Roles of thiol-redox pathways in bacteria

Disulfide bonds in proteins play various important roles. They are either f ormed as structural features to stabilize the protein or are found only tra nsiently as part of a catalytic or regulatory cycle. In vivo, the formation and reduction of disulfide bonds is catalyzed by specialized thiol-disulfi de exchanging enzymes that contain an active site with the sequence motif C ys-X-X-Cys. These proteins have structurally evolved to catalyze predominan tly either oxidative reactions or reductive steps. There is mounting eviden ce that, in addition to the thiol redox potential, the spatial distribution within different cell compartments and the overall redox state of the cell are equally important. In the cytoplasm, multiple pathways play overlappin g roles in the reduction of disulfide bonds and additionally, the expressio n of several components of thiol-redox pathways was shown to respond to the changes in the cellular thiol-redox equilibrium. In the periplasm, two sys tems coexist, one catalyzing thiol oxidation and the other disulfide reduct ion. Recent results suggest that two different mechanisms are used to trans locate reducing power from the cytoplasm or to dissipate the electrons afte r oxidation.