2. An octapeptide was determined to have the following amino acid composition: L
ID: 62279 • Letter: 2
Question
2. An octapeptide was determined to have the following amino acid composition:
Lys (2), Phe (2), Gly (1), His (1), Leu (1), Met (1). The native peptide was run through one cycle of the Edman degradation and the PTH-leucine derivative was identified by HPLC. When the native peptide was exposed to cyanogen bromide (BrCN), a heptapeptide and free glycine were recovered. Incubation of the native protein with trypsin gave a tetrapeptide, a tripeptide, and free lysine. The peptides were separated and each run through one cycle of the Edman degradation. The tetrapeptide yielded the PTH-leucine derivative, and the tripeptide yielded the PTH-phenylalanine derivative. Incubation of the native protein with pepsin produced a dipeptide and two tripeptides. The amino acid composition of the tripeptides (not the order) were determined to be (Phe, Gly, Met) and (Phe, Lys, Lys). What is the sequence of the octapeptide?
Show your work. 5 pts
Edman degredation—isolates N-terminal
Cyanogen Bromide—isolates C-terminal
Specificities of Proteases
BrCN cuts at the C-terminal side of Met
Trypsin cuts at the C-terminal side of Lys or Arg
Pepsin cuts at the N-terminal side of Phe, Trp or Tyr
Explanation / Answer
Please consider following points-
1. Reaction with CnBr resulted in Heptapeptide and a free Glycine, that means Glycine will be at the last of the sequence while Methionine come before that.
2. Trypsin treatment yielded free Lysine , means one Lysine will be at other end of sequence. and other Lysine will be at the mid of Sequence.
3. Tetrapetide yielding the PTH Leu means , Leu will be at other end just ater Lysine.
4. Pepsin treatment suggest that Phe will be at 3rd and 6th position of the order.
So Sequence order should be-
Lys-His-Phe-Lys-Leu-Phe-Met-Gly
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