Biology Forums - Study Force

calculate lysin and methionine isoelectric point?
and explain how!

You'll need to know the pKa values for the amino acids in question. For example:

pKa1 is 2.2

pKa2 is 9.0

pKa3 is 10.5

This gives you an isoelectric point of (pI) of 9.8.

The isoelectric point, pI, is the pH of an aqueous solution of an amino acid (or peptide) at which the molecules on average have no net charge. In other words, the positively charged groups are exactly balanced by the negatively charged groups. For simple amino acids such as alanine, the pI is an average of the pKa's of the carboxyl (2.34) and ammonium (9.69) groups. Thus, the pI for alanine is calculated to be: (2.34 + 9.69)/2 = 6.02, the experimentally determined value. If additional acidic or basic groups are present as side-chain functions, the pI is the average of the pKa's of the two most similar acids. To assist in determining similarity we define two classes of acids. The first consists of acids that are neutral in their protonated form (e.g. CO2H & SH). The second includes acids that are positively charged in their protonated state (e.g. -NH3+). In the case of aspartic acid, the similar acids are the alpha-carboxyl function (pKa = 2.1) and the side-chain carboxyl function (pKa = 3.9), so pI = (2.1 + 3.9)/2 = 3.0. For arginine, the similar acids are the guanidinium species on the side-chain (pKa = 12.5) and the alpha-ammonium function (pKa = 9.0), so the calculated pI = (12.5 + 9.0)/2 = 10.75.

For simple amino acids, the pI is (pK1+pK2)/2. If you have three pKs, e.g. in lysine, it really boils down to an average of the pKs for the amine groups. Thus, if lysine is fully protonated, the carboxyl group will not have a charge, and the 2 amine groups will be positive. Thus, you have 2 positive charges, and the pI would be approximated as pKaminegroup1 + pKaminegroup2/2.