4. A pentapeptide was found to have the sequence) Aa pentapeptide s reagent, fol

Question

4. A pentapeptide was found to have the sequence) Aa pentapeptide s reagent, folloposition (not sequence) Ala Arg Gly Pro Trp. Reaction of the s, gave the DNP derivative of proline. Treatment of the pentapeptide a aproduced alanine. Treatment of the pentapeptide with trypsin gave a tetrapeptide hat is the sequence of the pentapeptide? (Spts) which, when treated with chymotrypsin, produced a tripeptide. W tide. However, after treating it with 5. A student isolated a protein and assumed it was made up of a single polypep excess dithiothreitol (DTT; see structure below), two polypeptides were obtained. One of the polypeptides contained 78 amino acids and the other contained 125 amino acids. Both polypeptides contained 1 Cys and 1 Met Using the structure of DTT below, sketch what the protein likely looked like before treatment with DTT (label/name any key amino acids and the ends of your polypeptide). (2pts) A. B. Now, draw/sketch ALL of the likely products of the reaction labeling/naming any key amino acids. (3pts) Protein?? Products?? HO DTT

Explanation / Answer

Ans 4. The sequence of peptide is (N ter)Pro-Gly-Trp-Arg-Ala(C ter)

Sanger's reagent reacts with free amino group of N terminal amino acid residue of a peptide and form a DNP derivative which is released from peptide by hydrolysis. So DNP derivative of proline indicates that Pro is the first residue at N terminus.

Carboxypeptidases cleaves C terminus amino acid residue, So Ala generation indicates that it is the first residue at C terminus of peptide.

Trypsin cleaves at carboxyl side of Lys and Arg but the next residue should not be Pro. Hence in this sequence Arg should be at 4th position from N terminus so that after treatment with trypsin, it results in a tetrapeptide by cleaving bond between Arg-Ala.

Chymotrypsin cleaves at carboxyl side of aromatic amino acid residues but the next residue should not be Pro. Hence Trp should be present next to Arg so that after treatment it can results in a tripeptide formation.

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