Roles of a conserved arginine residue of DsbB in linking protein disulfide-bond-formation pathway to the respiratory chain of Escherichia coli

The active-site cysteines of DsbA, the periplasmic disulfide-bond-forming e nzyme of Escherichia coil, are kept oxidized by the cytoplasmic membrane pr otein DsbB. DsbB, in turn, is oxidized by two kinds of quinones (ubiquinone for aerobic and menaquinone for anaerobic growth) in the electron-transpor t chain. We describe the isolation of dsbB missense mutations that change a highly conserved arginine residue at position 48 to histidine or cysteine. In these mutants. DsbB functions reasonably well aerobically but poorly an aerobically. Consistent with this conditional phenotype. purified R48H exhi bits very low activity with menaquinone and an apparent Michaelis constant (K-m) for ubiquinone seven times greater than that of the wild-type DsbB. w hile keeping an apparent K-m for DsbA similar to that of wild-type enzyme. From these results, we propose that this highly conserved arginine residue of DsbB plays an important role in the catalysis of disulfide bond formatio n through its role in the interaction of DsbB with quinones.