Dihydrogen peroxide

The use of six equivalents of dihydrogen peroxide leads to a clean conversion of the dithiolate complex to the disulfonate compound. Earlier studies on oxidation of nickel thiolates showed that oxidations with dioxygen stop at monosulfinates. Our observation and the characterization of the first chelating bis-sulfonato nickel complex formed from the direct oxidation of a mononuclear nickel dithiolate, may also provide new insight into the chemistry of sulfur-rich nickel-containing enzymes in the presence of oxygen. 

The standard procedure for removal of active-site vanadium(V) has been to incubate V-BrPO in 0.1 Mphosphate-citrate buffer pH 3.8, containing 10 mM ethylenediaminetetra-acetic acid (edta). These conditions remove over 95% of the vanadium, which produces the inactive apo-BrPO derivative [1,45]. The essential component of the apoprotein preparation is the phosphate, without which vanadium is not completely removed and the enzyme is not completely inactivated [46], In fact phosphate in the absence of edta is sufficient for preparation of apo-BrPO [46] inactivation by phosphate is much faster at low pH (pH 4) than at neutral or higher pH. However, phosphate inactivation does not occur in the presence of dihydrogen peroxide [46],... 

The standard assay for haloperoxidase activity is the halogenation of monochloro-dimedone (mcd) (2-chloro-5,5-dimethyl-l,3-dimedone) using dihydrogen peroxide as the oxidant of the halide (Figure 4) [48],... 

Bromoperoxidase activity is expressed as micromoles of mcd brominated per minute per milligram of enzyme (U/mg). The early work on V-BrPO employed the oxidation of iodide by dihydrogen peroxide [1], forming triiodide (Ir), which was followed spectrophotometrically at 353 nm ( = 26,400 M 1cm 1). However, this reaction is less desirable for quantitation of haloperoxidase activity because of competing side reactions, such as the nonenzymatic oxidation of iodide by dihydrogen peroxide and reduction of triiodide by dihydrogen peroxide (discussed later). The specific activity of mcd bromination for V-BrPO isolated from A. nodosum is 170 U/mg (at pH 6.5, 2 mM H202, 0.1 M Br , 50 nM mcd,... 

The stoichiometry of the V-BrPO-catalyzed bromination reaction is the oxidation of one equivalent of bromide by one equivalent of dihydrogen peroxide, producing one equivalent of brominated organic substrate ... 

In the absence of an organic halogen acceptor, the oxidized bromine intermediate is reduced by a second equivalent of dihydrogen peroxide, producing bromide and dioxygen [52]. The net reaction is the disproportionation of dihydrogen peroxide to dioxygen and water... 

The irreversible inactivation of V-BrPO that occurs at low pH [55] was found to produce 2-oxohistidine as identified by high-performance liquid chromatography (HPLC) using electrochemical detection [60], Inactivation of V-BrPO and formation of 2-oxohistidine require all the components of turnover (i.e., bromide, dihydrogen peroxide, and V-BrPO) as well as low pH (Figure 5). Neither... 

The interesting feature of the V-BrPO reactions with peracetic acid in the presence of amines (i.e., added amines or amine-containing buffers) is the formation of bromamines (BrNHR) [29]. Bromamines have been proposed as possible intermediates in haloperoxidase reactions [61,62] however, their direct detection had not been reported previously. Bromamine formation is not observed when the peroxide source is dihydrogen peroxide because the bromamine is rapidly reduced by dihydrogen peroxide, forming dioxygen and bromide. 

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. 

III.B.1. Chlorination and Chloride-Assisted Disproportionation of Dihydrogen Peroxide... 

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... 

In addition to mcd, V-BrPO catalyzes the chlorination of phenol red to tetrachlorophenol blue [64], Chlorination of amines (e.g., taurine, ammonia, valine, serine, leucine) is also catalyzed by V-BrPO forming the stable chloramine derivative, even in the presence of dihydrogen peroxide. Unlike bromamines, the chloramine is not reduced by dihydrogen peroxide, or is reduced only very slowly [53],... 

The mcd chlorination kinetics for V-CIPO (C. inaequalis) also fit a substrate inhibited bi-bi Ping-Pong mechanism. The kinetic constant for chloride, Kmc is reported to be 0.25 mMat pH 4.5 [3], The kinetic constant for dihydrogen peroxide, KmU2° varies as a function of pH 0.5 mM at pH 3.2 to 0.01 mM at pH 5 [59], As with V-BrPO (A. nodosum), chloride is both a substrate for and... 

Initial studies on functional mimics of V-BrPO were driven by the lack of spectroscopic techniques capable of observation of the vanadium(V) site in the enzyme. Early on it was found that acidic solutions of cw-dioxovanadium(V) (cis-V02+) catalyzed the oxidation of halides by dihydrogen peroxide, resulting in halogenation of an organic substrate and halide-assisted disproportionation of dihydrogen peroxide [73],... 

In this biomimetic system, cis-V02+ coordinates dihydrogen peroxide, forming the monoperoxo or diperoxo species, in ratios dependent on the dihydrogen peroxide and acid concentrations [74],... 

Other groups have also examined vanadium(V) catalyzed oxidation of bromide by dihydrogen peroxide in acidic aqueous or aqueous/organic mixtures, although without examining the detailed speciation of the vanadium peroxo compounds in solution [77-82], These reports have focused more on the nature of the substrate brominated and the product distribution under different conditions. 

Analogous to the preceding cw-dioxovanadium(V)-catalyzed system of bromide oxidation by dihydrogen peroxide, Secco carried out a detailed kinetic analysis of vanadate-catalyzed oxidation of iodide by dihydrogen peroxide [75], Peroxovanadate species (i.e., V0[02]+ and V0[02]2 ) or their hydronated forms oxidize iodide in acidic aqueous solution, forming V02+ and V0(02)+, respectively however, once V0(02)+ and V0(02)2 are consumed, iodide reduces... 

The electron transfer role of vanadium has possible relevance to vanadium bro-moperoxidase, although this system and V-BrPO differ in that V-BrPO requires dihydrogen peroxide for catalytic activity. A speculative catalytic cycle has been proposed to be... 

The vanadium haloperoxidases function first by coordination of dihydrogen peroxide followed by oxidation of the halide (Scheme 7). The consensus seems to be that the vanadium center functions as a Lewis acid, remaining in the 5+ oxidation state, as opposed to functioning as an electron transfer catalyst, although it should be pointed out that the reduction potential of the vanadate center has not been measured. 

Most other peroxidases are Fe-heme-containing systems, which function as two-electron redox catalysts (Scheme 8). Dihydrogen peroxide oxidizes the Fe-heme moiety by two electrons, forming Compound 1 (a heme + FeIV=0 species) [97], Compound 1 oxidizes the halide ion, forming the active halogenating species. This mechanism cannot be operative in V-BrPO because the vanadium is already in its highest accessible oxidation state. Moreover, native V-BrPO does not oxidize bromide without an acceptable peroxide source. However, it should... 

The catalytic dismutation of superoxide is actually more complicated in E. coli [42] and B. thermophilus [43] Mn-SODs than that of either Cu or Fe proteins since it may involve an inactive form of the enzyme. The inactive form is believed [44] to contain a Mnm-side-on peroxo unit (of the type shown in Figure 29) formed within the hydrophobic environment of MnSOD, in the absence of H+, by the oxidative addition of the superoxide ion to the Mn11 center. When H+ ions are present, an active, end-on peroxo complex forms, yielding successively a bound hydroperoxide ion and free dihydrogen peroxide (cf. Figure 3). Thus, the key parameter that turns the reaction off or on may be the absence or presence of a H+ ion [44],... 

Several functional ligninase models that do not contain key structural elements of LiP or MnP are worth mentioning since they do not use dihydrogen peroxide in the delignification process. Among them, robust polyoxometallate catalysts have been shown [64] to work well with dioxygen, thus making this chemistry commercially attractive. 

The catalases catalyze the disproportionation reaction of dihydrogen peroxide according to the following equation (Eq. 5) [5] ... 

In general, as can be seen from the actual parameters for dihydrogen peroxide decomposition (Table 3), manganese catalases are less active than their heme iron counterparts. Catalytic rates comparable to these are a target for functional models of the Mn enzyme. 

Many manganese complexes decompose dihydrogen peroxide, but we limit our discussion to the functional dinuclear ones the catalase activity of bis-dinuclear (tetranuclear) photosystem II (PSII) models is discussed later. Furthermore, mainly model complexes reported in the last 5 years are discussed in detail since previous work is covered in several excellent reviews [1,2b,3a,5,8-12],... 

Table 3 Steady-State Kinetic Parameters of Dihydrogen Peroxide Decomposition by Mn Catalases...

Nonblue. These include galactose oxidase (GO) and amine oxidases (e.g., plasma amine oxidase, diamine oxidase, lysyl oxidase), which produce dihydrogen peroxide by the two-electron reduction of 02 [33], For GO (stereospecific primary alcohol oxidation), spectroscopic studies by Whittaker [70,71] suggest that the two-electron oxidation carried out by a mononuclear copper center is aided by a stabilized ligand-protein radical (i.e., (L)Cu(I) + 02 —> (L +)Cu(lI) + H202), obviating the need to get to Cu(III) in the catalytic cycle. Protein x-ray structures [33,72] reveal a novel copper protein cofactor, which would seem... 

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