Protein purification: Protein purification is a series of processes used to isolate a
specific type of protein from a complex mixture. Protein purification is important
for the characterization of the function, structure and interactions of the protein
of interest. The various steps in the purification process may include the isolation
of the protein from a matrix, separate the protein and non-protein parts of the
mixture, and finally separate the desired protein from all other proteins. Separation
of the proteins depends on protein size, physio-chemical properties, binding
affinity and biological activity (Burgess. 2008).
- Ion-exchange chromatography separates proteins based on charge. Columns
can either be prepared for anion exchange or cation exchange. Elution of
the target proteins is done by changing the pH in the column, which results
in a change or neutralisation of the charged functional groups of each protein
- Size-exclusion chromatography (gel filtration) separates larger proteins from
small ones, since the larger molecules travel faster through the cross-linked
polymer in the chromatography column. The large proteins do not fit into
the pores of the polymer whereas smaller proteins do, and take longer to
travel through the chromatography column, via their less direct route.
- Affinity chromatography is a very useful technique for completing the protein
purification process. Beads in the chromatography column are cross-linked
to ligands that bind specifically to the target protein. The protein is then
removed from the column by rinsing with a solution containing free ligands.
This method generally gives the purest results and highest specific activity
compared to other techniques (Wilson and Walker. 2006).
Blotting: is a method of transferring proteins, DNA or RNA, onto a carrier (for
example, a nitrocellulose, polyvinylidine fluoride (PVDF) or nylon membrane.
Electrophoresis is the main biochemical technique for molecular separation.
Electrophoresis can be one dimensional or two dimensional. One dimensional
electrophoresis is used for most routine protein and nucleic acid separations.
Two dimensional separation of proteins is used for finger printing , and when
properly constructed can be extremely accurate in resolving all of the proteins
present within a cell (greater than l,500).When the detergent SDS (sodium
dodecyl sulfate) is used with proteins, C all of the proteins become negatively
charged by their attachment to the SDS anions. When separated on a
polyacrylamide gel, the procedure is known as SDS—PAGE (Sodium Dodecyl
Sulfate- Poly Acrylamide Gel Electrophoresis). The technique has become a
standard means for molecular weight determination (Lodish et al. 2004).
Clinical testing for a biochemical disease utilizes techniques that examine the protein instead of the gene. Many biochemical genetic diseases are known as “inborn errors of metabolism” because they are present at birth and disrupt a key metabolic pathway. Depending on the disease, tests can be developed to directly measure protein activity (direct measurement of enzyme activity), level of metabolites (indirect measurement of enzyme activity), and the size or quantity of protein (protein structure). These tests require a tissue sample in which the protein is present, typically blood, urine, amniotic fluid, or cerebrospinal fluid. Because gene products may be more unstable than DNA or RNA and can degrade quickly, the sample must be collected, stored properly, and shipped promptly according to the laboratory’s specifications.
A variety of technologies such as high performance liquid chromatography (HPLC), gas chromatography/mass spectrometry (GC/MS), and tandem mass spectrometry (MS/MS) enable both qualitative detection and quantitative determination of metabolites. In addition, bioassays may employ flourometric, radioisotopic, or thin-layer chromatography methods.