Question

Lysine is an amino acid with three ionizable group. These are the $\alpha$ -COOH ,$\alpha$- amino and $\epsilon$-amino group with pKa value of $2.2,9.2$ and $10.8$,respectively . The isoelectric point (pI) for Iysine is

• A. $5.7$
• B. $6.5$
• C. $9.2$
• D. $10.0$

Answer 1

The correct option is D $10.0$

For calculating iso electric point, if the side chain of amino acid does not have an ionizable group, then the pI is simply the average of the αNH3 and α-COOH pKa values. But if the side chain has an ionizable group then all three pKa values must be considered. If the side chain is acidic (asp and glu), then average the side chain pKa with the α-COOH pKa. For other ionizable groups (tyr and cys), determine which is the middle pKa and average it with the α-COOH pKa. If the side chain is basic (his, arg, and lys), then average the sidechain pKa with the α-NH3 pKa. So the average of α- amino and ε-amino group with pKa value of 9.2 and 10.8 is 10.0

So the correct answer is '10.0'.