Mary K. Campbell
Shawn O. Farrell
http://academic.cengage.com/chemistry/campbell
Chapter Five
Protein Purification and
Characterization Techniques
Paul D. Adams • University of Arkansas
Isolation of Proteins from Cells
Many different proteins exists within one cell
• Many steps needed to extract protein of interest, and
separate from many contaminants
• Before purification begins, protein must be released
from cell by homogenization
How We Get Proteins Out of Cells
Salting Out
• After Proteins solubilized, they can be purified based
on solubility (usually dependent on overall charge,
ionic strength, polarity
• Ammonium sulfate (NH4SO4) commonly used to “salt
out”
• Takes away water by interacting with it, makes protein
less soluble because hydrophobic interactions among
proteins increases
• Different aliquots taken as function of salt
concentration to get closer to desired protein sample
of interest (30, 40, 50, 75% increments)
• One fraction has protein of interest
Differential Centrifugation
• Sample is spun, after
lysis, to separate
unbroken cells, nuclei,
other organelles and
particles not soluble in
buffer used
• Different speeds of
spin allow for particle
separation
Column Chromatography
• Basis of Chromatography
• Different compounds distribute themselves to a varying
extent between different phases
• Interact/distribute themselves
• In different phases
• 2 phases:
• Stationary: samples interacts with this phase
• Mobile: Flows over the stationary phase and carries
along with it the sample to be separated
Column Chromatography
Size-Exclusion/Gel-Filtration
• Separates molecules based on size.
• Stationary phase composed of cross-linked gel
particles.
• Extent of cross-linking can be controlled to determine
pore size
• Smaller molecules enter the pores and are delayed in
elution time. Larger molecules do not enter and elute
from column before smaller ones.
Size Exclusion/Gel-filtration (Cont’d)
Affinity Chromatography
• Uses specific binding properties of molecules/proteins
• Stationary phase has a polymer that can be covalently
linked to a compound called a ligand that specifically
binds to protein
Ion Exchange
• Interaction based on overall charge
(less specific than affinity)
• Cation exchange
• Anion exchange
Electrophoresis
• Electrophoresis- charged particles migrate in electric
field toward opposite charge
• Proteins have different mobility:
• Charge
• Size
• Shape
• Agarose used as matrix for nucleic acids
• Polyacrylamide used mostly for proteins
Electrophoresis (Cont’d)
• Polyacrylamide has more resistance towards larger
molecules than smaller
• Protein is treated with detergent (SDS) sodium
dodecyl sulfate
• Smaller proteins move through faster (charge and
shape usually similar)
Isoelectric Focusing
• Isolectric focusing- based on differing isoelectric pts.
(pI) of proteins
• Gel is prepared with pH gradient that parallels electricfield. What does this do?
• Charge on the protein changes as it migrates.
• When it gets to pI, has no charge and stops
Primary Structure Determination
How is 1˚ structure determined?
1) Determine which amino acids are present (amino
acid analysis)
2) Determine the N- and C- termini of the sequence
(a.a sequencing)
3) Determine the sequence of smaller peptide
fragments (most proteins > 100 a.a)
4) Some type of cleavage into smaller units necessary
Primary Structure Determination
Protein Cleavage
Protein cleaved at specific sites by:
1) Enzymes- Trypsin, Chymotrypsin
2) Chemical reagents- Cyanogen bromide
Enzymes:
Trypsin- Cleaves @ C-terminal of (+) charged side
chains
Chymotrypsin- Cleaves @ C-terminal of aromatics
Peptide Digestion
Cleavage by CnBr
Cleaves @ C-terminal of INTERNAL methionines
Determining Protein Sequence
After cleavage, mixture of peptide fragments produced.
• Can be separated by HPLC or other chromatographic
techniques
• Use different cleavage reagents to help in 1˚ determination
Peptide Sequencing
• Can be accomplished by Edman Degradation
• Relatively short sequences (30-40 amino acids) can
be determined quickly
• So efficient, today N-/C-terminal residues usually not
done by enzymatic/chemical cleavage
Peptide Sequencing