Mercury Trithiolate Binding (HgS) to a de Novo Designed Cyclic Decapeptide with Three Preoriented Cysteine Side Chains.

Mercury(II) is an unphysiological soft ion with high binding affinity for thiolate ligands. Its toxicity lies in the interactions with low molecular weight thiols including glutathione and cysteine-containing proteins that disrupt the thiol balance and alter vital functions. However, mercury can also be detoxified via interactions with Hg(II)-responsive regulatory proteins such as MerR, which coordinates Hg(II) with three cysteine residues in a trigonal planar fashion (HgS coordination). The model cyclodecapeptide P, c(GCTCSGCSRP) was designed to promote Hg(II) chelation in a HgS coordination environment through the parallel orientation of three cysteine side chains. The binding motif is derived from the dicysteine P cyclodecapeptide validated previously as a model for d metal transporters containing the binding sequence CxxC. The formation of the mononuclear HgP complex with a HgS coordination is demonstrated using electrospray ionization mass spectrometry, UV absorption, and Hg NMR. Hg L-edge extended X-ray absorption fine structure (EXAFS) spectroscopy indicates that the Hg(II) coordination environment is T-shaped with two short Hg-S distances at 2.45 Å and one longer distance at 2.60 Å. The solution structure of the HgP complex was refined based on H-H NMR constraints and EXAFS results. The cyclic peptide scaffold has a rectangular shape with the three binding cysteine side chains pointing toward Hg(II). The HgPH complex has a p K of 4.3, indicating that the HgS coordination mode is stable over a large range of pH. This low p K value suggests that the preorientation of the three cysteine groups is particularly well-achieved for Hg(II) trithiolate coordination in P.