Analysis of the blood of a catatonic football fan revealed large concentrations

Question

Analysis of the blood of a catatonic football fan revealed large concentrations of a psychotoxic peptide. Hydrolysis in 6M HCl yielded the amino acid composition: 2 Ala 1 Arg 1 Asn 1 Met 2 Tyr 1 Val 1 NH4+
[a] Partial hydrolysis of the octapeptide yielded a dipeptide of the structure:

[b] Chymotrypsin treatment yielded two tetrapeptides, each containing an alanine residue
[c] Trypsin treatment of one of the two tetrapeptide fragments from b. gave two dipeptides
[d] Cyanogen bromide yielded of the same peptide gave tripeptide and free Tyr.
[e] End group analysis of the other peptide from b. gave an Asp.

WHAT is the amino acid sequence of this octapeptide? Use the one letter code for amino acids to answer this question and explain the information each analysis gave you.

Explanation / Answer

We have the following amino acids in the protein sequence:

2 Ala 1 Arg 1 Asn 1 Met 2 Tyr 1 Val   1 NH4+

i.e. 2A, 1R, 1N, 1 M, 2 T, 1 V

a) The first statement says that partial hydrolysis of the octapeptide yielded a dipeptide which has a structure of that of Ala Val dipeptide. So we know that AV should be side by side in the sequence.

b) The property of Chymotrypsin is to cleave at aromatic amino acids side chains. Since the information given states that this treatment yielded two tetrapeptides, each containing an alanine residue, we can conclude that the two Tyrosine residues must be placed one at the C terminus and one at the R terminus. Only this arrangement can lead to two tetrapeptides each with alanine.

So we assume the following must be the protein sequence:

_ _ _ Y _ _ _ Y

c) Now the next statement says that Trypsin treatment of one of the two tetrapeptide fragments from b. gave two dipeptides. As we know that Trypsin cleaves after an Arginine residue, there could be only 2 positions where the Arginine residue could be placed in our sequence.

Either _ R_Y_ _ _ Y or _ _ _ Y _ R _ Y

d) It is a known fact that Cyanogen bromide cleaves the C terminal of Methionine, hence to get a tripeptide and free Tyr, the position of the methionine-tyrosine has to be placed adjacent to the arginine residue in our sequence.

So we either get a sequence _RMY_ _ _Y or _ _ _Y_RMY

e) Now the End group analysis of the other peptide from b. gave an Asp. So, we would assume the amino acid to be aspartic acid. However, we also know that the end result was an extra NH4 group, which states that the amino acid is asparagine N and not aspartic acid D.

Hence we have the following sequence: Either _RMYN _ _ Y or N _ _ Y_RMY

Since we have to place only 3 more missing amino acids in this sequence, we start with the result of a, wherein, we concluded that AV should occur side by side.

Hence, we get the following sequence: Either _RMYNAVY or NAVY_RMY

Since only one more A needs to fit in the sequence, we get the final protein sequence as

Either ARMYNAVY or NAVYARMY

Related Questions

Navigate

• Browse (All)
• Browse A
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.