FIGURE 4.

A, composite summary of amino acid sequences identified in peptide fragments of R60A CDO generated by chymotrypsin digestion of the upper right and lower left spots from two-dimensional gel electrophoresis followed by nanoLC-MS/MS analysis. Amino acid residues positively identified in peptide fragments are indicated in black, and those that were not detected are indicated in gray. Residues Cys⁹³ and Tyr¹⁵⁷ are indicated in an enlarged font. The mutated residue (R60A) is indicated by an asterisk. B, MALDI evidence for the presence of a Cys⁹³–Tyr¹⁵⁷ cross-linked peptide in the lower left spot but not in the upper right spot of R60A CDO. C, MALDI data indicating a reduced cutting efficiency of chymotrypsin in the lower left spot of R60A CDO at the covalent bond between Tyr¹⁵⁷ and Ser¹⁵⁸, resulting in a poor yield of the SPPFDTCHAF peptide compared with the yield for the upper right spot of R60A CDO.