Hydrolytic dehalogenation

Alcaligenes denitrificans strain NTB-1 is able to use 4-chloro-, 4-bromo-, and 4-iodoben-zoates as sole sources of carbon and energy. The pathway involves hydrolytic dehalogenation to 4-hydroxybenzoate followed by hydroxylation to 3,4-dihydroxybenzoate (van den Tweel et al. 1987). 

Uotila JS, VH Kitunen, T Saastamoinen, T Coote, MM Hagblom, M Salkinoja-Salonen (1992) Characterization of aromatic dehalogenases of Mycobacterium fortuitum CG-2. J Bacteriol 174 5669-5675. van der Tweel WJJ, JB Kok, JAM de Bont (1987) Reductive dechlorination of 2,4-dichlorohenzoate to 4-chlorobenzoate and hydrolytic dehalogenation of 4-chloro-, 4-hromo-, and 4-iodohenzoate hyAcalig-enes denitrificans NTB-1. App/ Environ Microbiol 53 810-815. 

Fig. 11.3. Comparison of a) hydrolytic dehalogenation and b) oxidative dehalogenation. The products of Reaction a (an alcohol) and Reaction b (a carbonyl) may be interconverted by de-hydrogenation/hydrogenation (Reaction c). When these products are a primary alcohol and an aldehyde, further oxidation to the acid is possible (Reaction c).

The relevance of this mechanism to mammalian enzymes is an important question, but we are not aware of any detailed study that affords a definitive answer. Proof that reactions of hydrolytic dehalogenation ofhaloalkyl groups occur in animals is presented in the next subsection, but much remains to be discovered regarding the enzymes involved or the reaction mechanisms. Furthermore, nonenzymatic reactions remain a distinct possibility when the C-atom bearing the halogen is sufficiently electrophilic, as seen, e.g., with (2-chloroethyl)amino derivatives (see Sect. 11.4.2). 

This reaction should not be confused with hydrolytic dehalogenation, despite apparent similarities [58] although both hydrolytic and oxidative dehalogenation routes may produce the alcohol and carbonyl derivatives, the product that is formed as the primary or secondary metabolite is different in the two pathways. Further, it is clear that the enzymes involved cannot be identical. 

The metabolism of 1,2,3-trichloropropane (11.27), an industrial solvent that undergoes biotransformation via dechlorination at C(l) and C(2) [60], is a clearer case of oxidative dehalogenation. Following incubation with human or rat liver microsomes, the compound was converted to 1,3-dichloroacetone (11.29), which could a priori be produced by oxidative dehalogenation (i. e., via 11.28) or by hydrolytic dehalogenation. In this study, evidence was found... 

The above evidence should not be interpreted as meaning that hydrolytic dehalogenation can never be demonstrated unambiguously. Indeed, the next subsection will document reactions of multiple dehalogenation that are only consistent with a hydrolytic mechanism. But before doing so, we examine here the hydrolysis of medicinal nitrogen mustards and of their sulfur analogue, the infamous sulfur mustard. 

Yang et al reeently reported on the meehanism of 4-chlorobenzoyl eoenzyme A dehalogenase, an enzyme that catalyzes the hydrolytic dehalogenation of 4-chlo-robenzoyl coenzyme A (4-CBA-CoA) to form 4-hydro-xybenzoyl coenzyme A (4-HBA-CoA). The mechanism involves attack of an active site carboxylate at C4 of the substrate benzoyl ring to form a Meisenheimer complex (shown above). Loss of chloride ion from this intermediate then forms an arylated enzyme intermediate that is hydrolyzed to free enzyme plus 4-HBA-CoA by the addition of water at the acyl carbon. In later work, Taylor et al examined the activation of the 4-CB A-CoA toward nucleophilic attack by the active site carboxylate group. 

Chloro- -triazines have been shown to be metabolized in plants by one of four competing processes hydrolytic dehalogenation, oxidative /V-dcalkylation, nucleophilic displacement of the chlorine atom with glutathione, and ami-nation or deamination reactions. Much of the early research focused on the first three processes and attempted to determine the relative importance of each process to herbicide tolerance. The relevant research undertaken between 1961 and 1973 will be discussed. 

Hydrolytic Dehalogenation Simazine was shown by Roth (1957) to degrade in the presence of com extracts, but was stable in the presence of extracts from a susceptible wheat crop. Castelfranco et al. (1961) described a similar nonenzyme constituent of expressed com sap that hydrolyzed simazine to hydroxysimazine [2,4-bis(ethylamino)-6-hydroxy-y-triazine]. Wahlroos and Virtanen (1959) and Hamilton (1964) have established that the catalytic conversion of simazine to hydroxysimazine in roots and shoots of resistant species is caused by 2,4-dihydroxy-7-methoxy-l,4-benzoxazin-3-one (benzoxazinone) or its 2-glucoside as follows ... 

Hydrolysis of esters and ethers, hydrolytic cleavage of C—N single bonds, hydrolytic cleavage of nonaromatic heterocycles, hydration and dehydration at m ultiple bonds, new atomic linkages resulting from dehydration reactions, hydrolytic dehalogenation removal of hydrogen halide molecules, various reactions. 

Hydrolytic dehalogenation catalyzed by dehalogenases (or halido-hydrolases ) proceeds by formal nucleophihc substitution of the halogen atom with a hydroxyl ion [1810, 1811]. Neither cofactors nor metal ions are required for the enzymatic activity. Depending on the enzyme source, the reaction may either proceed with retention or inversion of configuration. It is this stereospecificity which makes... 

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