Packing of α-helices: Geometrical constraints and contact areas

The helical packing in sperm whale myoglobin has been examined. Using cylindrical co-ordinates based on each helix axis in turn, the overlap of the side-chain atoms of a helix with the surrounding atoms from other parts of the structure was 2.3 Å, but the distribution was not at all uniform and severe overlap occurred in at least one location for each helix. Simple axial translations or rotations of any helix in the native structure are not permitted motions. Translation perpendicular to the helix axis in at least one direction is not restricted by interlocking side-chains.The approach of two helices along the contact normal connecting their axes produces solvent exclusion effects at a distance of about 6 Å from the final position. The solvent-excluded area found in such interaction sites is equivalent to a large hydrophobic contribution to the free energy of association. The six principal sites correspond by themselves to 40% of the total area change in going from the extended sausage model to the native structure. The mean atom-packing densities for these sites and the standard deviations of these values are similar and are equal to that found for the protein as a whole.Helices of close-packed spheres form useful approximations to actual peptide helices. The helix of index number four corresponds closely to an α-helix. The required sphere size corresponds in volume to residues such as leucine or methionine. The predicted packing scheme for such helices corresponds to the three general classes of interactions actually seen.Making use of the geometry implied by the close-packed sphere helix, an algorithm is proposed for picking potentially strong helix-helix interaction sites in peptide chains of known sequence. When combined with preliminary secondary structure predictions, it is suggested that this algorithm might usefully restrict the search for these specific types of contact in the docking portion of a general folding program.