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Updated: Aug 7, 2026

Modeling Ligands into Maps Derived from Electron Cryomicroscopy
Published on: July 19, 2024
Automated ligand fitting by core-fragment fitting and extension into density
Thomas C Terwilliger1, Herbert Klei, Paul D Adams
1Los Alamos National Laboratory, Mailstop M888, Los Alamos, NM 87545, USA. terwilliger@lanl.gov
A new method efficiently fits ligands into electron-density maps by first placing a core fragment and then extending the rest. This procedure successfully positioned over half of tested ligands within 2 Å of their original coordinates.
Area of Science:
- Structural Biology
- Computational Chemistry
- Biochemistry
Background:
- Accurate ligand fitting into electron-density maps is crucial for understanding molecular interactions.
- Existing methods may face challenges with complex ligands or varying crystallographic resolutions.
Purpose of the Study:
- To present a novel, two-step procedure for fitting ligands into electron-density maps.
- To evaluate the performance and robustness of this fitting approach.
Main Methods:
- A sequential fitting strategy: first, a core ligand fragment is fitted, followed by extending the remainder.
- Validation using 9327 ligands from the Protein Data Bank (PDB) across diverse resolutions (0.8-4.8 Å).
- Difference electron-density maps ((Fo - Fc)exp(i phi(c))) were utilized for fitting.
Main Results:
- The procedure successfully placed 58% of ligands within 2 Å root-mean-square deviation (r.m.s.d.) of their reference PDB coordinates.
- Fitting success showed moderate sensitivity to crystallographic resolution.
- Ligand size (10-100 non-hydrogen atoms) had a limited impact on the procedure's effectiveness.
Conclusions:
- The presented ligand fitting procedure offers a reliable method for analyzing structural data.
- The approach demonstrates good performance across various ligand sizes and resolutions, aiding structural biology research.
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