The galactose represser protein from E. coli has a pI of about 5.9. While purification protocols were being designed, it was found to bind to a Mono-S column at pH values of 7 and below. (Mono-S columns have S-type sulfonic acid groups attached to the resin and are strong cation exchangers.)
What is unusual about this observation, and explain these results?
As you know, all proteins possess a net charge in solution, primarily dependent upon three things: the pH of the solution, their structure, and their isoelectric point (or pI).
The pI of a protein is the pH at which the net charge of the protein is zero. The word "net" is important, as we will see below. The "net" charge does not mean that there are no charged groups on the protein, but rather, the total of all the charges, both negative and positive charges, equals zero.
Therefore, a protein will have a net positive charge, and therefore, bind to a cation exchanger in solutions where the pH is below their pI. This is easy to remember, since in pH's below their pI, there are lots more H+ ions around, and therefore, you can link that to the extra postive ...
This solution is provided in 546 words. It discusses pH, protein structure, and isoelectric points to discuss the galactose represser protein.