The application of molecular-symmetry constraints to the constrained-restrained parameter structure-factor least-squares refinement program CORELS

A method is described to apply molecular symmetry constraints to the crystallographic refinement of a macromolecule at low resolution using the constrained-restrained parameter structure-factor least-squares refinement program CORELS [Sussman, Holbrook, Church & Kim (1977). Acta Cryst. A33, 800-804]. The parameter shifts calculated by CORELS are adjusted to ensure that the refined structure possesses exactly the desired molecular symmetry. This procedure has been used to refine the structure of apo-D-glyceraldehyde 3-phosphate dehyodrogenase from Bacillus stearothermophilus at 4 Å resolution, imposing 222 molecular symmetry. The starting model was the partially refined structure of the holoenzyme at 2.7 A resolution. The possible advantages of using CORELS refinement when dealing with more general problems involving two distinct molecular conformations which are related by simple rigid-body domain movements are also considered.