%0 Generic
%A Baldus, Ilona Beatrice
%C Anorganisch-Chemisches Institut, Universität Heidelberg
%D 2013
%F heidok:14398
%R 10.11588/heidok.00014398
%T Mechanochemistry of Disulfide Bonds in Proteins
%U https://archiv.ub.uni-heidelberg.de/volltextserver/14398/
%X Mechanical force alters a protein's stability not only due to its ability to unfold the biomolecule. As soon as a disulfide bond cross-linking the protein is exposed to force, its reduction rate is altered. Our first aim was quantifying the direct effect of force onto the chemical reactivity of sulphur-sulphur bonds in contrast to  indirect, e.g. steric or mechanistic, influences. To this  end, we evaluated the dependency of a disulfide bond's redox potential on a pulling force applied along the system. Our hybrid quantum and molecular mechanics simulations of cystine as a model system take conformational dynamics and explicit solvation into account and show that redox potentials increase over the whole range of forces probed here (30 - 3320 pN), and thus even in the absence of a significant disulfide bond elongation(<500 pN). Instead, at low forces, dihedrals and angles as the softer degrees of freedom are stretched and contribute to the destabilization of the oxidized state. We find physiological forces to be likely to tune the disulfide's redox potentials to an extent similar to the tuning within proteins by point mutations.  Next, we asked how internal strain resulting from the protein structure tunes redox potentials using free energy calculations, more precisely nonequilibrium Molecular Mechanics transformations and the Crooks Gaussian Intersection method. We added a residue to the Charmm force field that models a disulfide bond in the reference state and that can be transformed into a thiol in the product state. To our knowledge, this is the first approach to open a covalent bond by means of free energy transformation. We tested our method on E. coli and S. aureus thioredoxin, and could partly reproduce relative redox potentials of the wild-type and some mutants. We discuss promising routes to improve the accuracy of these challenging calculations.  Finally, we investigated the impact on force-induced unfolding by a special type of disulfide bond, a vicinal disulfide that links two adjacent cysteines. Our model system here is the von Willebrand factor (vWF) A2 domain.  We observe similar stabilities in equilibrium for both the native system and its analogue with the disulfide bond broken and also similar collective motions. Application of an external force, however, induces a difference: Unfolding of the vWF A2 domain with the vicinal disulfide bond present  leads to higher rupture forces than when it is missing. This indicates that the vicinal disulfide bond prevents the domain from unintentional unfolding.