(This is supposed to show a disulfide bond between cysteines in positions 6 and
ID: 33538 • Letter: #
Question
(This is supposed to show a disulfide bond between cysteines in positions 6 and 8 of the top chain, and between cysteine 2 of the top chain and cysteine 2 of the bottom chain.) List the peptides (abbreviations are OK; complete chemical structures are NOT required) that would be produced upon treatment of the above peptide in the following ways. (Each of the following is a separate digestion.): a) Digestion with chymotrypsin b) Treatment with mercaptoethanol, then digestion with trypsin c) Treatment with dithiothreitol, then iodoacetic acid, then cyanogen bromide 2. Bovine ribonuclease A has the following amino acid composition: 12 ala, 4 arg, 10 asn, 5 asp, 8 cys, 7 gIn, 5 glu, 3 gly, 4 his, 3 ile, 2 leu, 10 lys, 4 met, 3 phe, 4 pro, 15 ser, 10 thr, 0 trp, 6 tyr, and 9 val. a) The enzyme is treated with TCEP and then digested with trypsin. Predict the number of peptides that would be produced. b) Would your answer be different if you knew that the C-terminal amino acid was arginine? Explain. c) Predict the number of peptides that would be produced as a result of reaction with cyanogen bromide.Explanation / Answer
1. a) chymotrypsin acts on large hydrophobic residues such as tryptophan, tyrosine and phenylalanine. so, in the above peptide , on digestion with chymotrypsin, 3 peptides will be formed (note; in the top chain phe and trp are present.)
b ) trypsin cuts between lys ad arg. One arg is present in top chain and one in bottom chain. so, three fragments will be formed.
c) cyanogen bromide cuts at met. In bottom chain, one met is present. so, 2 fragments will be formed.
2 a) 11 peptides will be formed (10 lys are there; note: 4 more cuts at arg are not there, because arg is proceeded by pro).
b) If C- terminal was arg, one more cut would be produced . so, number of peptides will be one more.
c) 5 peptides will be formed (there are four positions for met)
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