thank you A good example of using everything you know about

thank you

A good example of using everything you know about amino acids to help identify the sequence. The following observations were made on a tetrapeptide obtained by treatment of a protein with pepsin. The tetrapeptide has an absorption band at 280 nm. Four equivalents of base are required to titrate the tetrapeptide from pH 1 to pH 13. Treatment of the tetrapeptide with hydrazine led to the identification of lys as the only unmodified amino acid. Treatment of the tetrapeptide with cyanogens bromide yielded two dipeptides. Treatment of the tetrapeptide with carboxypeptidase B had no effect. Determine the sequence of this tetrapeptide.

Solution

Since the tetrapeptide had an absorption band at 280nm, it means there are aromatic aminoacids present i.e tyrosine and tryptophan.When tetrapeptide is treated with hydrazine,the carboxy terminal residue remains free and in this case the residue is lysine that remains free.So lysine should be present at carboxyl end.Cyanogen bromide cleaves at C terminal of methionine and in this case it gives two dipeptides.So the sequence can be predicted as :

NH2 trp-----met----tyr----- lys COOH or

NH2 tyr-----met------trp-----lys COOH