Molecular dynamics study of secondary structure motions in proteins: application to myohemerythrin

The concept of secondary structure motions is examined in a molecular dynamics simulation of the protein myohemerythrin. We extracted from the simulation a corresponding trajectory of helices and demonstrated that the fluctuations of the protein are dominated by a rigid shift of these secondary structure elements. The relative motions of the helices are irregular, with no clear periodicity. They are bounded by approximately 2 A for the center of mass motions and by 20 degrees for the relative orientations. The potential of mean force for the interactions of the helices was calculated, and the correlations between the different extended motions were investigated. It is shown that the one-dimensional mean force potentials are close to quadratic for most of the helices coordinates. The anharmonicity is reflected by changes in the direction of the normal modes as a function of the energy and by the existence of multiple free energy minima for the helices packing. The multiple conformations are associated with a single type of secondary structure coordinate: the angle that describes the relative orientation of the helices in a plane perpendicular to the line connecting their center of mass.