Question 4: (8 marks)

Peptides hold great potential for the cancer therapy because of their ease of rational design and target specificity. During your first undergraduate research project, you conducted a screen to find peptides that inhibit a key signal transduction pathway that promotes proliferation and differentiation of cancer cells. From a large library of 8-amino acid residue-long peptides, you got a hit! Unfortunately, someone mislabeled the vial containing the active peptide, so now you cant be sure of its entire sequence. The third amino acid could be any one of the 20 amino acids!! Peptide: L C _ S Y E R D

Luckily, you collected information about the net charge of all the peptides in your library as a function of pH!

a)Based on the information below, what is the full sequence of this peptide? (please provide a rationale for your answer)

(5 marks)

At pH 3, net charge = +2 At pH 7, net charge = -1 At pH 12, net charge = -4

b) Estimate the isoelectic point (pI) of the peptide and show how you determined this value. (3 marks)