Question

I need someone to help explain this to me please in great detail! Explain it to me very simply. thank you!

31) A nonapeptide was determined to have the following amino acid composition: (Lys)2, (Gly)2, (Phe) 2, His, Leu, Met. The following are possible sequences for the peptide. A) G-F-K-K-G-L-M -F-H B) H-L-G-K-K-F-F-G-M C) H-L-F-G-K-K-F-M -G D) H-F-L-G-K-K-F-M-G E) M-L-F-K-F-G-G-K-H The following observations were made during sequence determination. (Hint: FDNB reacts with N-terminal amino groups and specificities of proteases listed below are given on the front page of the exam). (a) The native peptide was incubated with 1-fluoro-2,4-dinitrobenzene (FDNB) and then hydrolyzed; 2,4-dinitrophenylhistidine was identified in the sample. (b) When the native peptide was exposed to cyanogen bromide (CNBr), an octapeptide and free glycine were recovered. (c) Incubation of the native peptide with trypsin gave a pentapeptide, a tripeptide, and free Lys. 2,4-Dinitrophenylhistidine was recovered from the pentapeptide, and 2,4- dinitrophenylphenylalanine was recovered from the tripeptide. (d) Digestion with the enzyme chymotrypsin produced a dipeptide, a tripeptide, and a tetrapeptide. The tetrapeptide was composed of (Lys) 2, Phe, and Gly. Which of the 5 sequences above is most consistent with the data?

Answer 1

Answer ) C H--L- F-G-K -K-F- M-G

a) When native peptide incubated with FDNB, it was hydrolysed and 2,4 dinitrophenylhistidine was observed FDNB reactwith N terminal aminoacid. So in native peptide the N terminal aminoacid is Histidine (H) H--2-3-4-5-6-7-8-9 b) Native peptide exposed to cynogenbromide, octapeptide and free glycine were observed Cynogen bromide is cleave the peptide bond of the c terminus of methionine His--2-3-4-5-6-7- Methionine- Glycine His--2-3-4-5-6-7- Methionine gly H--2-3-4-5-6-7- M-G Incubation of native peptide with Trypsin-Trypsin can cleave the carboxl end of lysine or arginine c) After treatment with trypsin, a pentapeptide, tripeptide, lysine were obtained H--2-3-4-5-6-7- M-G H-2-3-4-K-K-7- M-G H--2-3-4-K -K-7- M-G H--2-3-4-K -K- 7- M-G H--2-3-4-K (Pentapeptide)- FDNB treatment Histidine was removed, 7- M-G (TRIPEPTIDE)- FDNB treatment Phenyl alanine was removed so F-M-G D) Digestion with chymotripsine-dipeptide,tripeptide and tetrapeptide was observed Chymotripsin cleave Cterminus of aromatic amino acids H-2-3-4-K -K-F- M-G, Tetrapeptide contain 2 Lysin, phe and Gly- H-L- (di) F-Gly-K -K--(tetra) F-M-G (tri) H--L- F-G-K -K-F- M-G