FIG 4 .

The disulfide bridge in the Cys-rich motif affects the SCHIC-heme interaction. (A) A disulfide bridge was identified within peptide AppA_397-409. Under oxidized conditions, neither Cys³⁹⁹ nor Cys⁴⁰⁶ can be modified by iodoacetamide (IAM). Under reduced conditions, the disulfide bridge formed between Cys³⁹⁹ and Cys⁴⁰⁶ is reduced, and both residues can be modified by iodoacetamide (DTT-IAM). (B) LC-MS-MS with double-labeled AppA identified the Cys³⁹⁹-Cys⁴⁰⁶ disulfide bond (see Fig. S2 in the supplemental material). (C) The cysteine residues in AppA are reduced by DTT (AppA_R). When light excited, the Soret peak of AppA_R-heme is redshifted from 412 nm to 418 nm. (D) When light excited, the Soret peak of AppA_C399A-heme is redshifted from 412 nm to 418 nm.