Protein–Protein Interaction Binder Design Using PPIFLOW

1. Overview PPIFLOW is a computational pipeline for designing de novo protein binders targeting specific protein–protein interaction (PPI) interfaces. It focuses on generating binders that are structurally compatible, interaction-optimized, and experimentally viable.

2. Pipeline Workflow The workflow consists of four main stages: Backbone generation Sequence design Filtering (sequence + structure) Validation and ranking

This staged process progressively refines candidates from large-scale generation to a small set of high-confidence binders.

1. Backbone Generation PPIFLOW generates interface-aware backbone structures conditioned on the target binding site. This ensures that generated binders are geometrically aligned with the interaction region and capable of forming stable contacts.

2. Sequence Design Backbones are converted into amino acid sequences using inverse folding models. Multiple sequences are sampled per backbone, and candidates with better structural compatibility and energetic favorability are selected.

3. Sequence-Level Filtering Sequences are filtered based on: Length and validity constraints Physicochemical properties (hydrophobicity, charge balance) Stability metrics (instability index, aromaticity) Developability risks (aggregation, PTMs, cysteine issues)

Only sequences passing strict thresholds move forward.

1. Novelty and Diversity Candidates are screened against protein databases to ensure novelty. Clustering methods are used to maintain diversity, avoiding redundant designs and improving coverage of sequence space.

2. Structure-Based Validation Predicted binder–target complexes are evaluated using: Binding confidence metrics (ipTM, pDockQ) Interface accuracy (ipSAE) Predicted binding affinity (ΔG) Structural quality (pTM, PAE, buried surface area)

Weak or inconsistent binders are removed.

1. Ranking and Selection Final candidates are ranked using a composite scoring strategy, prioritizing interface quality, binding strength, and structural stability. Top-performing binders are selected for downstream use.

Conclusion PPIFLOW is a structured and scalable framework for designing novel, diverse, and high-affinity protein binders, with strong emphasis on interaction quality and real-world feasibility.