Assume you have isolated a decapeptide (a 10-residue peptide) called "fighting illini (FI)," which has anticancer activity. Determine the sequence of the peptide from the following information. (Note that amino acids are separated by commas when their sequence is not known.). Hint: peptide bonds are not the only type of bonds linking the amino acids to each other in a protein. See insulin as an example.
(a) One cycle of Edman degradation of intact FI yields 2 mol of PTH-aspartate per mole of FI.
(b) Treatment with ?-mercaptoethanol yields two pentapeptides.
(c) Treatment of FI with trypsin in the presence of ?-mercaptoethanol yields three peptides with the following composition: (Ala,Cys,Phe), (Arg,Asp), and (Asp,Cys,Gly,Met,Phe). The intact (Ala,Cys,Phe) peptide yields PTH-cysteine in the first cycle of Edman degradation.
(d) Treatment of 1 mol of the intact FI with carboxypeptidase yields 2 mol of phenylalanine.
(e) Treatment of the intact pentapeptide (Asp,Cys,Gly,Met,Phe) with CNBr yields two peptides, a dimer and a trimer. The tripeptide yields PTH-glycine in the first round of Edman degradation.