Tools for protein study Flashcards
Define purification of a protein
‘Isolation of one particular protein from all others in the starting material’
Why purify proteins?
To determine the structure: x-ray crystallography, nmr
AND/OR
To determine the function: activity, interactions
How can we purify proteins?
Exploit the protein’s:
Solubility/charge
Size
Specific interactions
What are the steps of protein purification?
- Selection of source
- Cell disruption
- Protein stabilisation
- PURIFICATION
- Protein identification/measure of purity
- Protein characterisation
How do you select the source for protein purification?
Tissue/organism producing large amounts of ‘the protein’, e.g. haemoglobin - red blood cells, bacterial proteins (100 litre fermenters)
Recombinant DNA technology - proteins expressed in bacterial/eukaryotic cells, e.g. commercially lipases (washing powder), insulin
How do you disrupt the cells for protein purification?
Rupture of plasma membrane - homogenisation, sonication, French press, osmotic lysis, freeze/thaw
Subcellular fractionation
Outline subcellular fractionation
Differential centrifugation
Homogenate is centrifuged step-by-step
Denser material sediments at lower centrifugal forces
How do you stabilise the proteins during protein purification?
Temperature (4-6 degrees centigrade)
pH (7-8?)
Inhibit protease activity (protease inhibitors, e.g. PMSF)
Bacterial contamination (decrease temperature, Na azide)
Prevent oxidation (thiol reagents, e.g. dithiothreitol)
Prevent heavy metal binding (EDTA)
Solubilise membrane proteins (Triton X-100)
What does an excess of dithiothreitol do to disulphide-linked chains?
It causes the disulphide-linked chains to become separated reduced chains
How can we carry out the purification stage of protein purification?
Ammonium sulphate fractionation
Dialysis
Column chromatography
What is the biological basis of ammonium sulphate fractionation?
Solubility of most proteins decreases with increasing (salt)
What is the biological basis of dialysis?
NOT a purification stage but an important step in the purification protocol, e.g. for removing (NH4)2SO4 imidazole etc. before ion-exchange chromatography
Separation of small molecules from large, using semi-permeable membrane
Give the names of some types of column chromatography
Gel filtration
Ion exchange
Hydrophobic
Affinity
In a water molecule, what dipoles are on the atoms?
(+) dipole on the hydrogen atoms
(-) dipole on the oxygen atom
How does ammonium sulphate fractionation work?
Salt ions compete with protein for interaction with water. As (salt) increases, protein precipitates. ‘Salting out’.
At what wavelength is the protein elution from column chromatography detected?
At 280nm
State how gel filtration chromatography works
(Gel permeation/molecular exclusion)
Separates proteins according to their size
State how ion-exchange chromatography works
Separates proteins according to their NET charge
CM-cellulose = cation exchanger (cations are +ve)
DEAE-cellulose = anion exchanger (anions are -ve)