A pentapeptide is composed of Ala, Cys, Lys, Phe, and Ser. NH2-terminal analysis of the peptide revealed PTH-serine. Trypsin digestion produces a dipeptide and a tripeptide. The tripeptide has an NH2-terminal Cys whereas the dipeptide had an NH2-terminal Ser. Finally, chymotrypsin digestion produced free alanine and a tetrapeptide. What is the primary structure of the pentapeptide?