[jazzydunks]

I am studying for a Biochemistry I class and I have hit a brick wall and I would greatly appreciate some help.

Problem:
A peptide is composed of Tyr, Tyr, Arg, Asp, Thr, His, Met, and Lys.

• Treatment with cyanogen bromide yielded a free Met and a heptapeptide.
• Treatment with Trypsin yielded a tetrapeptide with a net positive charge at a pH of 5, a free Thr, and a tripeptide.
• Chymotrypsin treatment yielded a tripeptide , a dipeptide, and tripeptide with a net negative charge at pH of 12.

What is the sequence?

What I know:

• CNBr cleaves Met at the C-terminus
• Trypsin cleaves Arg and Lys at the C-terminus
• Chymotrypsin cleaves Tyr, Phe, Trp at the C-terminus
• The pKa of His at its imidazole is ~6
• The pKa of Lys at its amine is ~10
• The pKA of Arg at its amine is ~12
• The pKA of Asp at its side chain carboxylic acid is ~3

My attempt at a solution:
I have determined that Met is the first in the sequence, because it is cleaved at the C-term by CNBr. Ser must be the last in the sequence because Trypsin must cleave at the C-term of Lys and Arg. The Tetrapeptide must contain histidine since it has a net positive charge at pKa 5. I don't know where to go from here because there are several possibilities for cleavage. I don't know what useful information net negative charge at pH 12 of  one tripeptide as a result of Chymotrypsin treatment is suppose to give me.

[Babcock_Hall]

I think you mean Thr, not Ser, is the last amino acid residue.  Also, when cyanogen bromide acts on a Met residue, it produces homoserine lactone.  You teacher may have left that out for the sake of simplicity, but I thought I should mention it.

[jazzydunks]

yeah I meant Thr, not Ser. also, your reply doesn't really help me more than where I am now.

[Babcock_Hall]

Are any other charged amino acids present, besides the ones you have already mentioned?  Do you know the cleavage specificity of chymotrypsin?

[Babcock_Hall]

There may not be enough information to answer the question.