloride ions.

Staining and Quantitative Analysis

• Coomassie Blue Dye:

  • A primary dye for protein visualization.

  • Penetrates the gel and complexes with the proteins, turning them blue.

• Fixing the Gel:

  • Before staining, the gel is treated with a solution of methanol and acetic acid.

  • This precipitates the proteins, locking them in place so they do not migrate out of the gel when placed in water or staining solution.

• Staining/Destaining:

  • Staining often occurs overnight. If the entire gel turns blue, a destaining buffer is used to pull the excess dye from the matrix while leaving the dye complexed with the proteins in the bands.

• Calculating Molecular Weight:

  • A set of protein standards (markers) is run alongside the samples.

  • Relative Mobility ($RF$): Calculated using the formula:       $RF = \frac{\text{Distance of sample migration}}{\text{Distance of migration for tracking dye}}$

  • The tracking dye used is usually bromophenol blue.

  • An $RF$ versus molecular weight standard curve is plotted to determine the weights of unknown samples.

Troubleshooting: The Bad Gel "Hall of Shame"

• 1. The Bad Pour:

  • Occurs if polymerization begins while the gel is being poured.

  • Results in uneven densities; proteins may "take a right turn" or migrate diagonally instead of straight down.

• 2. The Overloaded Gel:

  • Caused by loading too much protein into a well.

  • Results in proteins bleeding between lanes, making individual bands indistinguishable.

• 3. The Ripped Gel:

  • Polyacrylamide gels are very thin and fragile compared to robust, "bouncy" agarose gels.

  • Common errors include ripping the gel when prying apart the glass plates or dropping the gel during transport.

  • Extreme care is required at the final handling stages to avoid losing data.