CRITICAL POINT ANALYSIS IN PROTEIN DENSITY MAP INTERPRETATION. Suzanne Fortier^*&, Antony Chiverton* and Janice Glasgow^&. ^*Depts. of Chemistry and ^&Computing and Information Science, Queen's University, Kingston, Canada, K7L 3N6, and Laurence Leherte, Facultés Universitaires Notre-Dame de la Paix, Namur, Belgium

Critical point analysis has been investigated as a method to assist in the interpretation of protein electron density maps, as part of a project in molecular scene analysis. In particular, the usefulness of the topological approach for the segmentation of medium resolution (3Å) maps of proteins and their interpretation in terms of structural motifs has been assessed. Using the program ORCRIT (1), we have analysed the topology of calculated and experimental electron density maps through the location, identification and linkage of their critical points. The study has shown that critical point mapping can serve as a segmentation and pattern recognition tool in medium resolution maps of protein and that, in particular, it can be used for tracing the main chain of the protein and identifying secondary structure motifs. Furthermore, it was observed that at 3Å resolution, each residue in the polypeptide chains is, in general, associated with only one peak in the main branch of the critical point networks. Thus the approach allows for the parsing of the electron density maps into super atoms corresponding to the residues. The critical point method is now being combined with the treading approach, commonly used in protein structure prediction work, to help align sequence onto structure.

1. Johnson, C.K. (1976). ORCRIT. The Oak Ridge Critical Point Network Program. Chemistry Division, Oak Ridge National Laboratory, USA.