Lecture 8 Flashcards

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1
Q

HOW PROTEINS ARE PRODUCED & PURIFIED

A
  1. Select a source of protein
  2. Extract protein from the source (e.g. lysis)
  3. Separate proteins (preparative steps)
    • Dialysis or ultrafiltration
    • Differential solubilities
    • Column chromatography
     Size‐exclusion (gel filtration/permeation)
     Ion‐exchange
     Affinity (inc. purifying fusion proteins)
     Hydrophobic interaction (HIC)
  4. Assess purity and efficacy of purification (analytical steps)
    • Protein quantitation to determine yield
    • Analysis of separation to indicate purity
    • Enzyme assay to indicate purity
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2
Q

Cells can be lysed/disrupted by

A

• Physical force (sonication,
pressure or grinding)
• Enzymes and/or osmotic pressure
• Detergents (e.g. SDS or tergitol)

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3
Q

How do you extract protein

A

1: Mix cells
2: Break open cells (lyse) to release contents of cell
(including protein)
3: Centrifuge cells
Supernatant containing protein is called “crude extract” (or Cell Free Extract, CFE)

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4
Q

What does the extraction buffers include

A
  • Buffer (e.g. Tris: pH 7.5–8.5)
  • Protease Inhibitors (e.g. EDTA: binds divalent metal ions required by metal proteases for activity)
  • Reducing agents (e.g. ‐mercaptoethanol or DTT: inhibit disulphide bond formation)
  • Salt
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5
Q

Reasons for separating proteins during purification:

A

1: To remove unwanted salts or ions: e.g. Dialysis & Buffer exchange using Ultrafiltration devices
2: To remove protein of interest from other contaminating proteins

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6
Q

What is dialysis

A

Procedure that separates proteins from small solutes by taking advantage of the protein’ larger size
Principle: passive diffusion through a semi‐ permeable membrane

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7
Q

How does dialysis purify

A
  • Partially purified extract placed in semi-permeable membrane
  • Suspended in large volume of buffered solution
  • Membrane allows exchange of salt and buffer, not proteins
  • Dialysis retains large proteins within the membranous bag
  • Small molecules diffuse freely across membrane until they reach equilibrium with the solution outside the membrane
  • Repeat until salt concentration is minimal
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8
Q

How does ULTRAFILTRATION of protein work

A

• Buffer exchange can be achieved by ultrafiltration
• Process involves: passing protein solution through a membrane filter under centrifugal force
• Molecules and salts smaller than MWCO pass through membrane
• Molecules larger than MWCO are retained within filter device
• Concentration of proteins can also be achieved by this
process

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9
Q

Ways to do Purification by Differential Solubilities

A

Precipitating proteins

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10
Q

Precipitating proteins using pH

A
  • Proteins are least soluble at their isoelectric point (pI)
  • Proteins have varying pI’s: can precipitate proteins differentially based on pI
  • Can also use differences in pI in purification and analysis
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11
Q
  1. Precipitation of proteins using salt
A

• Protein solubility ‐ lowered at high [salt]
‘salting out’
• Salts can selectively precipitate proteins
(e.g. Ammonium sulfate (NH
4)2SO
4 ‐ highly soluble)
• Removed protein from those remaining in solution by precipitation and low speed centrifugation
• Can also be used to concentrate dilute proteins

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12
Q

What is the principle of Precipitation of proteins using salt

A

Principle • High [salt] removes hydrogen shell from
around protein
 exposes ”hydrophobic” patches on protein surface
• Proteins aggregate via hydrophobic patches
• Water solvates proteins – forms hydration shell
• Polar/ionisable residues of protein interact with H2O through dipoles
• Most hydrophobic amino acid residues are found
within folded proteins, but some found on surface
• At hydrophobic patches, water forms ordered structure  specific van der waals interactions with the protein
• Result: water is weakly bound at hydrophobic patches
• Addition of ammonium sulphate (NH4)2SO4 :
• dH2O drawn away from proteins
• co‐ordinated around the NH4+ and SO42‐ ions
• Exposed hydrophobic patches on protein interact  form protein aggregates
• Protein precipitates

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13
Q

standard element used for COLUMN CHROMATOGRAPHY in separating protein

A
  • Reservoir – supplies constant flow of buffer solution
  • Stationary phase – porous solid matrix with chemical properties supported inside a column (glass or plastic)
  • Mobile phase – buffered solution that flows through matrix of the stationary phase
  • Eluent – solution that passes out of the bottom of the column
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14
Q

How do you do COLUMN CHROMATOGRAPHY

A

• Protein sample is layered on top of column
• Buffer is added from reservoir (constant flow)
• Proteins migrate through column, retarded to
different degrees depending on interaction with stationary phase
• Individual proteins separate from each other
• Eluted material is collected at bottom of column (in a specific elution volume, Ve)

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15
Q

How does Size-exclusion chromatography work

A

Separates proteins based on size
principle: movement within a liquid phase through a stationary porous medium
Column is matrix: beads made from cross‐linked polymers to form a porous matrix

Stationary phase:
Solution inside the porous beads
Mobile phase: 
Solution outside the bead
• Large molecules cannot enter 
porous beads: have less 
volume to travel down the 
column
• Small molecules enter beads: 
have access to a larger 
volume
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16
Q

Elution profile of proteins:

A

Vo = void volume: the elution volume of the molecules excluded from the size‐exclusion column
because they are larger than the largest pores
Ve = elution volume: measured from the chromatogram, relating to the molecular size of the eluted
molecule
Vt = total column volume: equivalent to the volume of the packed column bed

17
Q

What can size exclusion chromatograhy use to determine

A

Mr

18
Q

Advantage and disadvantage of size exclusion chromatograhy

A

Remove salt (or small mol wt) contaminants
e.g. Gel filtration on Sephadex G25, fractionation range (1 to 5 kDa)
Advantage: fast (<5 minutes)
Disadvantage: Only suitable for small volumes Dilute the protein sample