PROTAC FAQ

PROTAC FAQ

These answers cover PROTAC design concepts and tradeoffs — how PROTACs differ from molecular glues, choosing between CRBN and VHL, linker length, Lipinski violations, cooperativity, DC50 and Dmax prediction accuracy, and running retrosynthesis on a finished degrader.

What is the difference between a PROTAC and a molecular glue?

PROTACs are bifunctional molecules with two distinct binding moieties (warhead + E3 ligand) connected by a linker. They are typically larger (700-1200 Da) and can be rationally designed. Molecular glues are smaller monovalent molecules that stabilize a protein-protein interaction between the target and E3 ligase without a linker. Molecular glues are harder to design computationally because they rely on induced protein-protein interactions. Use the PROTAC tools for bifunctional degrader design; use generative chemistry tools (genmol, reinvent4) for molecular glue exploration.

Which E3 ligase system should I choose -- CRBN or VHL?

CRBN (cereblon) is the most widely used system with the broadest validation data. It has broad tissue expression and many published ligands (pomalidomide, lenalidomide, thalidomide derivatives). VHL (von Hippel-Lindau) offers higher selectivity and avoids the risk of neosubstrate degradation that can occur with IMiD-based CRBN ligands. Choose CRBN for initial exploration and broad applicability. Choose VHL when selectivity is critical or when targeting tissues with low CRBN expression.

How long should the linker be?

Optimal linker length depends on the distance between the warhead exit vector and the E3 ligand exit vector in the ternary complex. Most successful PROTACs use linkers of 6-10 atoms. Start with min_linker_length: 4 and max_linker_length: 12 to explore the full range, then narrow based on ternary complex modeling results. Very short linkers (< 4 atoms) often cause steric clashes. Very long linkers (> 14 atoms) tend to have poor cell permeability and drug-likeness.

My PROTACs violate Lipinski's Rule of Five. Is that a problem?

PROTACs are inherently beyond Lipinski space due to their bifunctional nature. Molecular weights of 700-1200 Da, cLogP values up to 10, and multiple hydrogen bond donors/acceptors are normal. However, PROTACs can still achieve oral bioavailability through macrocyclic conformations and intramolecular hydrogen bonding that reduce their effective polar surface area. The degradation scoring tool accounts for PROTAC-specific permeability in its assessment.

Can I use a custom E3 ligase instead of CRBN or VHL?

The current toolkit supports CRBN and VHL as these are the two most validated E3 ligase systems with the most structural data. Support for additional E3 ligases (IAP/cIAP1, DCAF15, DCAF16, RNF114) is planned for future releases. For custom E3 systems, you can upload your own E3 ligase structure for ternary modeling, but degradation scoring accuracy will be reduced without system-specific training data.

What does cooperativity (alpha) mean for my PROTAC?

The cooperativity factor alpha describes whether the PROTAC-mediated ternary complex is more stable (alpha > 1, positive cooperativity) or less stable (alpha < 1, negative cooperativity) than predicted from the individual binary binding affinities alone. Positive cooperativity means the POI-E3 interface is favorable and stabilizes the complex. An alpha > 5 is considered strong positive cooperativity. PROTACs with higher cooperativity tend to have better DC50 values and lower hook effect risk.

How accurate are the DC50 and Dmax predictions?

The degradation scoring model was trained on published PROTAC data and achieves reasonable correlation with experimental values (R^2 ~ 0.5-0.7 for DC50, R^2 ~ 0.6-0.8 for Dmax on held-out test sets). Predictions are most accurate for CRBN and VHL-based PROTACs targeting well-characterized proteins. Use the predictions for ranking and prioritization rather than absolute potency estimation. The model is less accurate for novel E3 systems, very large PROTACs (> 1200 Da), or targets with unusual degradation kinetics.

Can I run retrosynthesis on a PROTAC?

Yes. After identifying a promising PROTAC from the design pipeline, pass the full PROTAC SMILES to the retrosynthesis tool. The retrosynthesis engine will propose disconnection strategies, often splitting the PROTAC at the linker-warhead and linker-E3 ligand junctions. This aligns with common PROTAC synthesis strategies where the warhead, linker, and E3 ligand are prepared separately and coupled in late-stage steps.

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