title: Optimum superimposition of protein structures: ambiguities and implications.
abstract: BACKGROUND
  Techniques for comparison and optimum superimposition of protein structures are indispensable tools, providing the basis for statistical analysis, modeling, prediction and classification of protein folds. Observed similarity of structures is frequently interpreted as an indication of evolutionary relatedness. A variety of advanced techniques are available, but so far the important issue of uniqueness of structural superimposition has been largely neglected. We set out to investigate this issue by implementing an efficient algorithm for structure superimposition enabling routine searches for alternative alignments.
  
  
  RESULTS
  The algorithm is based on optimum superimposition of structures and dynamic programming. The implementation is tested and validated using published results. In particular, an automatic classification of all protein folds in a recent release of the protein data bank is performed. The results obtained are closely related to published data. Surprisingly, for many protein pairs alternative alignments are obtained. These alignments are indistinguishable in terms of number of equivalent residues and root mean square error of superimposition, but the respective sets of equivalent residue pairs are completely distinct. Alternative alignments are observed for all protein architectures, including mixed alpha/beta folds.
  
  
  CONCLUSIONS
  Superimposition of protein folds is frequently ambiguous. This has several implications on the interpretation of structural similarity with respect to evolutionary relatedness and it restricts the range of applicability of superimposed structures in statistical analysis. In particular, studies based on the implicit assumption that optimum superimposition of structures is unique are bound to be misleading.
authors.affiliation: Center for Applied Molecular Engineering, University of Salzburg, Australia.
authors.forename: Z K
authors.initials: ZK
authors.lastname: Feng
authors.forename: M J
authors.initials: MJ
authors.lastname: Sippl
mesh_terms: D000465:Algorithms; D000595:Amino Acid Sequence; D005069:Evaluation Studies as Topic; D019143:Evolution, Molecular; D008958:Models, Molecular; D008969:Molecular Sequence Data; D015394:Molecular Structure; D017510:Protein Folding; D017433:Protein Structure, Secondary; D011506:Proteins / Q000737:chemistry* / Q000235:genetics; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid
chemical_list: D011506:Proteins