Fluorination irreversible inhibition

The ability of fluorinated substituents to prevent the development of a positive charge on the a position has been used to slow down or even inhibit enzymatic processes involving positively charged transition states. According to the case, the observed result can be (1) slowing down the reaction, (2) reversible inhibition, or (3) irreversible inhibition. Various examples are now given. 

Biotransformation, especially phase I metabolic reactions, cannot be assumed to be synonymous with detoxification because some drugs (although a minority) and xenobiotics are converted to potentially toxic metabolites (e.g. parathion, fluorine-containing volatile anaesthetics) or chemically reactive intermediates that produce toxicity (e.g. paracetamol in cats). The term lethal synthesis refers to the biochemical process whereby a non-toxic substance is metabolically converted to a toxic form. The poisonous plant Dichapetalum cymosum contains monofluoroacetate which, following gastrointestinal absorption, enters the tricarboxylic acid (Krebs) cycle in which it becomes converted to monofluorocitrate. The latter compound causes toxicity in animals due to irreversible inhibition of the enzyme aconitase. The selective toxicity of flucytosine for susceptible yeasts (Cryptococcus neoformans, Candida spp.) is attributable to its conversion (deamination) to 5-fluorouracil, which is incorporated into messenger RNA. 

These transformations are efficient under biological conditions only if another activating group is present (carbonyl, aryl, etc.) [77]. Such an activating group is important to render the elimination product (resulting from the loss of a fluoride ion) a better Michael acceptor. Moreover, if one or several fluorine atoms are present on the double bond, the latter is also more reactive (Fig. 22). Thus, the elimination, promoted by the enzyme, of a fluoride ion from a jS-fluoro amino acid leads to a very reactive Michael acceptor. The latter can undergo an irreversible addition of a nucleophile residue of the active site of the enzyme, which is thus inhibited (Fig. 23) [78,79]. 

In the 1970s, Fuller examined the effects of jS,/f-difluoro substitution on the biological properties of a series of arylethylamines. Among the compounds prepared were p,p-difluoroamphetamine (19), )S,)S-difluoro phenethylamine (20), and A/-cyclopropyl-4-chloro-jS,jS-difluorophenylethylamine (21). A drop in amine pKg of about 2.5 pH units resulted from the fluorine substitution. Included in biological studies were effects on activities toward MAO. In vitro )S,)S-difluoroamphetamine was a less active inhibitor of MAO than amphetamine and )S,)S-difluorophenylethylamine was a poorer substrate for deamination than phenylethylamine. A/-Cyclopropyl-4-chlorophenylethylamine is an irreversible inhibitor of MAO. There was little difference in vivo in MAO inhibition in various tissues of the rat [72]. 

The antibacterial and antiviral properties of mono-, di-, and trifluoroalanines are connected to their capacity to inhibit, in an irreversible way, many enzymes (cf. Chapter 1) They have been the subject of numerous synthetic studies. We only report on the most recent and significant ones, without detailing the general methods of amino acid chemistry or the techniques of electrophilic fluorination, which have recently been reviewed. ... 

Enzyme inhibitors can be designed through different ways taking into account the effects of fluorine substitution on the behavior of a substrate toward the enzyme. Many fluorinated inhibitors (reversible or irreversible) have been studied. In this chapter, we only consider cases in which fluorine atoms play a determinant role in the inhibition and which are of importance in dmg discovery. We successively focus on the following ... 

Pseudosubstrates in which the fluorinated group is introduced to destabilize a reaction intermediate or a transition state of the enzymatic transformation. This can provoke inhibition or irreversible inactivation of the enzyme. 

A number of conformationally restricted fluorinated inhibitors have been synthesized and evaluated. These smdies show that (1) subtle conformational differences of the substrates affect the inhibition (potency, reversible or irreversible character) (Figure 7.50), (2) a third inhibition process involving an aromatization mechanism could take place (Figure 7.51). When the Michael addition and enamine pathways lead to a covalently modified active site residue, the aromatization pathway produces a modified coenzyme able to produce a tight binding complex with the enzyme, responsible for the inhibition (Figure 7.51). ... 

Neuroamines are biosynthesized in the central nervous system by decarboxylation of the corresponding amino acids by the amino acid decarboxylases (AADCs), which are present in nerve endings. In consequence, inhibition of the AADCs could be a means to regulate concentration in neuroamines. Research has been based on the hypothesis that a S-fluoromethylene amino acid could be used as a precursor of the inhibitor. If this fluorinated amino acid was a substrate for the AADC, it would then be transformed in situ into a S-fluoromethylene amine, which is an irreversible inhibitor of MAOs (Figure 7.54). ° ... 

A detailed study of the inhibition of MAOs by fluorophenyl cyclopropyl amines shows that the presence of fluorine has very important effects on this inhibition. While some of the regioisomers are inhibitors of the CAO (copper-containing amine oxidase), some other ones, such as 2-fluoro-l-arylcylopro-pyl amines, are excellent selective and irreversible inhibitors of MAO A. In this latter case, the nonfluorinated parent compound is a poor inhibitor of MAO B (Figure 7.55). ° " ... 

Scheme 4.33 Mechanism of the suicide inhibition of tymidylate synthase by 5-fluorouracil. The reaction pathway with the natural substrate (dUMP) is depicted on the left, the analogous sequence with 5-fluoro-dUMP on the right. The key to the irreversible blocking of the enzyme reaction site is the inability of fluorine to functionally replace hydrogen in proton-transfer reactions, for example the -elimination liberating the enzyme thiolate group [10], In addition, the transient positive charge on the methylene group during hydride transfer is destabilized by the jff-fluorine.

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