Question

Which one of teh following peptide is likely to have a higher PI and why?

Arg-val-val-val-val or Val-val-val-val-Arg

Answer 1

Calculation of isoelectric point

1.) if the side chain does not have an ionizable group then PI is simply the average of alpha- NH₃ and alpha-COOH pKa values

2.) if the side chain has an ionizable group then all three pka values must be considered.

3.) if the side chain is acidic (asp and glu) then average the sidechain pka with alpha-COOH pka

4.) If the side chain is basic (his,arg,lys), then average the sidechain pka with alpha-NH₃ Pka value.

5.) for other ionizable group (tyr and cys) determine which is the middle Pka and average it with alpha-COOH pka.

By following this

val-val-val-val-arg has high PI value.