Regulation mechanism of the DsbA/DsbB system that catalyzes disulfine bond formation

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In the Escherichia coli protein disulphide bond formation pathway, a membrane protein DsbB reoxidizes reducd periplasmic DsbA, the disulphide boud-introducing enzyme. Our results indicate that the oxidizing equivalent for disulphide bond formation is provided by oxygen through the respiratory electron transfer system in aerobically growin E. coli cells. The Cys-41-Val-Leu-Cys-44 motif in DsbB is strongly oxidized by respiratory quinone molecules. The results of our insertion mutagenesis indicated that a segment just C-terminally adjacent to CXXC motif is crucial for the respiration-coupled oxidation of DsbB(4)Ala substitution for one or all of the I le45-tyr46-Glu47-Arg48 residues, which are franked by the CXXC motif and the presumed membrane-panning region, did not abolish the respiratory coupling. In contrast,deletion of one or more residues from this segment as well as insertions of one or more Ala into it severely impaired the oxidation.Thus,the latter mutant proteins accumulated as reduced forms or last the characteristic dithiothreitol resistance. We prose that the importance of the Ile45-tyr46-Glu47-Arg48 segment may lie in its physical length rather than the chemical nature of each amino acid. It may be conceivable that a proper positioning of the CXXC motif relative to the membrane surface is crucial for its efficient interaction with membrane-anchored quinonc molecules.The reaction Could then be driven by the large difference between the redox potentials of ubiquinone ubiquinol vs(CXXC)ox/(CXXC)red.