Ch5CP - Practice Problems PDF

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Class Problems - Chapter 5 1. Mechanism to convert ninhydrin to aldimine? 2. What peptides are expected to be produced when the melanocyte-stimulating peptide hormone -melanotropin (Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val) is cleaved by (a) trypsin, (b) chymotrypsin, or (c) cyanogen bromide? Write whole sequence using one-letter abbreviations: Write peptide fragments after cleavage: (a) (b) (c) 3. The following reagents are often used in protein chemistry. Match the reagent with the purpose for which it is best suited. Some answers may be used more than once or not at all; more than one reagent may be suitable for a given purpose. (a) CNBr (cyanogen bromide)

(e) performic acid

(b) Edman reagent (phenylisothiocyanate) (c) FDNB

(f) chymotrypsin

(g) trypsin

(d) dithiothreitol ___ hydrolysis of peptide bonds (carboxyl side of Lys and Arg) ___ cleavage of peptide bonds on the carboxyl side of Met ___ breakage of disulfide (—S—S—) bonds ___ determination of the amino acid sequence of a peptide ___ determining the amino-terminal amino acid in a polypeptide 4. Sequence of Leucine Enkephalin, a brain opioid peptide. a. Complete hydrolysis by 6M HCl at 110oC followed by amino acid analysis indicated the presence of G (Gly), L (Leu), F (Phe), and Y (Tyr) in a 2:1:1:1 molar ratio. b. Treatment of the peptide with 1-fluoro-2,4, dinitrobenzene (Sanger’s Reagent) followed by complete hydrolysis and chromatography indicated the presence of 2,4dinitrophenyl derivative of tyrosine. No free tyrosine could be found. c. Complete digestion of the peptide with chymotrypsin followed by chromatography yielded free tyrosine and leucine with a tripeptide containing Phe and Gly in a 1:2 ratio. a. b. c.

5. A mutant form of polypeptide hormone angiotensin II has the amino acid composition (Asp, Arg, Ile, Met, Phe, Pro, Tyr, Val) The following observations are made: a) Trypsin yields a dipeptide containing Asp and Arg, and a hexapeptide with all the rest. b) Cyanogen bromide cleavage yields a dipeptide containing Phe and Pro, and a hexapeptide containing all the others. c) Chymotrypsin cleaves the hormone into two tetrapeptides, of composition (Asp, Arg, Tyr, Val) and (Ile, Met, Phe, Pro) d) The dipeptide of composition (Pro, Phe) cannot be cleaved by either chymotrypsin or carboxypeptidase. What is the sequence of angiotensin II? One-letter abbreviation amino acid composition (not sequence): Fragments after cleavage: a) b) c) d)

Trypsin: CNBr: Chymotrypsin: Chymotrypsin or carboxypeptidase:

Sequence of angiotensin II:...