Heme-haloperoxidase

Heme haloperoxidases can also use peroxide and halide ions to halogenate an activated (benzylic/allylic) carbon. The halide is first oxidized to an active halogenating intermediate (Fig. 10.4, pathway (2)). The substrate is halogenated in the next step. The overall reaction is... 

Scheme II. Mechanism for Fe-heme haloperoxidase-catalyzed production of...

Some of the non-heme haloperoxidases are very stable with respect to organic solvents115 which is of great importance when the substrates that are to be halogenated are not very soluble in water. In these cases water missible organic solvents can be added to the reaction mixture or a two phase-system can be used. 

Some of the non-heme haloperoxidases and perhydrolases are very stable, even against organic solvents, and easy to use as they do not need any cofactors. However, care has to be taken not to use too high concentrations of hydrogen peroxide, as this could lead to oxidation of the substrate. 

Some of the major enzyme groups that facilitate this transformation are heme-containing MOs of the cytochrome P450 type [111], alkane hydroxylases, xylene monooxygenases, styrene monooxygenases [105], and haloperoxidases [112],... 

Heme-dependent haloperoxidases generate HOX as reactive species from H2O2 and X, which represents an X+ equivalent capable of undergoing electrophilic addition at electron-rich centers [270,271]. Aprototype biocatalyst of this group is the chloroperoxidase from Caldariomyces Jumago [272]. In many natural systems, such enzymes are responsible for the halogenation of electron-rich aromatic cores. 

Haloperoxidases are peroxidases capable of halogenating substrates in the presence of halide and hydrogen peroxide [14] or other reactions such as sulfoxidation, epoxidation and aromatic hydroxylation. Here, the halide ion is initially bound to the active site which may incorporate heme or vanadium or be metal free. The halide ion is incorporated into the substrate after electron transfer... 

For electron-rich substrates, nature often uses flavin-dependent halogenases (e.g. chlorotetracycline), vanadium haloperoxidases (snyderol, Figure 7.4) or heme-iron haloperoxidases (tetraiodothyronine. Figure 7.4) for this role (Figure 7.5). For electron-deficient molecules such as alkanes, mononuclear iron... 

Vanadoproteins are found in most marine algae and seaweed and in some lichens.616 Among these are haloperoxidases,252 253 617 618b enzymes that are quite different from the corresponding heme peroxidases discussed in Section A,6. The vanadium is bound as... 

Hofrichter M, Ullrich R (2006) Heme-Thiolate Haloperoxidases Versatile Biocatalysts with Biotechnological and Environmental Significance. Appl Microbiol Biotechnol 71 276... 

General acid-base catalysis is often the controlling factor in many mechanisms and acts via highly efficient and sometimes intricate proton transfers. Whereas log K versus pH profiles for conventional acid-base catalyzed chemical processes pass through a minimum around pH 7.0, this pH value for enzyme reactions is often the maximum. In enzymes, the transition metal ion Zn2+ usually displays the classic role of a Lewis acid, however, metal-free examples such as lysozyme are known too. Good examples of acid-base catalysis are the mechanisms of carbonic anhydrase II and both heme- and vanadium-containing haloperoxidase. 

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... 

Most haloperoxidases, including, for example, EPO, CPO, LPO, TPO and HPO, consist of a glycoprotein and ferriprotoporphyrin IX (18) as a heme component. The metal present carries out the heterolytic cleavage of H202 and stores one oxidizing equivalent, the... 

There are several examples of haloperoxidases containing metals other than iron, and many of these are non-heme enzymes. For example, a series of bromoperoxidases have been isolated recently from seaweed17,18 and an actinomycete19 that require vanadium for halogenating activity. 

Fig. 2.15 Phylogeny of heme-containing haloperoxidases. The reconstructed tree from the ME-method of the MEGA package [13] is presented. A very similar tree was obtained with the ProML method of the PHYLIP package [16]. Numbers in the nodes indicate bootstrap values for ME and ML, respectively. Abbreviations of protein names correspond to PeroxiBase...

Hofrichter, M. and Ullrich, R. 2006. Heme-thiolate haloperoxidases versatile biocatalysts with biotechnological and environmental significance. Applied Microbiology and Biotechnology, 71 276-88. 

A novel class of haloperoxidases, in which a heme prosthetic group was absent, was detected in brown algae (Phaeophyceae) by Vilter and coworkers (13-15). These publications escaped the attention of most biochemists involved in peroxidase research. Only when some of this work was published in the journal Phytochemistry (16) was there as increasing awareness of these findings. A clue for the involvement of vanadium was also published (16). It was shown that the bromoperoxidase could be inactivated at low pH and reactivated by vanadate. These results were subsequently confirmed (17, 18) when it was shown that vanadium was present in a number of bromoperoxidases from different sources and was essential for enzymatic activity. To date, these sources include the enzymes from the brown seaweed Ascophyllum nodosum... 

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