Hierarchical analysis of promolecular full electron-density distributions: Description of protein structure fragments

A theoretical method is applied to describe protein structures in terms of hierarchically related substructures. The approach is based on the location of local maxima (peaks) in promolecular electron-density (ED) distributions established at various smoothing levels. Promolecular ED distributions are generated using either the Promolecular Atom Shell Approximation (PASA) representation or the ED are calculated using the Cromer-Mann coefficients. The analysis of the decomposition patterns of a protein structure in its native and hypothetical extended conformations showed that the amino-acid residues have a similar decomposition pattern regardless of their position in the protein sequence, the protein conformation and the influence of the crystal packing. A link is proposed with model-building tools in protein structure determination from diffraction data.