Noncovalent inhibitors

This class of inhibitors usually acts irreversibly by permanently blocking the active site of an enzyme upon covalent bond formation with an amino acid residue. Very tight-binding, noncovalent inhibitors often also act in an irreversible fashion with half-Hves of the enzyme-inhibitor complex on the order of days or weeks. At these limits, distinction between covalent and noncovalent becomes functionally irrelevant. The mode of inactivation of this class of inhibitors can be divided into two phases the inhibitors first bind to the enzyme in a noncovalent fashion, and then undergo subsequent covalent bond formation. 

AmpC P-Lactamase. A map of hot spots was constructed from the X-ray structure of AmpC P-lactamase and a university version of the program DOCK was used to search for noncovalent inhibitors in 229,810 compounds of the ACD database. Of 56 tested compounds three had values <650pM, for example, compound 41 Ki = 26pM Fig. 16.6) [117]. The experimental X-ray structure of its complex with AmpC P-lactamase closely resembles the predicted binding mode. 

Powers RA, Morandi F, Shoichet BK. Structure-based discovery of a novel, noncovalent inhibitor of AmpC beta-lactamase. Structure 2002 10 1013-23. 

Sheehan and coworkers [47] have also prepared a novel isocyanide derivative, which was used in an Ugi-4CR to prepare noncovalent inhibitors of factor Xa, a key element in the coagulation process [48]. 

Over a decade ago, work on the enzyme aldolase reductase elegantly demonstrated this point. The noncovalent inhibitor alrestatin was modified to contain various electrophiles a-chloroacetamide, a-bromoacetamide or a-iodoacetamide. Noncovalent interactions between inhibitors and protein would not have changed, but molecules behaved differently based on the electrophile the weakest showed reversible inhibition, whereas the iodoacetamide displayed almost complete irreversible inhibition.1401 These results are an important warning if a reaction is too facile, irreversible reactions can obscure true binding affinities. 

The x-ray stmcture of complexes of TcAChE with HA and other AChE inhibitors displayed that these noncovalent inhibitors vary greatly in their stmctures and bind to different sites of the enzyme, offering many different starting points for future dmg design. To rationalize the stmcmral requirements of AChE inhibitors, Kaur and Zhang attempted to derive a coherent AChE-inhibitor recognition pattern based on literature data of molecular modeling and quantitative SAR analyses. 

Exposure to a toxic dose of OP results in inhibition of acetylcholinesterase and butyrylcholinesterase activities. The most common method to measure OP exposure is to assay acetylcholinesterase and butyrylcholinesterase activities in blood using a spectrophotometric method (EUman et al, 1961 Wilson et al, 2005 Worek et al, 1999). The drawbacks of activity assays are that they do not identily the OP. They show that the poison is a cholinesterase inhibitor but do not distinguish between nerve agents, OP pesticides, carbamate pesticides, and tightly bound, noncovalent inhibitors like tacrine and other anti-Alzheimer drugs. In addition, low-dose exposure, which inhibits less than 20% of the cholinesterase, carmot be determined by measuring acetylcholinesterase and butyrylcholinesterase activity because individual variability in activity levels is higher than the percent inhibition. 

Binda C, Wang J, Pisani L, Caccia C, Carotti A, Salvati P, Edmondson DE, Mattevi A. Structures of human monoamine oxidase B complexes with selective noncovalent inhibitors sa-finamide and coumarin analogs. J. Med. Chem. 2007 50 5848-5852. 

L Irreversible inactivation. Inactivation by affinity labels leads to irreversible covalent bond formation between the enzyme and the inhibitor. Unlike the complex between and enzyme and a rapid, reversible inhibitor, the covalent enzyme-inhibitor complex is no longer in equilibrium with free enzyme and inhibitor. Therefore, exhaustive dialysis or gel filtration of the covalent enzyme-inhibitor complex cannot lead to the recovery of free, active enzyme. However, such experiments do not allow distinction among tight-binding, noncovalent inhibitors, affinity labels, and mechanism-based inactivators. 

Systematic synthesis was carried out by Smith Khne French to identify freely reversible, noncovalent inhibitors of gastric H /K -ATPase with a mode of action comparable with SCH 28080. It was expected that gastric HVK -ATPase inhibitors with a shorter duration of action could be of therapeutic interest [155, 156]. This research resulted in the selection of compound SK F 96067 [157] Figure 4.10). 

Reversible, Noncovalent inhibitors Related to 1,2,3,4-Tetrahydro-9-Aminoacridine... 

Recently, more selective noncovalent inhibitors of proteasome have been developed. TMC-95A 15 is a potent and reversible selective inhibitor of the chymotrypsin-like, trypsinlike, and caspaselike activities of the 20S proteasome. Comparatively, TMC-95A shows no inhibition of calpain, cathepsin, or trypsin. 

CHEMISTRY AND STRUCTURE-ACTIVITY RELATIONSHIPS Noncovalent Inhibitors... 

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