Influence of sulfur oxidation state and steric bulk upon trifluoromethyl ketone (TFK) binding kinetics to carboxylesterases and fatty acid amide hydrolase (FAAH)

Abstract

Carboxylesterases metabolize numerous exogenous and endogenous ester-containing compounds including the chemotherapeutic agent CPT-11, anti-influenza viral agent oseltamivir and many agrochemicals. Trifluoromethyl ketone (TFK)-containing compounds with a sulfur atom beta to the ketone moiety are some of the most potent carboxylesterase and amidase inhibitors identified to date. This study examined the effects of alkyl chain length (i.e., steric effects) and sulfur oxidation state upon TFK inhibitor potency (IC 50 ) and binding kinetics (k i ). The selective carboxylesterase inhibitor benzil was used as a non-TFK containing control. These effects were examined using two commercial esterases (porcine and rabbit liver esterase) and two human recombinant esterases (hCE-1 and hCE-2) as well as human recombinant fatty acid amide hydrolase (FAAH). In addition, the inhibition mechanism was examined using a combination of 1 H NMR, X-ray crystallography and ab initio calculations. Overall, the data show that while sulfur oxidation state profoundly affects both inhibitor potency and binding kinetics, the steric effects dominate and override the contributions of sulfur oxidation. In addition, the data suggest that inclusion of a sulfur atom beta to the ketone contributes an increase (~5-fold) in inhibitor potency due to effects upon ketone hydration and/or intramolecular hydrogen bond formation. These results provide further information on the nature of the TFK binding interaction and will be useful in increasing our understanding of this basic biochemical process.

Juvenile hormone esterase: biochemistry and structure

“…The physical properties of TFK inhibitors such as mimicry of the carbon backbone of JH, role of sulfur or sulfone at the β position, effect of ketone hydration, and lipophilicity are discussed previously. 3,33,36,37) The most potent TFK inhibitors have long aliphatic tails that mimic the length of the JH backbone and a sulfur 29) or sulfone 30) at the β position of the inhibitor (Fig. 1D).…”

Reactivity versus steric effects in fluorinated ketones as esterase inhibitors: a quantum mechanical and molecular dynamics study

“…Based on the preceding findings and their implication for the mechanism of action of TFMKs, we feel that selectivity of inhibitors with a central pharmacophore of a polarized ketone for different target enzymes can be modulated through the inclusion of suitable substituents in the chemical structure that contains the ketone moiety. Finally, it is likely that general conclusions drawn from the interaction of TFMK inhibitors and CEs can be extended to the design of inhibitors of other targets such as fatty acid amide hydrolase [15] and diacyl glycerol [16]. …”

“…An important group of chemicals that promote CE inhibition are fluorinated ketones, which inhibit a variety of esterases such as acetylcholinesterase [9], juvenile hormone esterase (JHE) [10] and human liver microsomal CEs [11, 12], as well as enzymes such as chymotrypsin and trypsin [13, 14] and enzymes that metabolize chemical mediators including fatty acid amide hydrolase and diacyl glycerol [15, 16]. Trifluoromethyl ketones (TFMKs) also reversibly inhibit pheromone-degrading esterases in male olfactory tissues [17–19].…”

Identification and optimization of soluble epoxide hydrolase inhibitors with dual potency towards fatty acid amide hydrolase

“…Finally, modifications to the 4-substituent on ring C determine potency based on the interactions required to generate an acyl-intermediate; thus, we would predict a highly electrophilic organophosphate or trifluoromethylketone analogues that mimic these acyl-intermediates may improve the potency of future inhibitors. 42, 43…”