How Reactive are Druggable Cysteines in Protein Kinases?

Targeted covalent inhibitors (TCIs) have been successfully developed as high-affinity and selective inhibitors of enzymes of the protein kinase family. These drugs typically act by undergoing an electrophilic addition with an active-site cysteine residue, so design of a TCI begins with the identification of a "druggable" cysteine. These electrophilic additions generally require deprotonation of the thiol to form a reactive anionic thiolate, so the acidity of the residue is a critical factor. Few experimental measurements of the p K_a's of druggable cysteines have been reported, so computational prediction could prove to be very important in selecting reactive cysteine targets. Here we report the computed p K_a's of druggable cysteines in selected protein kinases that are of clinical relevance for targeted therapies. The p K_a's of the cysteines were calculated using advanced computational methods based on all-atom replica-exchange thermodynamic integration molecular dynamics simulations in explicit solvent. We found that the acidities of druggable cysteines within protein kinases are diverse and elevated, indicating enormous differences in their reactivity. Constant-pH molecular dynamics simulations were also performed on selected protein kinases, and the results confirmed this varied range in the acidities of druggable cysteines. Many of these active-site cysteines have low exposure to solvent molecules, elevating their p K_a values. Electrostatic interactions with nearby anionic residues also elevate the p K_a's of cysteine residues in the active site. The results suggest that some cysteine residues within kinase binding sites will be slow to react with a TCI because of their low acidity. Several oncogenic kinase mutations were also modeled and found to have p K_a's similar to that of the wild-type kinase.