Reversible inhibition transition state analogues

Herbicidal Inhibition of Enzymes. The Hst of known en2yme inhibitors contains five principal categories group-specific reagents substrate or ground-state analogues, ie, rapidly reversible inhibitors affinity and photo-affinity labels suicide substrate, or inhibitors and transition-state, or reaction-intermediate, analogues, ie, slowly reversible inhibitors (106). 

Trifluoromethyl /1-thioalkyls and /1-amino alcohols are often good reversible inhibitors of esterases and proteases, respectively. Depending on the enzymes (serine or aspartyl enzymes), fluorinated alcohols are often less efficient inhibitors than the corresponding ketones, which act as analogues of the transition state (vide infra). Nevertheless, fluoroalcohols inhibit hydrolytic enzymes with high inhibition constants (Figure 7.25)." ... 

Circiunstantial support for this mechanism was supplied by the fact that A-tosyl-Phe-CMK, a specific inhibitor of chymotrypsin, did not react with anhydrochymotrypsin [104]. Although both X-ray crystallographic and NMR studies supported the alkylated hemiketal as the structure of the inhibited enzyme, those studies did not prove whether alkylation or hemiketal formation oecurred first [105, 98]. Carbon-13 NMR studies were also used to determine the pKa (7.88-8.1) of the hemiketal hydroxyl and this finding provided the first evidence that serine proteinases could stabilize the ionized form of the alkylated hemiketal, via hydrogen bonds in the oxyanion hole [106,107]. A series of more recent papers has confirmed that hemiketal formation precedes the alkylation step and has shown that the initial, reversible part of the interaction is made up of two discrete stages (a) formation of a Michaelis complex, followed by (b) hemiketal formation [102, 108]. The requirement of an intermediate hemiketal may mean that chloromethyl ketone (CMK) inhibitors should be considered as transition-state [109] analogue inhibitors (see diseussion in seetion on Aldehydes). 

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