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Biology Biomolecules Part 3

IEF separates proteins based on their isoelectric point (pI), where they have no net charge, in a pH gradient from high to low pH. Native PAGE separates proteins based on both size and charge, while SDS-PAGE separates based only on size by using SDS to provide uniform negative charge. SDS-PAGE can be non-reducing to separate subunits not linked by disulfide bonds, or reducing to separate all subunits by breaking disulfide bonds.

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40 views3 pages

Biology Biomolecules Part 3

IEF separates proteins based on their isoelectric point (pI), where they have no net charge, in a pH gradient from high to low pH. Native PAGE separates proteins based on both size and charge, while SDS-PAGE separates based only on size by using SDS to provide uniform negative charge. SDS-PAGE can be non-reducing to separate subunits not linked by disulfide bonds, or reducing to separate all subunits by breaking disulfide bonds.

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Ralph Kenneth Luna
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Isoelectric focusing (IEF)

 An electrophoresis technique that separates proteins based on their isoelectric point


(pI).
 A pH gradient is established, such that one end has a high pH (– charge, cathode) and
the other has a low pH (+ charge, anode)
o Recall: for IEF and electrophoresis, the labeling of cathode and anode are
opposite to what you would think (since they are electrolytic cells)
 Once a protein reaches its pI (the place in the pH field whereby the protein has no net
charge), it will no longer move in the electric field.

Anode Cathode

Anode Cathode
Polyacrylamide Agarose Gel Electrophoresis (PAGE)
 Mobility of a protein or nucleic acid is a function of the length, conformation, and
charge of the molecule.
 Native PAGE
o Preserves the molecules’ higher order structure
o Separates based on both size and charge
 SDS PAGE
o SDS (sodium dodecyl sulfate) is a negatively charged chemical that binds to
proteins in a set ratio, approximately one molecule of SDS for every 2 amino
acids.
o In this way, the detergent provides all proteins with a uniform charge-to- mass
ratio, independently of their original charge (therefore charge is no longer a
factor)
o By binding to the proteins, the detergent denatures proteins by destroying their
2°, 3°, and/or 4° structure, turning them into negatively charged linear poly
peptide chains.
o Thus, SDS-Page separates based exclusively on size
o Two types:
 Non-reducing
 Can separate out subunits, provided they are not covalently linked
by disulfide bonds
 Reducing
 Removes disulfide bonds (R-S-S-R –> R-SH + SH-R), further
denaturing the protein
 Can separate out subunits, even if they are covalently linked by
disulfide bonds.

Summary:

Native-PAGE Separates based on size and charge

SDS-PAGE (non-reducing) Separates based on size only


Separates out SUs, provided they’re not covalently linked by S-S bonds

SDS-PAGE (reducing) Separates based on size only


Separates out SUs, even if they are covalently linked by S-S bonds

Isoelectric focusing (IEF) Separates based on charge only


Blots

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