SDS-PAGE (SDS-Polyacrylamide Gel Electrophoresis)

Core Concept

SDS-PAGE separates proteins by molecular weight. The anionic detergent sodium dodecyl sulfate (SDS) unfolds proteins and coats them with a roughly uniform negative charge per unit mass, so each protein's charge-to-mass ratio is nearly constant. When an electric field is applied, the SDS-coated proteins migrate through a porous polyacrylamide gel as a function of size alone: small proteins thread through the pores quickly, large ones slowly. The result is a set of bands, each representing proteins of a given molecular weight. SDS-PAGE reports size and relative amount of proteins, but not their identity, function, or native state.

Key Components

SDS (sodium dodecyl sulfate)

• An anionic detergent that binds ~1.4 g SDS per gram of protein, denaturing the polypeptide and imparting a negative charge proportional to length (mass). This swamps the protein's own charge.

Reducing agent (β-mercaptoethanol or DTT)

• Breaks disulfide bonds, so multi-subunit or internally cross-linked proteins fully unfold into single polypeptide chains; without it, disulfide-linked complexes migrate at higher apparent mass.

Polyacrylamide gel (stacking + resolving)

• A cross-linked polymer whose pore size is set by the acrylamide percentage (higher % = smaller pores = better for small proteins). A low-% stacking gel concentrates samples into a sharp band before they enter the higher-% resolving gel where separation occurs.

Molecular-weight markers

• A ladder of proteins of known sizes run alongside samples to calibrate band masses.

Staining

• Coomassie blue (simple, microgram sensitivity) or silver stain (nanogram sensitivity) makes separated bands visible for imaging.

Mechanism

1. Denature and coat. Sample is boiled in loading buffer with SDS and a reducing agent; SDS unfolds the protein and wraps it in uniform negative charge; DTT/β-ME reduces disulfides.
2. Load and stack. Samples enter the stacking gel, where glycine/chloride ion fronts compress proteins into a thin, sharp band.
3. Resolve by size. In the resolving gel, SDS-coated proteins migrate toward the anode (+); smaller proteins move faster through the pores, so bands separate by molecular weight.
4. Stain and measure. The gel is stained; band positions are compared to the ladder to estimate molecular weights.

Energy and Directionality

Separation is driven entirely by the electric field: negatively charged SDS-protein complexes migrate toward the positive electrode. No ATP or GTP is consumed — the "energy" is electrical work moving charged particles through a resistive gel. Migration distance is roughly inversely proportional to the log of molecular weight (smaller proteins travel farther). The detergent's uniform charge coating is what converts the field into a pure size sieve rather than a size-and-charge separation.

Experimental Evidence

• What it measures: the molecular weight and relative abundance of proteins in a mixture.
• Principle: SDS denatures proteins and equalizes charge-per-mass; polyacrylamide sieves them by size in an electric field.
• Input: protein lysate + SDS + reducing agent + polyacrylamide gel. Output: stained bands at positions corresponding to molecular weight.
• What it can prove: the apparent molecular weight of a polypeptide; the number of major protein species and their relative amounts; purity of a preparation; (with reduction vs. non-reduction comparison) the presence of disulfide-linked subunits.
• What it cannot prove: protein identity (that requires a Western blot or mass spectrometry); function/activity (the protein is denatured); native oligomeric state; and it cannot detect post-translational modifications unless they shift mass detectably.
• Controls: a molecular-weight ladder (essential); known positive control protein of defined size; running non-reduced sample in parallel (to detect disulfide-linked complexes); proper boiling/denaturation (incomplete denaturation causes smears or wrong apparent sizes).
• Common mistakes: skipping the reducing agent (disulfide-linked species run at double/higher mass); under- or over-loading (distorted bands); wrong acrylamide percentage for the size range; running without a ladder; and forgetting that the observed mass is the denatured monomer, not the native complex.

Common confusions

• "SDS-PAGE identifies the protein" — No. It reports size and amount; a Western blot (antibody) or mass spectrometry identifies it.
• "The protein keeps its native structure" — No. SDS denatures it; native PAGE is the non-denaturing variant.
• "Charge differences separate the proteins" — SDS equalizes charge-per-mass, so separation is essentially by size.
• "No need for a reducing agent" — Without DTT/β-ME, disulfide-linked complexes migrate at inflated apparent mass.
• "A single band proves a pure, functional protein" — One band suggests one size/purity, not activity or correct folding.

Quick review

• SDS + reducing agent unfold and uniformly charge proteins → electric field sieves them by size in polyacrylamide.
• Smaller = farther; compare to a molecular-weight ladder.
• Reports size and relative amount; cannot identify or prove function (use Western/MS).