The influence of His94 and Pro149 in modulating the activity of V. cholerae DsbA.

DsbA is the primary catalyst of disulfide bond formation in the periplasm of gram-negative bacteria. Numerous theoretical and experimental studies have been undertaken to determine the molecular mechanisms by which DsbA acts as a potent oxidant, whereas the homologous cytoplasmic protein, thioredoxin, acts as a reductant. Many of these studies have focused on the nature of the two residues that lie between the active-site cysteines. Although these are clearly important, they are not solely responsible for the differences in activity between these thiol-disulfide oxidoreductases. Q97 in the helical domain of E. coli DsbA has been implicated in influencing the redox potential of E. coli DsbA. In V. cholerae DsbA, the analogous residue is H94. In this study, the effect of H94 on the oxidase activity of DsbA is examined, along with the role of the conserved cis-proline residue P149. The DsbA mutant H94L shows a nearly fourfold increase in activity over the wild-type enzyme. To our knowledge, this is the first time an increase in the normal activity of a thiol-disulfide oxidoreductase has been reported. Potential reasons for this increase in activity are discussed.