Question

BIOCHEM:

Tyrosine, When incorporated in a peptide backbone, approximately what fraction of tyrosine is ionized at pH 7.0?

Please explain, thank you. Will give a thumbs up if correct.​

Answer 1

For Tyrosine :

pKa₁= a-carboxyl group = 2.2

pKa₂ = a-ammonium ion = 9.11

and pKa₃ = side chain group = 10.07

Since the tyrosine is inserted into the protein backbone, then the ammonium ion and carboxy group are connected. So, there is no charge at this end of the amino acid. Only possible is side chain, but the side chain pKa3 = 10.01 and the solution pH = 7.0. (pH <pKa3). Therefore, the side chain is not charged at pH=7.0.

Therefore, the net charge on the tyrosine at pH = 7 is Zero (no charge). And the tyrosine is in total unionised form and no ionized form.

Hence, the fraction tyrosine ionized = zero