For 2425 An octapeptide isolated from brain tissue was shown

For 24-25) An octapeptide isolated from brain tissue was shown to have analgesic properties and bind to the samesites where endorphins, enkephalins and narcotics bind. Deduce the primary structure from the following data.• Amino acid analysis indicates stochiometric (equal) amounts of the following: val (V), phe (F), his (H),

asp (D), met (M), ala (A) lys (K), thr (T)• Two rounds of Edman degradation of the intact peptide gave first the PTH derivative of Val, and thesecond round a PTH derivative of Ala.• Cyanogen bromide cleavage resulted in a 7 amino acid fragment and a free His • Trypsin digestion produced two peptide fragments of 3 and 5 amino acids. The tripeptide was sequencedwith two rounds of Edman degradation to yield a sequence identical to that determined above for theintact peptide (i.e. Val/Ala). For the 5 amino acid fragment, three rounds of Edman degradation providedevidence for a Thr followed by a Phe, followed by Asp.

24). Which of the following amino acid sequenceswould most likely produce the above results (in 1 letter code).

a) AVKTFHDM b) AVKTFHMD

c) VAKTFDMH d) VAKTFDHM e) VAKDFTMH

25. For the correct peptide, which of the followingwould be closest to the overall charge if thispeptide were found in normal human blood.

a) 0      b) +1      c) -1     d) +2      e) -2  

Thank you!

Solution

24: Edman degradation is used in sequencing of polypeptide chains at the N terminus using phenylisothiocyanate as reagent which forms a phenylthiohydantion derivative.   

Cyanogen bromide cleavage is carried out the C termainal of methionine residues.

Trypsin cleaves at the C terminal of Lys or Arg residues.

Using the above information Ans. C