Elimination of all charged residues in the vicinity of the active-site helix of the disulfide oxidoreductase DsbA. Influence of electrostatic interactions on stability and redox properties

Abstract

Disulfide oxidoreductases are structurally related proteins that share the thioredoxin fold and a catalytic disulfide bond that is located at the N terminus of an alpha-helix. The different redox potentials of these enzymes varying from -270 mV for thioredoxin to -125 mV for DsbA have been attributed to the lowered pKa values of their nucleophilic, active-site cysteines and the difference in thermodynamic stability between their oxidized and reduced forms (DeltaDeltaGox/red). The lowered pKa of the nucleophilic cysteine thiols was proposed to result from favorable interactions with the helix dipole and charged residues in their vicinity. In this study, we have eliminated all charged residues in the neighborhood of the active-site disulfide of DsbA from Escherichia coli to analyze their contribution to the physicochemical properties of the protein. We show that the conserved charge network among residues Glu24, Glu37, and Lys58 stabilizes the oxidized form of DsbA and thus does not cause the high redox potential of the enzyme. The pKa values of the nucleophilic cysteine (Cys30) and the redox potentials of the DsbA variants E24Q, E37Q, K58M, E24Q/K58M, E37Q/K58M, E24Q/E37Q, E24Q/E37Q/K58M, and E24Q/E37Q/E38Q/K58M are similar to those of DsbA wild type. The redox potentials of the variants neither correlate with the Cys30 pKa values nor with the DeltaDeltaGox/red values, demonstrating that the relationship between these parameters is far more complex than previously thought.