What is the pI of most of proteins in cell? - (Mar/29/2008 )

Hi all,
I have a quite theoretical question. Do you know, what is the isoelectric point of most (majority) of proteins in mammalian cell? I have read somewhere, that most of proteins in cell are not neutral (pI approx. 7), but slightly acid, i.e. most of proteins have pI of 5-6. Is it right?

Thanks for your answer,
victor

Just by guessing a protein tends to precipitate at its pI, I would say if most proteins have their pI=7 it would make a lot of mess. I remember seeing something like most serum proteins are slightly acidic.

The pIs of denatured proteins have a roughly bimodal distribution with a peak around 4.8-6.8, and other one around 7.8-9.8. This pattern is caused by the uneven distribution of the pKas of the charged residues, as well as by the amino acid composition of each protein. Most extracellular human proteins are more acidic than their predictions, because they contain carbohydrates that end in several sialic acids, which are more acidic than aspartic or glutamic residues.

The pIs of native proteins can be quite different from the values of denatured proteins, but most measurements are done in denaturing conditions. I agree that most proteins have native pIs different from the pH of their environment, in order to avoid solubility and aggregation problems. So, most extracellular proteins are acidic, while most nuclear proteins are basic, presumably to favour interactions with the acidic DNA.

Best regards