BACE Protein Isolation and Quantification 2 — Questions and Answers
Question 1: What is the purpose of a protease inhibitor cocktail added during cell lysis for protein extraction?
- To enhance protein solubility by breaking disulfide bonds
- To prevent endogenous proteases released during cell lysis from degrading the target protein (Correct answer)
- To increase protein yield by improving lysis efficiency
- To precipitate unwanted lipids and nucleic acids from the lysate
Correct answer: To prevent endogenous proteases released during cell lysis from degrading the target protein
Cells contain proteases that are released during lysis; protease inhibitors block these enzymes, preventing degradation of the protein of interest during extraction.
During cell lysis, vacuolar and lysosomal proteases are released into the lysate. Without inhibition, they rapidly degrade proteins of interest. Protease inhibitor cocktails contain multiple inhibitors with different mechanisms: PMSF (serine protease inhibitor), EDTA (metalloprotease inhibitor), leupeptin (serine/cysteine protease inhibitor), pepstatin A (aspartyl protease inhibitor), and aprotinin. Working on ice and completing the extraction quickly also minimizes protease activity.
Question 2: In SDS-PAGE, why are proteins separated primarily by molecular weight?
- SDS gives each protein a unique net charge proportional to its amino acid composition
- SDS binds uniformly to proteins (approximately 1.4 g SDS per g protein), giving all proteins a uniform negative charge-to-mass ratio so that migration depends only on molecular size (Correct answer)
- The polyacrylamide gel selectively binds large proteins retarding their migration based on charge density
- Reducing agents in the sample buffer separate proteins by charge while SDS separates by size
Correct answer: SDS binds uniformly to proteins (approximately 1.4 g SDS per g protein), giving all proteins a uniform negative charge-to-mass ratio so that migration depends only on molecular size
SDS binds proteins at approximately 1.4 g/g, coating them with negative charges proportional to their mass, so all proteins migrate with the same charge-to-mass ratio and are separated only by the sieving effect of the polyacrylamide gel.
SDS is an anionic detergent that denatures proteins and binds uniformly to the denatured polypeptide backbone. The uniform charge-to-mass ratio means all proteins migrate toward the anode with equal electrical driving force per unit mass. The polyacrylamide gel acts as a molecular sieve: larger proteins are retarded more while smaller proteins migrate faster. A protein standard ladder allows estimation of unknown protein sizes.
Question 3: What does a Bradford assay (Coomassie Blue dye binding) measure, and what is its color shift?
- Total DNA concentration; the dye shifts from blue to clear in the presence of DNA
- Total protein concentration; Coomassie Blue shifts from reddish-brown to blue when it binds proteins under acidic conditions (Correct answer)
- Enzyme activity; the blue dye is consumed proportional to enzymatic turnover
- Lipid concentration; the dye shifts from blue to yellow in the presence of membrane lipids
Correct answer: Total protein concentration; Coomassie Blue shifts from reddish-brown to blue when it binds proteins under acidic conditions
The Bradford assay measures total protein by the color shift of Coomassie Brilliant Blue G-250 from its reddish-brown (free) form to its blue form upon binding to proteins — measured spectrophotometrically at 595 nm.
Coomassie Brilliant Blue G-250 in the Bradford reagent shifts to blue with maximum absorbance at 595 nm when it binds to basic and hydrophobic residues in proteins. The assay uses BSA as the standard protein for calibration. It is sensitive, fast (5 min), and relatively free of interference from common reagents. However, it is sensitive to detergents (SDS, Triton X-100) and varies with protein amino acid composition.
Question 4: What is the principle behind ammonium sulfate precipitation for protein purification?
- Ammonium sulfate reduces the pH to precipitate proteins at their isoelectric point
- High salt concentrations remove the hydration shell from proteins reducing their solubility until they precipitate (salting out) (Correct answer)
- Ammonium sulfate chelates metal cofactors causing metalloenzymes to aggregate and precipitate
- Ammonium sulfate forms covalent crosslinks between adjacent protein molecules causing bulk precipitation
Correct answer: High salt concentrations remove the hydration shell from proteins reducing their solubility until they precipitate (salting out)
High concentrations of ammonium sulfate compete with proteins for water molecules, removing their hydration shell; without hydration, protein-protein hydrophobic interactions dominate and they precipitate (salting out).
Proteins are soluble because their surface hydrophilic residues are hydrated by water molecules. When ammonium sulfate concentration is increased, its highly hydrated ions compete for water molecules. As the protein's hydration layer is removed, hydrophobic patches interact with those on other proteins, causing aggregation and precipitation. Different proteins precipitate at different ammonium sulfate concentrations (expressed as % saturation), enabling selective purification.
Question 5: In a Western blot procedure, what is the function of the blocking step (e.g., 5% milk or BSA)?
- To enhance antibody binding to the target protein on the membrane
- To saturate non-specific protein-binding sites on the membrane preventing antibody from binding non-specifically to the membrane background (Correct answer)
- To denature the primary antibody so it only detects denatured proteins on the blot
- To transfer blocking agent onto the gel before membrane transfer to improve protein retention
Correct answer: To saturate non-specific protein-binding sites on the membrane preventing antibody from binding non-specifically to the membrane background
Blocking covers non-specific protein-binding sites on the membrane with irrelevant proteins (milk casein, BSA), preventing primary antibody from sticking non-specifically to the membrane background.
Nitrocellulose and PVDF membranes bind proteins non-specifically. Without blocking, antibodies would bind to exposed membrane sites everywhere, creating high background. Blocking with 5% non-fat dry milk or 1-5% BSA covers all available non-specific binding sites. Milk is inappropriate for phospho-protein detection (casein is a phosphoprotein); BSA is used in those applications. Blocking time: typically 1 hour at room temperature.
Question 6: What does 'affinity chromatography' exploit to purify a specific protein from a complex mixture?
- Size differences between the target protein and contaminants
- The specific, reversible, non-covalent interaction between the target protein and an immobilized ligand (Correct answer)
- Differences in protein surface charge at a specific pH
- Differences in protein solubility in organic solvents
Correct answer: The specific, reversible, non-covalent interaction between the target protein and an immobilized ligand
Affinity chromatography uses a column containing an immobilized ligand that specifically binds the target protein; after washing away non-binding proteins, the target is eluted by disrupting the interaction.
Affinity chromatography is the most powerful single-step purification method, capable of 1,000-fold or greater purification. Common systems: Protein A/G columns (bind IgG antibodies), Ni-NTA columns (bind His-tagged recombinant proteins), glutathione-agarose (binds GST-fusion proteins). Loading: target binds, contaminants flow through. Elution: specific conditions (competitive inhibitor, pH shift) release pure target protein.
BACE Protein Isolation and Quantification 2
What is the purpose of a protease inhibitor cocktail added during cell lysis for protein extraction?