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You are trying to purify a protein that is soluble in a solution of 2 M ammonium sulfate. After centrifugation to remove other proteins that have precipitated at this high salt concentration, you recover the supernatant to assay the target protein's activity in a cell culture system.
(a) Explain why the cell dies when incubated with the supernatant.
(b) What procedure could you preform to correct the problem?


Sagot :

Answer:

a) The cell dies because it was in a hypertonic medium, and it loses water due to the difference in pressure and may even die from dehydration.

b) To correct this, the cells can first be placed in an isotonic solution (one that has the same concentration of solute as a solution separated or isolated by a semi-permeable barrier). A method to remove the salt is called dialysis.

Explanation:

A protein purification is a series of processes that allow the isolation of protein from mixture. Thereby this process is vital for the characterization of the function, structure and interactions of the protein of interest. It is therefore a very important procedure to perform correctly.

Ammonium sulfate is a salt whose chemical formula is (NH4)2SO4. The cells are in a 2M solution (2 moles of sulfuric acid per liter of solution). This is detrimental because the solution is hypertonic, it has a higher osmolarity in the external medium than inside the cell, so that a cell in such a solution loses water due to the pressure difference, i.e. osmotic pressure, and may even die from dehydration. The outflow of water from the cell continues until the osmotic pressure of the external medium and the cell are equal.

Centrifugation is a method by which solids can be separated from liquids of different density by means of a rotating force. After centrifugation, the supernatant will have a soluble part (proteins) and a pellet (cells and other insoluble parts). Thereby, the protein of interest would be recovered in the supernatant. But since the cell was not previously in a suitable medium, it could not carry out its metabolism and died and could not produce the protein of interest.

a) The cell dies because it was in a hypertonic medium, and it loses water due to the difference in pressure and may even die from dehydration.

b) To correct this, the cells can first be placed in an isotonic solution (one that has the same concentration of solute as a solution separated or isolated by a semi-permeable barrier). A method to remove the salt is called dialysis.

Dialysis is the process of separating molecules in a solution by the difference in their diffusion rates or osmotic pressure across a semipermeable membrane. A solution of various types of molecules is placed in a semi-permeable dialysis bag, such as, for example, on a cellulose membrane with pores, and the bag is sealed. The sealed bag is placed in a beaker with pure water or another solution. Molecules small enough to pass through the pores (often water, salts and other small molecules) tend to move in or out of the dialysis bag in the direction of the lower concentration. However, larger molecules (such as DNA, proteins or polysaccharides) that are larger than the pore diameter are retained within the dialysis bag.