How Thioredoxin Dissociates Its Mixed Disulfide
Figure 7
Cys32Trx is primed for nucleophilic attack in the Trx-ArsC activated complex.
Selected snapshots from an MD simulation of the Trx-ArsC complex (ionized Cys82ArsC), presenting the structural basis of the activation of Cys32Trx (green carbons, ball and sticks) for its nucleophilic attack onto Cys29Trx. In these snapshots, Cys32TrxSγ is hydrogen-bonded (magenta dotted lines) to the amide NH groups of both Trp28Trx and Cys29Trx. Also, the sulphur atoms of Cys32Trx and Cys29Trx are within 4.5 Å of each other. The hydrogen bonds satisfy the geometric criteria: Cys32TrxSγ—Trp28TrxN≤4 Å and Cys32TrxSγ—Cys29NTrx≤4 Å, with the corresponding angles between Sγ and the N−H vectors being ≥150 degrees. The hydrogen-bonds donated to the sulphur of Cys32Trx lower its pKa to 7.4 (Table 4), corresponding to a significant population of the thiolate form of Cys32Trx. This thiolate being close to Cys29Trx, it is in effect primed for nucleophilic attack onto Cys29Trx (black arrow). In combination with the supporting reaction analysis, pKa calculations and complex formation experiments (main text), the conformations and interactions shown here are proposed to underpin the dissociation mechanism of the Tx-ArsC complex.