Mechanism of thioredoxin-catalyzed disulfide reduction. Activation of the buried thiol and role of the variable active-site residues.

Thioredoxins (Trx) are enzymes with a characteristic CXYC active-site motif that catalyze the reduction of disulfide bonds in other proteins. We have theoretically explored this reaction mechanism, both in the gas phase and in water, using density functional theory. The mechanism of disulfide reduction involves two consecutive thiol-disulfide exchange reactions, that is, nucleophilic substitutions at sulfur (S(N)2@S): first, by one Trx cysteine-thiolate group (Cys-32) at a sulfur atom of the disulfide substrate and, second, by the other Trx cysteine-thiolate group (the buried thiol of Cys-35) at the sulfur atom of the first Trx cysteine. We have investigated the intrinsic nature of such S(N)2@S substitution using the simple CH3S(-) + CH3SSCH3 model and how it is affected by solvation in aqueous solution. Next, we have examined how the behavior of the elementary S(N)2@S steps changes in the more realistic enzyme-substrate model CGPC + CH3SSCH3, which contains the active-site of Trx. In all model reactions, solvation turns the hypervalent trisulfide anion (i.e., the S(N)2@S transition species) from a stable complex into a transition state. Importantly, our analyses suggest that the deprotonation of the buried thiol (which is required before the latter can enter into the second S(N)2@S step) is done by the leaving group evolving from the first S(N)2@S step. Finally, molecular dynamics (MD) simulations, in the gas phase and in water, of CGPC, CGGC, and the corresponding wild-type Trx and P34G Trx show that the activity of the thioredoxin active-site motif (CXYC) is determined not only by the structural rigidity associated with the particular variable residues (XY) but also by the number of amide N-H groups. The latter are involved in the stabilization of the Cys-32 thiolate and thus affect the acidity and nucleophilicity of this residue.