PROTEIN PROTEIN INTERACTION PPI docking

PPI docking predicts the 3D structure of a complex formed between two proteins — how they recognize each other, where they bind, and with what geometry. Unlike small molecule docking where a ligand explores a defined pocket, PPI docking involves two large, flexible macromolecules with interfaces spanning 1500–3000 Å² or more.

Why PPI docking is fundamentally harder

No pre-defined binding pocket. Small molecule docking targets a geometric cavity. PPI interfaces are flat or gently curved surface patches with no obvious binding funnel. Identifying the correct interface requires hotspot prediction tools like FTMap or KFC2 before docking even begins.

Search space is vastly larger. Small molecule docking searches 6 degrees of freedom plus ligand torsions. PPI docking searches the full rotational and translational space of a second macromolecule — the sampling problem is exponentially more complex.

Interface area and chemistry. Small molecule binding sites average 300–1000 Å². PPI interfaces average 1500–3000 Å² and can exceed 10,000 Å² in obligate complexes. Interactions are dominated by hydrophobic burial and distributed hydrogen bonds — not a concentrated pharmacophore geometry.

Scoring functions are not transferable. Functions validated for small molecule docking cannot be applied to PPI interfaces. The desolvation of large hydrophobic patches, entropic costs of restricting backbone motion, and long-range electrostatic steering all contribute differently to PPI binding.

Tools built specifically for PPI docking

HADDOCK — data-driven docking using experimental restraints from NMR, mutagenesis, or HDX; gold standard for protein-protein and antibody-antigen complexes

ClusPro — FFT-based rigid body docking; consistently top-ranked in CAPRI benchmarks

AlphaFold-Multimer / AlphaFold 3 — complex structure prediction; increasingly used as a starting conformation before physics-based refinement

Why it matters for drug discovery

An estimated 650,000 protein-protein interactions exist in the human interactome. Many of the most critical biological processes, oncogenesis, viral entry, immune checkpoint signaling are driven by PPI interfaces, not enzyme active sites. Targeting these interfaces with small molecule PPI inhibitors is one of the most active frontiers in drug discovery.

Computational PPI docking is the entry point.

Small molecule docking finds a key for a lock. PPI docking maps how two locks recognize each other and how to disrupt that recognition.

Are you working on any PPI targets in your current research?

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