Controlling and Fine Tuning the Physical Properties of Two Identical Metal Coordination Sites in De Novo Designed Three Stranded Coiled Coil Peptides

Abstract

Herein we report how de novo designed peptides can be used to investigate whether the position of a metal site along a linear sequence that folds into a three-stranded α-helical coiled coil defines the physical properties of Cd(II) ions in either CdS₃ or CdS₃O (O-being an exogenous water molecule) coordination environments. Peptides are presented that bind Cd(II) into two identical coordination sites that are located at different topological positions at the interior of these constructs. The peptide GRANDL16PenL19IL23PenL26I binds two Cd(II) as trigonal planar 3-coordinate CdS₃ structures whereas GRANDL12AL16CL26AL30C sequesters two Cd(II) as pseudotetrahedral 4-coordinate CdS₃O structures. We demonstrate how for the first peptide, having a more rigid structure, the location of the identical binding sites along the linear sequence does not affect the physical properties of the two bound Cd(II). However, the sites are not completely independent as Cd(II) bound to one of the sites (¹¹³Cd NMR chemical shift of 681 ppm) is perturbed by the metalation state (apo or [Cd(pep)(Hpep)₂]⁺ or [Cd(pep)₃]⁻) of the second center (¹¹³Cd NMR chemical shift of 686 ppm). GRANDL12AL16CL26AL30C shows a completely different behavior. The physical properties of the two bound Cd(II) ions indeed depend on the position of the metal center, having pK_a2 values for the equilibrium [Cd(pep)(Hpep)₂]⁺ → [Cd(pep)₃]⁻ + 2H⁺ (corresponding to deprotonation and coordination of cysteine thiols) that range from 9.9 to 13.9. In addition, the L26AL30C site shows dynamic behavior, which is not observed for the L12AL16C site. These results indicate that for these systems one cannot simply assign a “4-coordinate structure” and assume certain physical properties for that site since important factors such as packing of the adjacent Leu, size of the intended cavity (endo vs exo) and location of the metal site play crucial roles in determining the final properties of the bound Cd(II).