[help784]

I was given the following problem in class, I don't understand the answer my friend insists on being right.

"Given that the N terminus has a pK of 9.l0 and the C terminuns has a pK of 3.5.  Estimate the net charge of the polypeptide shown below if it were found in the small intestine at a pH of 8?

Tyr-Asp-Lys-Glu

N terminus +1
Tyr 0
Asp -1
Lys +1
Glu -1
C terminus -1

equals net charge of -1

I say it is a net charge of 0 because Tyr has a net charge of +1 at pH 8.  Who is right?

[Wreath]

As far as I know, the pK of tyrosine hydroxyl is around 10.0, so at pH = 8 it should be uncharged.