Discussion
Four proteins (P1, P2, P3 and P4) have 17, 10, 21 and 14 percent hydrophobic amino-acids respectively. The order of precipitation of these proteins using ammonium sulphate will be
Answer: A
The order of precipitation is P3, P1, P4, P2.
Ammonium sulphate precipitates proteins by 'salting out': as the salt concentration rises, the salt ions take up the water that was hydrating the protein surface, and exposed hydrophobic patches on neighbouring protein molecules begin to stick together. The more hydrophobic a protein, the less salt is needed before it aggregates.
Taking the hydrophobic content as the guide, the most hydrophobic protein precipitates first: P3 (21%), then P1 (17%), P4 (14%) and finally P2 (10%). The orders starting with P2 run from least to most hydrophobic, which is backwards, and P3, P1, P2, P4 puts P2 ahead of the more hydrophobic P4.
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