Proteins are usually least soluble in water at their isoelectric points. Explain.

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Hi everybody. Our next question says, why is the solubility of proteins in water lower at their isoelectric points. A proteins have a net positive charge and will repel water at their isoelectric points which makes them less soluble in water. B proteins have a net negative charge and will repel water at their isoelectric points which makes them less soluble in water. C proteins have a zero net charge and will promote protein protein interactions at their isoelectric point which makes them less soluble in water or D proteins are neutral ph equals seven and will promote protein protein interactions at their isoelectric points which makes them less soluble in water. Well, before we even think about defining isoelectric points and thinking about what happens with solubility when we look at choice A and B choice A says they have a net positive charge and will repel water. Choice B says have a net negative charge and will repel water. And this cannot be our answer. Even if you weren't sure about the net charge issue. A charged molecule will not repel water. We recall that the rule like dissolves like so charged or polar molecules are soluble in water. So a protein with a net negative or net positive charge will not repel water. So choices A and B can not be our answer. So we're left with C and D. So let's think about what that isoelectric point is. The isoelectric point. I'll just put IP for short is the ph at which a molecule is electrically neutral. So it's in a form where it has no net charge. So that is what choice C says proteins have a zero net charge. So we just need to see if the rest of the statement is correct, says it will promote protein protein interactions which makes them less soluble. Well, we know that the protein still will dissolve in water. Uh Proteins are polar molecules by the very fact, they're made up of amino acids but will this promote protein protein interactions? Well, this is correct because the fact that they are electrically neutral at their isoelectric point by definition means that there's less repulsion between protein molecules. So when they're charged, they repel each other, but when they're electrically neutral, they don't have that repulsion. So this does indeed promote protein protein interactions. It will cause them to aggregate rather than dissolve. So they will be less soluble. So choice C is the correct answer. We should look at choice CD just to be sure. Choice D says proteins are neutral, Ph equals seven and will promote protein protein interactions. The interaction part is OK. But the neutral PH H is not correct a protein with an isoelectric point that's in the neutral range will be neutral at its isoelectric point. But the PH will depend on the R groups. So a protein with a preponderance of acidic R groups at its isoelectric point will be at an acidic Ph A protein with many basic R groups at its isoelectric point will be at a high Ph. So that's why choice D is incorrect. So once again, why is the solubility proteins in water? Lower their isoelectric points. That is choice C proteins have a zero net charge and will promote protein protein interactions at their isoelectric points which makes them less soluble in water. See you next video.

Proteins are usually least soluble in water at their isoelectric points. Explain.

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Key Concepts

Isoelectric Point (pI)

Protein Solubility

Protein Aggregation