Preparation and structure of the charge-transfer intermediate of the transmembrane redox catalyst DsbB.

Disulfide bond formation is a critical step in the folding of many secretory proteins. In bacteria, disulfide bonds are introduced by the periplasmic dithiol oxidase DsbA, which transfers its catalytic disulfide bond to folding polypeptides. Reduced DsbA is reoxidized by ubiquinone Q8, catalyzed by inner membrane quinone reductase DsbB. Here, we report the preparation of a kinetically stable ternary complex between wild-type DsbB, containing all essential cysteines, Q8 and DsbA covalently bound to DsbB. The crystal structure of this trapped DsbB reaction intermediate exhibits a charge-transfer interaction between Q8 and the Cys44 in the DsbB reaction center providing experimental evidence for the mechanism of de novo disulfide bond generation in DsbB.