Conformational Study of an Artificial Metal-Dependent Regulation Site for Use in Designer Proteins

This report describes the dimerisation of glutathione, and by extension, other cysteine-containing peptides or protein fragments, with a 5, 5’-disubstituted-2, 2’-bipyridine or 6, 6”-disubstituted-2, 2’:6’,2”-terpyridine unit. The resulting bipy-GS(2) and terpy-GS(2) were investigated as potential metal ion dependent switches in aqueous solution, and were found to predominantly adopt the transoïd conformation at physiological pH. Metal complexation with Cu(II) and Zn(II) at this pH has been studied by UV/Vis, CD, NMR and ion-mobility mass spectrometry. Zn(II) titrations are consistent with the formation of a 1:1 Zn(II):terpy-GS(2) complex at pH 7.4, but bipy-GS(2) was shown to form both 1:1 and 1:2 complexes with the former being predominant under dilute micromolar conditions. Formation constants for the resulting 1:1 complexes were determined to be log K(M) 6.86 (bipy-GS(2)) and 6.22 (terpy-GS(2)), consistent with a higher affinity for the unconstrained bipyridine, compared to the strained terpyridine. Cu(II) coordination involves the initial formation of 1:1 complexes, followed by 1.5Cu:1bipy-GS(2) and 2Cu:1terpy-GS(2) complexes at micromolar concentrations. Binding constants for formation of the 1:1 complexes (log K(M) 12.5 (bipy-GS(2)); 8.04 and 7.14 (terpy-GS(2))) indicate a higher affinity for Cu(II) than Zn(II). Finally, ion-mobility MS studies detected the free ligands in their protonated form, and were consistent with the formation of two different Cu adducts with different conformations in the gas-phase. We illustrate that the bipyridine and terpyridine dimerisation units can behave like conformational switches in response to Cu/Zn complexation, and propose that in future these can be employed in synthetic biology with larger peptide or protein fragments, to control large scale folding and related biological function.