you are trying to figure out the sequence 0f a decapeptide. Use the results belo

Question

you are trying to figure out the sequence 0f a decapeptide. Use the results below to figure out the sequence:

            Trypsin digestion gave 2 fragments with the following residues (not in order):

T1: A, R, F, P, T, W, Y

T2: K, M, V

            Chymotrypsin digestion gave three fragments with the following residues (not in order):

C1: A, R, F, P

C2: T, W

C3: K, M, Y, V

N-terminal sequencing: The N-terminal residues of T1 are: Y-T

CnBr gave only a single amino acid (M) and a nonapeptide

What is the sequence? (1-letter code, no spaces or breaks in the answer)

Explanation / Answer

The sequence of the decapeptide is MVKYTWARPF

Or MVKYTWRPAF

1st clue is the CnBr digestion which cleaves at the C-terminus of a Methionine(M)-

As we get an M+a nonamer, this clearly indicates that M is the 1st amino acid in the entire sequence.

Now in the case of T1- it is clearly mentioned that the N-terminal sequencing gave Y-T...

So, Y-T are the first 2 amino acids of T1...

So the sequence obtained thus far is M-X-X-Y-T-X-X-X-X-X

Now the C2 sequence is T,W... As we already know that T is preceeded by Y, we can then conclude that W follows T.

So the sequence now reads M-X-X-Y-T-W-X-X-X-X

Now, trypsin cleaves peptide bonds at the C-terminal of R,K except if they are followed by a P.

We already know that the T2 fragement is K,M,V. We already know that M is first. We also know that the N-terminal of T1 has Y and not P. So K has to be present in the 3rd position to give us the 3 amino acid fragment.

Now the sequence reads - M-V-K-Y-T-W-X-X-X-X

Now, chymotrypsin cleaves at the C-terminus of F,Y,W.

In the decapeptide given... thgere is 1 F, 1 Y,1 W. So we should have observed 4 digested fragments for Chymotrypsin instead of 3. This means that 1 out of F,Y,W must be the last amino acid in the sequence which is why we get just 3 bands... Therefore since we have already determined the positions of Y and W, F must be the last amino acid in the sequence.

So the sequence now reads - M-V-K-Y-T-W-X-X-X-F

Now onto the final part... You will notice that trypsin cleaves at the C-terminus of R,K only except if theres a P following it.

Now in the decapeptide given above, there is 1 R and 1 K. So we should have obtained 3 fragments instead ocf just 2 after trypsin digestion.

This means that either of them must be the last amino acid in the sequence...which is not the case as we have already previously determined the last amino acid to be F

OR one of them is followed by a P.

Now since we already know the position of K and that it is not followed by P... the only conclusion that now remains is that R is followed by P.

The last vacant spot remaining is taken up by A...

So the sequence now reads - M-V-K-Y-T-W-A-R-P-F

OR M-V-K-Y-T-W-R-P-A-F

The information provided is inconclusive for the exact position of A and thus you get 2 options and either of the above 2 sequence is the actual sequence of the decapeptide...

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