Vanadium bromoperoxidases bromide

Clague MJ, Keder NL, Butler A (1993) Biomimics of Vanadium Bromoperoxidase Vanadium(V)-Schiff Base Catalyzed Oxidation of Bromide by Hydrogen Peroxide. Inorg Chem 32 4754... 

The focus of this chapter will be on the mechanistic chemistry of vanadium compounds that catalyze the oxidation of bromide by hydrogen peroxide—that is, functional mimics of vanadium bromoperoxidase. The reader is referred to references 2-8 for more comprehensive reviews of the enzyme. 

For the vanadium bromoperoxidase from the lichen X. parietina it has been reported (23) that an excess (50 mM) of F", Cl", and Br" inhibited the enzymic activity. This enzjune from the terrestrial organism also had a surprisingly high affinity for bromide (Km = 0.03 mM), which is about two orders of magnitude higher than the values reported for the enzymes from seaweed (19,22,32, 33). This particular enzyme was also inhibited by low concentrations (1-5 mM) of nitrate (32). 

Alkyl hydroperoxides, including ethyl hydroperoxide, cuminyl hydroperoxide, and tert-butyl hydroperoxide, are not used by V-BrPO to catalyze bromination reactions [29], These alkyl hydroperoxides have the thermodynamic driving force to oxidize bromide however, they are kinetically slow. Several examples of vanadium(V) alkyl peroxide complexes have been well characterized [63], including [V(v)0(OOR)(oxo-2-oxidophenyl) salicylidenaminato] (R = i-Bu, CMe2Ph), which has been used in the selective oxidation of olefins to epoxides. The synthesis of these compounds seems to require elevated temperatures, and their oxidation under catalytic conditions has not been reported. We have found that alkyl hydroperoxides do not coordinate to vanadate in aqueous solution at neutral pH, conditions under which dihydrogen peroxide readily coordinates to vanadate and vanadium( V) complexes (de la Rosa and Butler, unpublished observations). Thus, the lack of bromoperoxidase reactivity with the alkyl hydroperoxides may arise from slow binding of the alkyl hydroperoxides to V-BrPO. 

In addition to bromide and iodide, V-BrPO can catalyze the oxidation of chloride [64]. As mentioned previously and discussed more fully later, a distinct enzyme, vanadium chloroperoxidase, has also been discovered. Originally it was thought that V-BrPO could only catalyze the oxidation of bromide and iodide by dihydrogen peroxide. In fact, under the standard mcd bromoperoxidase assay conditions, in which the V-BrPO concentration is ca. nanomolar, very little, if any, chlorination of mcd is observed. However, it seemed very unusual that V-BrPO could be inhibited by fluoride and bromide, but apparently not by chloride [27], In reinvestigating the halide specificity of V-BrPO, it was discovered that when the enzyme concentration is increased 100-fold to 0.1 pM, chlorination is observed at an appreciable rate [64], The specific chloroperoxidase activity is 0.76 U/mg (under conditions of 1 M certified 100% bromide-free KC1, 2 mMH202, 50 pM... 

The haloperoxidases are a class of enzymes that catalyze the oxidation of halides via a reactive peroxometal active site. These enzymes are named according to the most electronegative halide they are able to oxidize. Hence, a bromoperoxidase can oxidize bromide and iodide but not chloride, whereas a chloroperoxidase can oxidize all three. Haloperoxidases are found in most living organisms and predominately fall into two classes the iron heme-based and vanadium-dependent enzymes. Of these, heme-based enzymes are found in mammals, where they provide a vital... 

Bhattachaijee, M. 1992. Activation of bromide by vanadium pentoxide for the bro-mination of aromatic hydrocarbons Reaction mimic for the enzyme bromoperoxidase. Polyhedron 11 2817-2817. 

Interestingly, a study of the bromide-dependent chloroperoxidase bromination of tyrosine reveals that the active brominating agent is free bromine and not a bromine-enzyme complex [122]. The situation is very different for bromoperoxidase- and vanadium peroxidase-catalyzed brominations [106-108]. 

It would be of considerable interest to see whether vanadium-peroxo complexes are also able to oxidize bromide and display kinetic behavior similar to that of the vanadium-containing bromoperoxidases. In this respect the complexes reported by Li et al. (80) may provide a useful contribution. Conversely, some attention should be paid to whether bromoperoxidases show specificity only toward bromide or iodide. These enzymes may perhaps be tuned to catalyze the oxidation and oxygenation of other nucleophiles. 

Vanadium ions take part in some an important processes occurring in biological systems [62]. One vanadium-containing enzyme, bromoperoxidase, has been isolated from several species of marine brown algae. This enzyme catalyzes the oxidation of the bromide anion by hydrogen peroxide, resulting in the bromi-nation of organic compounds ... 

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