Why are protein-protein interactions traditionally considered 'undruggable' compared to enzyme active sites?
AProteins never interact with each other in cells
BPPI interfaces are typically large (1,500+ A^2), relatively flat, and lack the deep, well-defined pockets that small molecules can occupy — conventional drugs are too small to cover enough of the interface to compete with the natural protein partner
CPPI interfaces are always identical to enzyme active sites
DSmall molecules cannot exist inside cells
Enzyme active sites evolved to bind small substrates and typically feature deep, enclosed pockets with specific chemical environments — ideal for small-molecule drugs. PPI interfaces evolved to bind large protein surfaces and are typically broad, flat, and shallow — there is no obvious pocket for a small molecule to occupy. However, the hotspot concept has changed this view: because binding energy is concentrated at a few key residues, a small molecule that targets the hotspot region can potentially disrupt the interaction. Successful PPI inhibitors (venetoclax for Bcl-2/BH3, nutlins for MDM2/p53) bind at hotspot-containing sub-pockets on the interface.
Question 2 True / False
At a protein-protein interface, every residue in contact contributes equally to the binding affinity.
TTrue
FFalse
Answer: False
Alanine scanning mutagenesis studies (systematically mutating each interfacial residue to alanine and measuring the effect on binding affinity) revealed that binding energy is highly non-uniform. A small number of 'hotspot' residues — typically 3-10% of the interface — contribute most of the binding energy (each contributing >2 kcal/mol when mutated). The remaining interfacial residues contribute little individually. Hotspot residues tend to be at the center of the interface (surrounded by a 'O-ring' of residues that exclude water), are often aromatic (Trp, Tyr, Phe) or charged (Arg), and are more conserved across species than non-hotspot interfacial residues.
Question 3 Short Answer
How has the hotspot concept changed the approach to designing small-molecule PPI inhibitors?
Think about your answer, then reveal below.
Model answer: The hotspot concept showed that disrupting a PPI does not require covering the entire interface — targeting the hotspot region with a small molecule can be sufficient. Drug design efforts focus on identifying the hotspot residues (by alanine scanning or computational prediction), finding or designing small-molecule fragments that mimic the key hotspot interactions, and growing these fragments into drug-like molecules that occupy the hotspot region with high affinity. The hotspot often creates a small sub-pocket on the protein surface where the partner's hotspot residue (like a tryptophan or leucine) inserts — this pocket can serve as a conventional drug binding site. Fragment-based screening and structure-guided optimization from hotspot-binding fragments have produced several clinically approved PPI inhibitors.
The MDM2/p53 interaction illustrates this: p53 binds MDM2 through three hotspot residues (Phe19, Trp23, Leu26) that insert into a hydrophobic pocket on MDM2. Nutlin compounds mimic these three residues and fit into the same pocket, blocking the interaction with nanomolar affinity — a triumph of hotspot-targeted PPI drug design.