Suicide enzyme inactivators

Suicide Substrates —Mechanism-Based Enzyme Inactivators... 

SCHEME 11.3 Postulated mechanisms for the inhibition of serine proteases by coumarin derivatives. NuH nucleophile. Pathway a suicide-type inactivation (suicide substrate). Pathway b transient inactivation by formation of a stable acyl-enzyme (alternate substrate-inhibitor). 

Walsh, C. T. Suicide substrates mechanism-based enzyme inactivators recent developments. Ann. Rev. Biochem. 1984, 53, 493-535. 

The biological activity of a compound can often be affected dramatically by the presence of even a single fluorine substituent that is placed in a particular position within the molecule. There are diverse reasons for this, which have been discussed briefly in the preface and introduction of this book. A few illustrative examples of bioactive compounds containing a single fluorine substituent are given in Fig. 3.1. These include what is probably the first example of enhanced bioactivity due to fluorine substitution, that of the corticosteroid 3-1 below wherein Fried discovered, in 1954, that the enhanced acidity of the fluorohydrin enhanced the activity of the compound.1 Also pictured are the antibacterial (3-fluoro amino acid, FA (3-2), which acts as a suicide substrate enzyme inactivator, and the well-known anti-anthrax drug, CIPRO (3-3). 

However, an important problem arises during the peroxidative removal of phenols from aqueous solutions PX is inactivated by free radicals, as well as by oligomeric and polymeric products formed in the reaction, which attach themselves to the enzyme (Nazari and others 2007). This suicide peroxide inactivation has been shown to reduce the sensitivity and efficiency of PX. Several techniques have been introduced to reduce the extent of suicide inactivation and to improve the lifetime of the active enzyme, such as immobilization. Moreover, Nazari and others (2007) reported a mechanism to prevent and control the suicide peroxide inactivation of horseradish PX by means of the activation and stabilization effects of Ni2+ ion, which was found to be useful in processes such as phenol removal and peroxidative conversion of reducing substrates, in which a high concentration of hydrogen peroxide may lead to irreversible enzyme inactivation. 

Suicide substrates and quiescent affinity labels, unlike the other types of inhibitors discussed in this chapter, form covalent bonds with active site nucleophiles and thereby irreversibly inactivate their target enzymes. A suicide substrate,191 also described by Silverman in a comprehensive review1101 as a mechanism-based inactivator, is a molecule that resembles its target enzyme s true substrate but contains a latent (relatively unreactive) electrophile. When the target enzyme attempts to turn over the... 

An interesting dinically useful prodrug is 5-fluorouracil, which is converted in vivo to 5-fluoro-2 -deoxyuridine 5 -monophosphate, a potent irreversible inactivator of thymidylate synthase It is sometimes charaderized as a dead end inactivator rather than a suicide substrate since no electrophile is unmasked during attempted catalytic turnover. Rathei since a fluorine atom replaces the proton found on the normal substrate enzyme-catalyzed deprotonation at the 5 -position of uracil cannot occur. The enzyme-inactivator covalent addud (analogous to the normal enzyme-substrate covalent intermediate) therefore cannot break down and has reached a dead end (R. R. Rando, Mechanism-Based Enzyme Inadivators , Pharm. Rev. 1984,36,111-142). 

Walsh, C.T. (1984) Suicide Substrates, Mechanism Based Enzyme Inactivators -Recent Developments. Annual Review of Biochemistry, 53, 493-535. 

The synthesis of suicide inhibitors and the kinetics of enzyme inactivation have greatly advanced the study of enzyme mechanisms as well as the design of drugs and antimetabolites. 

Linear furanocoumarins (psoralens) inhibit P450s as mechanism-based inactivators (suicide inhibitors). Thus, species that produce psoralens may have evolved C4H enzymes with enhanced tolerance to these compounds. Recombinant C4H from the psoralen-producing species R. graveolens showed markedly slower inhibition kinetics with psoralens, and possibly biologically significant tolerance, compared to C4H from a species that does not produce the compounds (H. tuberosus) ... 

A special class of irreversible inhibitors is the suicide inactivators. These compounds are relatively un-reactive until they bind to the active site of a specific enzyme. A suicide inactivator undergoes the first few chemical steps of the normal enzymatic reaction, but instead of being transformed into the normal product, the... 

As these inhibitors owe their activity to the kcat term (i.e., the enzyme s usual mode of action) they have been designated "kcat inhibitors" by Rando (1), while Abeles and Maycock (2 ) have used the term "Suicide Enzyme Inactivators" because the enzyme, in accepting such a "booby-trapped" substrate commits suicide by its own mechanism of action. 

Abeles and Maycock (2 ) and Walsh (5) as well as to a symposium proceedings which offers the most recent and comprehensive reviews in the area (6 ). This review will restrict itself to the irreversible inhibition of pyridoxal phosphate (PyCHO)-dependent enzymes, a class of enzymes which has proven to be generally susceptible to inhibition by suicide enzyme inactivators. 

In conclusion, suicide enzyme inactivators offer a powerful method for the selective irreversible inhibition of enzymes. Although this review has concentrated on pyridoxal phosphate-dependent enzymes the approach is also valid for the irreversible inhibition of other types of enzymes (6 ) and may offer a means for the rational design of therapeutically-useful substances. 

The catalytic aspect of enzymes has prompted the development of new types of enzyme inactivators, which have been called suicide inhibitors . These inhibitors are substrates which are modified by masking the reactive functions. If they are unmasked by the catalytic action of the target enzyme, reaction of the reactive group with an active site or cofactor is made possible and this leads to the inactivation of the enzyme. Since the chemically reactive group is only liberated at the active site of the target enzyme, reactions with foreign molecules cannot occur. A number of examples of suicide inhibitors (or K -at inhibitors) can be found in a paper by Rando... 

C. T. Walsh. 1984. Suicide substrates, mechanism-based enzyme inactivators Recent developments Rev. Biochem. 53 493-535. (PubMed)... 

Inhibition of xanthine oxidase by allopurinol, an analogue of hypoxanthine. Nv and Cg of hypoxanthine are reversed in allopurinol. Allopurinol, a suicide enzyme inactivator, is converted to alloxanthine, which binds tightly to the active site of the enzyme via Mo and interrupts the reoxidation of Mo + to Mo " needed to initiate the next catalytic cycle. 

Alloxanthine (suicide enzyme-inactivator of xanthine oxidase)... 

Mechanism of action of S-adenosylhomoeysteine hydrolase (enzyme) and its inhibition by deoxyadenosine. (a) The enzyme uses enzyme-bound NAD+ to temporarily oxidize substrate and eventually hydrolyze it to adenosine and homocysteine. [Reproduced with permission from R. H. Abeles, Suicide enzyme inactivators. Chem. Eng. News 61(38), 55 (September 19, 1983). 1983 by the American Chemical Society.] (b) Deoxyadenosine, a suicide substrate, is also oxidized by the enzyme with the formation of a ketosugar, which undergoes decomposition, with the product dissociating from the enzyme and leaving the enzyme in the reduced state (NADH). 

Suicide enzyme inactivators, R. H. Abeles, 1983, Chem. Eng. News, 61 4856. 

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