Chemical Biology 1 – Pharmacology 10-17-14
Dec 16, 2015
Methods for studying protein function – Loss of Function
• 1. Gene knockouts
• 2. Conditional knockouts
• 3. RNAi
• 4. Pharmacology (use of small molecules to turn off protein function)
pre-translational
Pharmacology
• Disadvantage– Unlike genetic methods it is difficult to identify
ligands that are highly selective for a target.
• Advantages1. Fast time scale
2. Only perturbs targeted sub-domains
3. graded dose response - tunability
4. Most drugs are small molecules
Weiss WA, Taylor SS, Shokat KM. “Recognizing and exploiting differences between RNAi and small-molecule inhibitors.” Nat Chem Biol. 2007 Dec;3(12):739-44.
Small molecules are subdomain specific
Example: PAK1 Kinase
Small molecules affect only one domain, while pre-translational methodsremove the entire protein from the cell.
Reverse Chemical Genetics (Pharmacology)
1. Identify a protein target of interest– Develop an activity assay (enzymes) or a binding
assay (protein-ligand interactions) to screen compounds
3. Optimize your initial lead compound by making analogs (SAR) and by using any additional biochemical/structural information. In parallel, screen optimized analogs against other targets (selectivity)
2. Test biased or unbiased panels of compounds against protein target of interest
Major challenges
• Druggability– Many proteins do not appear to make
favorable interactions with drug-like small molecules
Molecular Weight <900 Da
Kd < 1 M (∆G < -8.4 kcal/mol)
No more than one or two fixed charges
– Estimated that only ~10% of all proteins are druggable
Hopkins and Groom, Nat Reviews Drug Disc, 2002
Major challenges
• Selectivity– Finding selective agonists and antagonists is
very challenging– Knowing which other proteins to
counterscreen is difficult (easier for mechanism-based or enzyme family-directed ligands)
In some cases, chemistry and genetics can be used to circumvent these problems.
Knight ZA, Shokat KM. “Chemical genetics: where genetics and pharmacology meet. Cell. 2007 Feb 9;128(3):425-30.”
Koh JT. “Engineering selectivity and discrimination into ligand-receptor interfaces.”Chem Biol. 2002 Jan;9(1):17-23. Review.
Identification of small molecule inhibitors
2 classes– 1. Enzyme Inhibitors
• Many effective strategies for identifying enzyme inhibitors.
– 2. Protein-Protein Interaction Inhibitors• Difficult to identify potent inhibitors of protein-
protein interactions.
Methods for discovering enzyme inhibitors
• High throughput screening (parallel synthesis and combinatorial chemistry)
• Mechanism-based (incorporate a functionality that is unique for an enzyme enzyme class (For example, proteases)
• Privileged scaffolds (kinases, phosphodiesterases)
• Transition state analogs
Turk B.Targeting proteases: successes, failures and future prospects.Nat Rev Drug Discov. 2006 Sep;5(9):785-99.
HIV Protease InhibitorsINHIBITORS OF HIV-1 PROTEASE: A “Major Success of Structure-Assisted Drug Design” Alexander Wlodawer, Jiri Vondrasek. Annual Review of Biophysics and Biomolecular Structure. Volume 27, Page 249-284, 1998
More protease inhibitors
• Ketones (serine and cysteine proteases)
• Phosphonic and hydroxamic acids (metalloproteases)
transition state analog
hydroxamic acidschelatethe active sitezinc
Synthesis of kinase inhibitors
OlomucineCdc2/CyB: 1µMCdk2/CyA: 1µM
LibrarySynthesis
10,000
LibraryScreening
10,000
HitCdc2/CyB: 340 pMCdk2/CyA: 340 pM
Gray et. al. Science (1998) 281, 533-538.
Approved kinase inhibitors
28 small molecule kinase inhibitors are now in the clinicGleevec (Imatinib) was the first clinically approved kinase inhibitor (2003)
Protein-Protein Interaction (PPI) Inhibitors
• Identification of potent PPI inhibitors is very challenging. In general, standard screening strategies don’t work.
• Conversion of Peptides/Proteins to Small Molecules
• Innovative new strategies are needed– for example, SAR by NMR
“SAR” by NMR Abbott Laboratories (Stephen Fesik)
Fragment based approach- library of small compounds (several thousands)- build up larger ligands- n fragments may yield n2 compounds
NMR: 15N-HSQC of target protein (2D NMR)
Requirements3D structure of target protein (NMR or other)large quantities of 15N-labeled protein (> 100 mg)NMR assignments of backbone N and HN atomssize of protein <40 kDasolubility: protein and ligands
Principlestart with known protein structure and 15N assignments15N-HSQC of protein15N-HSQC of protein plus ligand: identify shifted peaksmap these on protein surface: binding site
Shuker, S. B.; Hajduk, P. J.; Meadows, R. P.;Fesik, S. W. Science 1996, 274, 1531.
Conversion of Peptides/Proteins to Small Molecules
“SAR” by NMR
1. Screen 100-5000 low molecular weight (150 -300 MW) ligands to identify weak binders. HSQC perturbations identigy the site of binding
2. Screen for a second site of binding in the presence of the first ligand
3. Use structural information to design a linkage between the two identified ligands
∆G(linked ligand) = ∆G(fragment 1) + ∆G(fragment 2) + ∆G(linker) + ∆G(cooperativity)
∆G(linker) usually positive (entropic cost)
∆G(cooperativity) is a non-additive effect