Question

You have isolated a peptide, which you wish to sequence. First you treat the peptide with CNBr and isolate two fragments. With a separate portion of the peptide you cleave with trypsin and obtain 3 fragments. You sequence each of the five peptides and get the following data:

Peptide 1: VALKTQAM
Peptide 2: LDGKM
Peptide 3: TQAM
Peptide 4: LDGK
Peptide 5: MVALK

The sequence of the original peptide is:

		LDGKMVALKTQAM
		VALKTQAMLDGKM
		VALKTQAMLDGKM
		MVALKTQAMLDGK

Answer 1

one of the peptide (the number 5) begins with MVALK which means that the portion VALK comes after a M, so, we can discart the 2nd and 3rd option.

Now between the 1 and 4 option, you can see that in peptide 4 is LDGK and peptide 2 is LDGKM, which means that the peptide should have that part ending in M. In option 4, LDGK ends with that, but in option 1, it begins with LDGKM and then the part of VALKTQAM. So the correct option would be the first one.

Hope this helps