How to Use Coot for Real-Space Refinement and Manipulation Coot (Crystallographic Object-Oriented Toolkit) is the industry-standard software for macromolecular model building, manipulation, and validation, widely used in both X-ray crystallography and cryo-EM. It allows for interactive, real-time adjustments of atomic models to fit electron density or electrostatic potential maps, bridging the gap between automated refinement and manual interpretation.
This article guides you through the essentials of using Coot for real-space refinement and interactive model manipulation. 1. Getting Started: Loading Data
Before manipulating your model, you must load your coordinate file (PDB/mmCIF) and your map file (MTZ or CCP4/MRC map). Load Coordinates: Go to File -> Open Coordinates. Load Maps: Go to File -> Auto Open CCP4 Map or Open Map. Navigation: Rotate: Left mouse click and drag. Zoom: Right mouse click and drag. Center on Atom: Middle mouse click on an atom. Change Contour Level: Scroll with the middle mouse wheel. 2. Real-Space Refinement: The Core Tool
Real-space refinement in Coot works by adjusting model parameters (atomic positions) to maximize the fit to the map while minimizing geometric distortions (bonds, angles, and torsions). How to perform Real-Space Refinement:
Select Action: Click the “Real Space Refinement” icon (or press R on the keyboard).
Select Residue/Zone: Click on a residue in the graphics window.
Refinement: Coot will automatically refine the clicked residue and its neighbors to fit the density.
Accept/Reject: If the result looks better, click Accept. If it introduces errors, click Reject. Advanced Refinement Settings:
Refinement Weight: You can adjust the weight between X-ray terms and geometry terms, allowing you to prioritize fitting density over perfect geometry.
Local Distance Restraints: Particularly useful for low-resolution maps (e.g., in cryo-EM), this uses local distance networks to maintain overall shape. 3. Interactive Manipulation and Editing
Beyond automatic refinement, Coot offers tools for manually fixing errors, such as misaligned side chains or incorrect backbone orientations.
Rotamer Fitting (Auto Fit Rotamer): Select a residue and click Auto Fit Rotamer to select the most probable sidechain orientation that fits the density.
Manual Sidechain Manipulation: Use Edit -> Chi Angles to manually rotate sidechain bonds.
Backbone Manipulation (Flip Peptide): If a peptide bond is oriented incorrectly, use Flip Peptide (or p on the keyboard) to flip the carbonyl and amide groups.
Manual Manipulation (Rotate/Translate Zone): Select Extensions -> Modelling -> Rotate/Translate Zone to move a large part of the model simultaneously. 4. Validation and Analysis
To ensure your refinement is valid, Coot provides immediate feedback.
Map Difference: Use the FOFC difference map (typically red/green) to identify where atoms are missing (positive density) or where atoms are incorrectly placed (negative density).
Ramachandran Plot: Use Validate -> Ramachandran Plot to detect backbone conformational outliers.
Density Fit Analysis: Use Validate -> Density Fit Analysis to identify regions with poor fitting. 5. Finalizing Changes
Once you have manipulated and refined your structure, save your results.
Go to File -> Save Coordinates to output the updated PDB/mmCIF file. Summary Table of Common Tools Tool / Key Refinement R key Real-space refine selected residue Fit Sidechain Auto Fit Rotamer Fit sidechain to density Move Part Rotate/Translate Move fragments manually Fix Peptide p key Flip peptide plane Delete Delete key Remove residues/atoms
By mastering these tools, you can significantly reduce the time required to build an accurate model into your density map.
If you tell me which, I can help you with specific tasks like: Building ligands from SMILES strings.
Applying Non-Crystallographic Symmetry (NCS) to speed up refinement. Using jiggle-fit for high-uncertainty regions.