Question

2) Suppose you have a solution of a protein, which contains a specific Tyr residue that has an actual (measured) pKa of 8.8. The protein binds a ligand by several noncovalent interactions, one of which is a hydrogen bond in which the Tyr phenolic hydroxyl group must serve as a hydrogen bond donor. Calculate the percentage of the protein molecules in which that tyrosyl residue's phenolic hydroxyl group could serve as a hydrogen bond donor at pH 8.5. (4 pt)

Answer 1

Accoring to the Henderson-Hasselbulch equation:

pH =  pKa + Log([Tyr.O^-]/[Tyr.OH])

i.e. 8.5 = 8.8 + Log([Tyr.O^-]/[Tyr.OH])

i.e. Log([Tyr.O^-]/[Tyr.OH]) = 8.5-8.8 = -0.3

i.e. [Tyr.O^-]/[Tyr.OH] = 10^-0.3 = 0.5

Now, [Tyr.O^-]/([Tyr.O^-] + [Tyr.OH]) = 0.334

Therefore, the required percentage = 0.334 * 100 = 33.4%