Isoelectric point

From Wikipedia, the free encyclopedia

The isoelectric point (pI) is the pH at which a molecule carries no net electrical charge. In order to have a sharp isoelectric point, a molecule must be amphoteric, meaning it must have both acidic and basic functional groups. Proteins and amino acids are common molecules that meet this requirement.

For an amino acid with only one amine and one carboxyl group, the pI can be calculated from the pKas of this molecule.

$pI = {{pK_1 + pK_2} \over 2}$

For amino acids with more than two ionizable groups, such as lysine for example, the same formula is used, but this time the two pKa's used are those of the two groups that lose and gain a charge from the neutral form of the amino acid. Lysine has a single carboxylic pKa and two amide pKa values (one of which is on the R-group), so fully protonated lysine has a +2 net charge. To get a neutral charge, we must deprotonate the lysine twice , and therefore use the R-group and amide pKa values (found at List of standard amino acids).

$pI = {{9.06 + 10.54} \over 2} = 9.80$

However, a more exact treatment of this requires advanced acid/base knowledge and calculations.

Proteins can be separated according to their isoelectric point in a process known as isoelectric focusing.

At a pH below the pI, proteins carry a net positive charge. Above the pI they carry a net negative charge. This has implications for running electrophoretic gels (see Agarose gel electrophoresis). The pH of an electrophoretic gel is determined by the buffer used for that gel. If the pH of the buffer is above the pI of the protein being run, the protein will migrate to the positive pole (negative charge is attracted to a positive pole). If the pH of the buffer is below the pI of the protein being run, the protein will migrate to the negative pole of the gel (positive charge is attracted to the negative pole). If the protein is run with a buffer pH that is equal to the pI, it will not migrate at all. This is also true for individual amino acids.