Homology modeling by the ICM method

Proteins. 1995 Nov;23(3):403-14. doi: 10.1002/prot.340230314.


Five models have been built by the ICM method for the Comparative Modeling section of the Meeting on the Critical Assessment of Techniques for Protein Structure Prediction. The targets have homologous proteins with known three-dimensional structure with sequence identity ranging from 25 to 77%. After alignment of the target sequence with the related three-dimensional structure, the modeling procedure consists of two subproblems: side-chain prediction and loop prediction. The ICM method approaches these problems with the following steps: (1) a starting model is created based on the homologous structure with the conserved portion fixed and the nonconserved portion having standard covalent geometry and free torsion angles; (2) the Biased Probability Monte Carlo (BPMC) procedure is applied to search the subspaces of either all the nonconservative side-chain torsion angles or torsion angles in a loop backbone and surrounding side chains. A special algorithm was designed to generate low-energy loop deformations. The BPMC procedure globally optimizes the energy function consisting of ECEPP/3 and solvation energy terms. Comparison of the predictions with the NMR or crystallographic solutions reveals a high proportion of correctly predicted side chains. The loops were not correctly predicted because imprinted distortions of the backbone increased the energy of the near-native conformation and thus made the solution unrecognizable. Interestingly, the energy terms were found to be reliable and the sampling of conformational space sufficient. The implications of this finding for the strategies of future comparative modeling are discussed.

MeSH terms

  • Antibodies, Anti-Idiotypic / chemistry
  • Bacterial Proteins*
  • Computer Graphics
  • Computer Simulation
  • Cytochrome P-450 Enzyme System / chemistry
  • Databases, Factual
  • Helix-Turn-Helix Motifs
  • Mixed Function Oxygenases / chemistry
  • Models, Molecular*
  • Monte Carlo Method
  • Muramidase / chemistry
  • Nucleoside-Diphosphate Kinase / chemistry
  • Phosphoenolpyruvate Sugar Phosphotransferase System / chemistry
  • Protein Conformation*
  • Protein Structure, Secondary
  • Protein Structure, Tertiary
  • Proteins / chemistry*
  • Receptors, Retinoic Acid / chemistry
  • Sequence Alignment
  • Sequence Homology, Amino Acid


  • Antibodies, Anti-Idiotypic
  • Bacterial Proteins
  • Proteins
  • Receptors, Retinoic Acid
  • Cytochrome P-450 Enzyme System
  • Mixed Function Oxygenases
  • eryF protein, Saccharopolyspora erythraea
  • Phosphoenolpyruvate Sugar Phosphotransferase System
  • phosphocarrier protein HPr
  • Nucleoside-Diphosphate Kinase
  • Muramidase