Evaluation of the energetic contribution of interhelical Coulombic interactions for coiled coil helix orientation specificity.

ABSTRACT

Coiled coils are formed by two or more alpha-helices that align in a parallel or an antiparallel relative orientation. The factors that determine a preference for a given relative helix orientation are incompletely understood. The helix orientation preference for the designed coiled coil, Acid-a1-Base-a1, was measured previously. This model system therefore provides a means for the experimental determination of the energetic contribution of a variety of interactions to helix orientation specificity. The antiparallel preference for Acid-a1-Base-a1 is imparted by a single buried polar interaction. Interhelical Coulombic interactions between residues at the e and g positions have been proposed to influence helix orientation preference. In the Acid-a1-Base-a1 heterodimer, potentially attractive Coulombic interactions are expected in both orientations. To determine the energetic consequences of Coulombic interactions for helix orientation preference, we have positioned a single charged residue in each peptide such that exclusively favorable interhelical Coulombic interactions can occur only in the parallel orientation. In contrast, two potentially repulsive interactions are expected in the antiparallel orientation. Because the buried polar interaction can occur only in the antiparallel orientation, interhelical Coulombic interactions favor the parallel orientation and the potential to form a buried polar interaction favors the antiparallel orientation. We find no clear preference for an antiparallel orientation in the resulting heterodimer, Acid-Ke-Base-Eg, suggesting that interhelical Coulombic interactions and a buried polar interaction are of approximately equal importance for helix orientation specificity. Stability measurements indicate that maintenance of all favorable electrostatic interactions and/or avoidance of two potentially repulsive interactions contributes approximately 2.1 kcal/mol to helix orientation preference.