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What are E/K Coils?

E/K coils are synthetic coiled-coil domains designed specifically to bind to each other with high affinity and specificity (Litowski and Hodges, 2002) (Figure 1). They are composed of a heptad repeat that forms a coil structures that are able to interact with each other. These coils are able to interact with each other in an anti-parallel fashion that makes them useful for applications such as peptide capture, protein purification and in biosensors. For our project we decided to make use of the IAAL E3/K3 coils due to the balance they offer between affinity and specificity (Table 1).

How do these Coils Work?

These E3/K3 coils are able to form heterodimers due to the hydrophobic residues contained within the heptad repeat. In our case these are isoleucine and leucine residues. Designated by empty arrows in the helical wheel diagram below (Figure 2) these residues form the core of the binding domain of the coils. In order to prevent the homodimerization of these coils charged residues are included in the design. The electrostatic interactions between glutamic acid and lysine residues prevent an E-coil from binding with an E-coil for example. We selected the use of E3/K3 coiled-coils over other synthetic E/K coils as the isoleucine residue present shows a significant increase in the heterodimer over valine found in other coils. The alpha-helical propensity of the residues outside of the core interacting residues is also increased by utilizing an alanine residue instead of the serine residue found in ISAL and VSAL E/K coils. This selection maximizes the stability and specificity of the coils used in our system.