Lysine Amino

8Jun/09Off

Lysine Amino

Calculating isoelectric point of amino acid in biochem?

How do you calculate the isoelectric point of amino acid glutamic acid given the following information.

Use the following pK values :

alpha-amino group 9.0
alpha-carboxyl 2.0

side chain groups:
lysine 10.0
arginine 13.0
histidine 6.0
aspartate 3.8
glutamate 4.0

So, normally in calculating the pI or isoelectric point (the pH at which the total average charge of the amino acid is 0), only alpha-amino and alpha-carboxyl groups are ionizable, so the two pK's 9.0 and 2.0 would be averaged and the answer would be 5.5.

In glutamic acid's case, though, the side chain, a C-C-COOH group, is also ionizable so mush also be considered. So, you have: 1. an alpha-amino group that can gain a positive charge, 2. an alpha-carboxyl group that can gain a negative charge, and 3. a carboxyl group that can also gain a negative charge as a side chain.

The information given tells you that the pK value of glutamate (the negatively charged form of glutamic acid) is 4.0.

With three pK values given to you: 9.0, 2.0, and 4.0 , it is safe to assume that at low pHs like 2.0 or 4.0, the alpha-amino group will be 100% protinated and hold a charge of +1 (it is at least 5 pH points away). To balance this +1 charge, ONE -1 charge between the TWO groups with lower pK's must be obtained.

Thus, to find where the two carboxyl groups will have one negative charge, the average must be taken. So:

(2+4)/2= 3. The pI is 3.0 with these values given.

Generally, when an amino acid has three given pK values (the side chain is ionizable), and there are two close numbers with one a considerable difference away, the average of the closer ones can be taken.

Good luck! Ochem isn't so bad if you get into it