Structure of reduced DsbA from Escherichia coli in solution.

The three-dimensional structure of reduced DsbA from Escherichia coli in aqueous solution has been determined by nuclear magnetic resonance (NMR) spectroscopy and is compared with the crystal structure of oxidized DsbA [Guddat, L. W., Bardwell, J. C. A., Zander, T., and Martin, J. L. (1997) Protein Sci. 6, 1148-1156]. DsbA is a monomeric 21 kDa protein which consists of 189 residues and is required for disulfide bond formation in the periplasm of E. coli. On the basis of sequence-specific 1H NMR assignments, 1664 nuclear Overhauser enhancement distance constraints, 118 hydrogen bond distance constraints, and 293 dihedral angle constraints were obtained as the input for the structure calculations by simulated annealing with the program X-PLOR. The enzyme is made up of two domains. The catalytic domain has a thioredoxin-like fold with a five-stranded beta-sheet and three alpha-helices, and the second domain consists of four alpha-helices and is inserted into the thioredoxin motif. The active site between Cys30 and Cys33 is located at the N terminus of the first alpha-helix in the thioredoxin-like domain. The solution structure of reduced DsbA is rather similar to the crystal structure of the oxidized enzyme but exhibits a different relative orientation of both domains. In addition, the conformations of the active site and a loop between strand beta5 and helix alpha7 are slightly different. These structural differences may reflect important functional requirements in the reaction cycle of DsbA as they appear to facilitate the release of oxidized polypeptides from reduced DsbA. The extremely low pKa value of the nucleophilic active site thiol of Cys30 in reduced DsbA is most likely caused by its interactions with the dipole of the active site helix and the side chain of His32, as no other charged residues are located next to the sulfur atom of Cys30 in the solution structure.