Biology · Biochemistry

Two experiments were performed on a peptide sample 'X'. In experiment 1, treatment of 'X' with dithiothreitol (DTT), followed by blocking of free sulfhydryl groups, yielded two polypeptides whose amino acid sequences are shown below: I. Ala-Phe-CysA3-Met-Tyr-CysA6-Leu-Trp-CysA9-Asn II. Val-CysB2-Trp-Val-Ile-Phe-Gly-CysB8-Lys In experiment 2, 'X' was treated with chymotrypsin, a protease that cleaves at the carboxy-terminal of aromatic residues. The amino acid compositions of five peptides obtained from this experiment are shown below: I. ([Ala], [Phe]) II. ([Asn], 2[Cys], [Met], [Tyr]) III. ([Cys], [Gly], [Lys]) IV. (2[Cys], [Leu], 2[Trp], [Val]) V. ([Ile], [Phe], [Val]) Based on the above results, which cysteines are linked by disulfide bonds in peptide 'X'?