1H NMR (300 MHz, D2O): 5.19 (d, JH-F = 44.7 Hz, 2H). outcomes, combined with total outcomes of solvent isotope impact and proton inventory research, strongly claim that the inactivation of PAD4 by F- and Cl-amidine proceeds with a multi-step system which involves the protonation and stabilization from the tetrahedral intermediate produced upon nucleophilic strike with the energetic site cysteine, i.e. Cys645. Stabilization of the intermediate would help get the halide-displacement response, which leads to the forming of a three-membered sulfonium band that eventually collapses to create the inactivated enzyme. This selecting – that protonation from the tetrahedral intermediate is normally very important to enzyme inactivation – could also claim that during catalysis, protonation of the analogous intermediate is required for efficient substrate turnover. Keywords: Deiminase, Citrulline, Cl-amidine, rheumatoid arthritis, inactivator In nature, a myriad of posttranslational modifications are found in proteins. These modifications, and the requisite PLCB4 modifying enzymes, can have far-reaching effects on living systems. Within the family of protein modifying enzymes are the Protein Arginine Deiminases (PADs). These enzymes catalyze the hydrolysis of arginine residues to form citrulline.[1C3] Much effort from our lab has been focused on gaining insight into the mechanism of the PADs, and, in particular, PAD4. [3C5] Our interest in PAD4, and the PADs in general, was piqued as evidence CEP dipeptide 1 linking dysregulated PAD activity to the increased incidence and severity of Rheumatoid Arthritis (RA) emerged.[2, 3, 6] This disease, which afflicts nearly 1% of the population, is an autoimmune disorder that appears to be triggered, at least in part, in response to aberrant citrullination, a result of dysregulated PAD activity. Based on this apparent CEP dipeptide 1 causal relationship, we set out to develop inhibitors/inactivators CEP dipeptide 1 that could be used to modulate PAD activity and disease progression. This effort led to the discovery of two mechanism-based inactivators, denoted F- and Cl-amidine (Physique 1), which are the most potent PAD inhibitors described to date.[7, 8] At the same time, Fast and colleagues reported that 2-chloroacetamidine, i.e. the warhead in Cl-amidine, also inactivates PAD4, and other members of the guanidinium modifying family of enzymes.[9] Open in a separate window Determine 1 Structures of compounds described in this communication and proposed mechanisms of PAD4 inactivation by Cl- or F-amidine. Much work has been done to characterize the mechanism of inactivation, including dialysis experiments verifying irreversible inhibition. Additionally, analysis of the kinetics of inactivation exhibited that F- and Cl-amidine possess kinact/KI values of 3,000 M?1min?1 and 13,000 M?1min?1, respectively.[7, 8] Subsequent crystallographic data confirmed that inactivation was due to the covalent modification of an active site cysteine (Cys645) that is critical for catalysis C this residue promotes arginine deimination by covalent catalysis using a mechanism that is analogous to the Cys proteases.[5] While this obtaining conclusively exhibited the mode of inactivation C alkylation of the thiolate C the precise mechanism of inactivation has yet to be established. While unclear, the mechanism of inactivation presumably proceeds through one of at least two routes: direct displacement of the halogen via an SN2 mechanism or initial attack around the imminium carbon, followed by displacement of the halide to form a sulfonium ring, and ending with concomitant reformation of the imine and opening of the sulfonium ring (Physique 1). While the former possibility is the most intuitive, the latter is usually analogous to the mechanism by which the fluoromethylketones inactivate the cysteine proteases[10C12] and therefore warranted further investigation. We began our investigations by examining the influence of pH on kincat/KI, i.e. the second order rate constant of enzyme inactivation. These studies were pursued because pH rate profiles CEP dipeptide 1 can suggest the identity of catalytically important functional groups, in the inactivator or free enzyme, up to and including the first irreversible step of the reaction. For these studies, kincat/KI values were decided for both Cl- and F-amidine over a pH.