[SunilNagpal]

If the pH of a protein is reduced to 1.5....below it's isoelectric point,which of it's group would be ionisable ?
i)COOH
ii)NH2
iii)Both of these
iv)Both would be neutral

At isoelectric point, amino acid exists in Zwitterion state...Reducing the pH below isoelectric point means addition of H+ ions to the medium....this would allow -COO(-) ion of zwitterion form to pick up the H+ ion and become -COOH, leaving behind -NH3(+) ionic form....

-NH2 is in ionized state as a result.