You have isolated an octopeptide from a rare fungus. This peptideprevents baldne

Question

You have isolated an octopeptide from a rare fungus. This peptideprevents baldness and you wish to determine its amino acidsequence. The composition is Lys2 , Asp, Tyr, Phe, Gly,Ser, Ala. Reaction of the intact peptide with FDNB yieldsDNP-alanine plis two moles of -DNP lysine among acidhydrolysis. Cleavage with trypsin yields peptides whosecompositions are (Lys, Ala, Ser) and (Gly, Phe, Lys) plus adipeptide. Reactions with chromotrypsin releases free asparticacid, a tetrapeptide with the composition (Lys, Ser, Phe, Ala) anda tripeptide whose composition following acid hydrolysis is (Gly,Lys, Tyr). What is the sequence?

Explanation / Answer

      FDNB reagent is used toidentify the N- terminal end of a peptide.       On treating the peptidewith FDNB yielded DNP- Alanine and two moles of Lysine.       This indicates that theN-terminal amino acid is Alanine.       Trypsin cleaves thepeptide at carboxyl group of the Lysine and Argine aminoacid.       In this experiment asLysine is present, trypsin cleaves the carboxyl group of Lysineresidues.       The two compositionsconsisted of Lys, Ala, Ser and Gly, Phe and Lys and the dipeptidewill have Asp and Tyr.       From this we can saythe sequence of one of the composition as Ala-Ser-Lys.       This is because theN-terminal has Ala and trypsin will cleave the carboxyl group ofLysine, hence Ser will be in the       middle and the sequencewill be as Ala-Ser-Lys.       The composition of thesecond sequence will be as either Gly-phe-Lys or Phe-Gly-Lys.       After trypsin cleavage thecomposition of the peptide may be as:       Ala-Ser-Lys + Gly-Phe-Lys/ Phe-Gly-Lys + Asp-Tyr / Tyr-Asp       Chymotrypsin cleaves thecarboxyl side of aromatic aminoacid.       In the given peptide thearomatic aminoacids present are Phe and Tyr.       The compostions obtainedfrom this cleavage are free Asp; tetrapeptide with Lys, Ser,Phe and Ala and a tri peptide       of Gly, Lys andTyr.       From earlier clevagesif is clear that the first fragment is Ala-Ser-Lys; fromchymotrypsin cleavage the tetrapeptide       has Phe in it so thefragment of tetrapeptide will be as Ala-Ser-Lys-Phe.       The second composition hasGly-Lys-Tyr.       From all these we can saythat the composition of the octapeptide isAla-Ser-Lys-Phe-Gly-Lys-Tyr-Asp.

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