Antibody–antigen docking

What is antibody–antigen docking?

Antibody–antigen docking is a protein complex modeling method for placing an antibody variable region against an antigen and generating candidate epitope and paratope contacts. The calculation searches relative orientation and ranks structural interfaces, with optional residue evidence to restrict the search.

The antibody's complementarity-determining regions usually define the likely paratope, but the antigen epitope may be unknown. CDR H3 and antigen loops can also be uncertain even when the antibody framework is well modeled. A rigid orientation search cannot repair a badly modeled loop.

The result is an interface hypothesis. It does not establish that the antibody binds, competes with another antibody or neutralizes the antigen. Glycans, membrane context and antigen oligomerization can block a proposed epitope that looks open in an isolated PDB.

When to use antibody–antigen docking

Use docking when structural information can narrow an experimental question:

• An antibody and antigen have structures but no complex structure.
• Epitope mapping or escape mutations suggest interface residues.
• You want alternative orientations to guide contact tests.
• A candidate epitope needs a glycan or accessibility check.

Strengths and limitations

Antibody-aware preparation and residue restraints can focus the search on experimentally plausible contacts. Multiple interface models help choose mutations that distinguish competing epitope hypotheses.

Uncertain CDR loops, antigen conformational change and missing glycans can dominate the result. An unrestrained global search produces many geometrically plausible false interfaces, while a restrained search can only be as sound as its input evidence.

How antibody–antigen docking works

The featured workflow compares two macromolecular docking methods on the same antibody and antigen PDB files.

Tool	Search	Best use
LightDock	Swarm search across an antigen surface, optionally restrained	Explore alternative orientations, or add supported paratope or epitope residues as restraints
DFMDock	Learned rigid-body sampling from the two structures	Compare an unrestrained orientation from a different search strategy

Preparing an antibody interface

Check heavy and light chain identity and inspect the CDR loops before docking. ImmuneBuilder can supply a variable-region model when no experimental structure exists, but that model's uncertain loops remain uncertain in the complex. Paratope predictions can guide search; they are not experimental epitope evidence.

LightDock can accept residue restraints while retaining a broader surface search. DFMDock treats both partners as rigid, so it is most informative when the CDR loops are already close to their bound shape. Report which contacts were imposed and which emerged from the calculation.

Applications of antibody–antigen docking

Docking can suggest residues to mutate on an antigen when binding has already been observed. Distinct predicted epitopes may motivate competition experiments or targeted structural studies. Compare candidate contacts with escape mutations, peptide mapping and known antigen accessibility.

For therapeutic antibodies, the proposed complex can identify a potential steric conflict with a glycan or neighboring domain. The absence of a conflict in an isolated model is weak evidence: expression context and antigen state still matter. Docking alone says nothing about neutralization or developability.

How to do antibody–antigen docking online

1. Prepare the antibody. Provide a variable-region PDB, verify chain assignments and inspect CDR loop geometry.
2. Prepare the antigen. Use the relevant biological assembly and retain glycans or neighboring chains when they affect epitope accessibility.
3. Record interface evidence. Identify measured epitope residues or credible paratope contacts and distinguish them from predictions.
4. Run the docking workflow. Give the antibody and antigen PDB files to LightDock and DFMDock, applying justified restraints in LightDock where supported.
5. Compare the interfaces. Group predicted orientations and check whether CDRs contact accessible antigen surface without severe clashes.
6. Assess geometry and references. Use MolProbity for structural outliers and DockQ only when a native complex is available.

How to interpret antibody–antigen docking results

The ranked complexes represent each tool's search and scoring assumptions. A model supported by imposed restraints is not an independent confirmation of those restraints. Review CDR geometry, antigen accessibility and agreement with evidence that was not used to guide docking.

DockQ can quantify similarity to a known native complex; it cannot certify a new epitope without one. Prioritize models for binding, competition or structural experiments. A pose does not confirm recognition, neutralization, specificity, safety or function.

How the antibody–antigen docking workflow works

Compare LightDock and DFMDock orientations from the same antibody and antigen structures without requiring a native reference.

1. Model and inspect the antibody. Upload a variable-region structure or build one from correctly paired heavy- and light-chain sequences. Inspect CDR loops and engineered residues before docking.
2. Prepare the antigen. Choose the relevant antigen construct, chains, oligomeric state, and glycans. Repair missing atoms and note unresolved regions near candidate epitopes.
3. Map paratope or epitope evidence. Use ParaSurf predictions or experimental mapping to define possible interface residues, and keep the confidence and source of each restraint explicit.
4. Dock complementary searches. Compare LightDock and DFMDock from the same structures. Run restrained and global LightDock searches as separate analyses when the evidence is uncertain.
5. Review interfaces. Inspect CDR participation, antigen contacts, clashes, buried polar groups, glycan conflicts, and agreement with escape or competition data before exporting complexes.

Inputs and outputs

Check formats before running, then inspect and download the result from every workflow step.

Inputs

Structural inputs

PDBSDFSMILES

Antibody variable-region and antigen PDB structures, optionally with epitope or paratope residue evidence.

Method context

Binding-site evidence, restraints, receptor-state provenance, known ligands, or reference complexes when available.

Outputs

Docked structures

PDBPDBQTSDF

Candidate complexes, interface contacts, method scores, and reference-based DockQ results when a native structure is available.

Review evidence

Method-native rankings, confidence, logs, interaction context, failures, and files for reproducible follow-up.