Vanadium containing bromoperoxidase – Insights into the enzymatic mechanism using X-ray crystallography

Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

Environmental Control of Vanadium Haloperoxidases and Halocarbon Emissions in Macroalgae

Vanadium-dependent haloperoxidases (V-HPO), able to catalyze the reaction of halide ions (Cl^-, Br^-, I^-) with hydrogen peroxide, have a great influence on the production of halocarbons, which in turn are involved in atmospheric ozone destruction and global warming. The production of these haloperoxidases in macroalgae is influenced by changes in the surrounding environment. The first reported vanadium bromoperoxidase was discovered 40 years ago in the brown alga Ascophyllum nodosum. Since that discovery, more studies have been conducted on the structure and mechanism of the enzyme, mainly focused on three types of V-HPO, the chloro- and bromoperoxidases and, more recently, the iodoperoxidase. Since aspects of environmental regulation of haloperoxidases are less well known, the present paper will focus on reviewing the factors which influence the production of these enzymes in macroalgae, particularly their interactions with reactive oxygen species (ROS).

X-ray Diffraction and Density Functional Theory Provide Insight into Vanadate Binding to Homohexameric Bromoperoxidase II and the Mechanism of Bromide Oxidation

X-ray diffraction of native bromoperoxidase II (EC 1.11.1.18) from the brown alga Ascophyllum nodosum reveals at a resolution of 2.26 Å details of orthovanadate binding and homohexameric protein organization. Three dimers interwoven in contact regions and tightened by hydrogen-bond-clamped guanidinium stacks along with regularly aligned water molecules form the basic structure of the enyzme. Intra- and intermolecular disulfide bridges further stabilize the enzyme preventing altogether the protein from denaturing up to a temperature of 90 °C, as evident from dynamic light scattering and the on-gel ortho-dianisidine assay. Every monomer binds one equivalent of orthovanadate in a cavity formed from side chains of three histidines, two arginines, one lysine, serine, and tryptophan. Protein binding occurs primarily through hydrogen bridges and superimposed by Coulomb attraction according to thermochemical model on density functional level of theory (B3LYP/6-311++G**). The strongest attractor is the arginine side chain mimic N-methylguanidinium, enhancing in positive cooperative manner hydrogen bridges toward weaker acceptors, such as residues from lysine and serine. Activating hydrogen peroxide occurs in the thermochemical model by side-on binding in orthovanadium peroxoic acid, oxidizing bromide with virtually no activation energy to hydrogen bonded hypobromous acid.

The role of vanadium in biology

Vanadium is special in at least two respects: on the one hand, the tetrahedral anion vanadate(v) is similar to the phosphate anion; vanadate can thus interact with various physiological substrates that are otherwise functionalized by phosphate. On the other hand, the transition metal vanadium can easily expand its sphere beyond tetrahedral coordination, and switch between the oxidation states +v, +iv and +iii in a physiological environment. The similarity between vanadate and phosphate may account for the antidiabetic potential of vanadium compounds with carrier ligands such as maltolate and picolinate, and also for vanadium's mediation in cardiovascular and neuronal defects. Other potential medicinal applications of more complex vanadium coordination compounds, for example in the treatment of parasitic tropical diseases, may also be rooted in the specific properties of the ligand sphere. The ease of the change in the oxidation state of vanadium is employed by prokarya (bacteria and cyanobacteria) as well as by eukarya (algae and fungi) in respiratory and enzymatic functions. Macroalgae (seaweeds), fungi, lichens and Streptomyces bacteria have available haloperoxidases, and hence enzymes that enable the 2-electron oxidation of halide X(-) with peroxide, catalyzed by a Lewis-acidic V(V) center. The X(+) species thus formed can be employed to oxidatively halogenate organic substrates, a fact with implications also for the chemical processes in the atmosphere. Vanadium-dependent nitrogenases in bacteria (Azotobacter) and cyanobacteria (Anabaena) convert N2 + H(+) to NH4(+) + H2, but are also receptive for alternative substrates such as CO and C2H2. Among the enigmas to be solved with respect to the utilization of vanadium in nature is the accumulation of V(III) by some sea squirts and fan worms, as well as the purport of the nonoxido V(IV) compound amavadin in the fly agaric.

The Vanadium Iodoperoxidase from the marine flavobacteriaceae species Zobellia galactanivorans reveals novel molecular and evolutionary features of halide specificity in the vanadium haloperoxidase enzyme family

Vanadium haloperoxidases (VHPO) are key enzymes that oxidize halides and are involved in the biosynthesis of organo-halogens. Until now, only chloroperoxidases (VCPO) and bromoperoxidases (VBPO) have been characterized structurally, mainly from eukaryotic species. Three putative VHPO genes were predicted in the genome of the flavobacterium Zobellia galactanivorans, a marine bacterium associated with macroalgae. In a phylogenetic analysis, these putative bacterial VHPO were closely related to other VHPO from diverse bacterial phyla but clustered independently from eukaryotic algal VBPO and fungal VCPO. Two of these bacterial VHPO, heterogeneously produced in Escherichia coli, were found to be strictly specific for iodide oxidation. The crystal structure of one of these vanadium-dependent iodoperoxidases, Zg-VIPO1, was solved by multiwavelength anomalous diffraction at 1.8 Å, revealing a monomeric structure mainly folded into α-helices. This three-dimensional structure is relatively similar to those of VCPO of the fungus Curvularia inaequalis and of Streptomyces sp. and is superimposable onto the dimeric structure of algal VBPO. Surprisingly, the vanadate binding site of Zg-VIPO1 is strictly conserved with the fungal VCPO active site. Using site-directed mutagenesis, we showed that specific amino acids and the associated hydrogen bonding network around the vanadate center are essential for the catalytic properties and also the iodide specificity of Zg-VIPO1. Altogether, phylogeny and structure-function data support the finding that iodoperoxidase activities evolved independently in bacterial and algal lineages, and this sheds light on the evolution of the VHPO enzyme family.

Synthesis of (+)-luzofuran and (-)-ancistrofuran

The first synthesis of the furan-containing snyderane, (+)-luzofuran, is reported. The key step in this approach was an electrophilic brominative cyclization, which was accomplished using a nucleophilic N-heterocycle-flanked phosphoramidite catalyst in combination with the common laboratory reagent N-bromosuccinimide.

Formation of environmentally relevant brominated dioxins from 2,4,6,-tribromophenol via bromoperoxidase-catalyzed dimerization

Polybrominated dibenzo-p-dioxins (PBDD) are emerging environmental pollutants with structural similarities to the highly characterized toxicants polychlorinated dibenzo-p-dioxins. The geographical and temporal variations of PBDD in biota samples from the Baltic Sea do not display features that are normally related to anthropogenic sources such as incineration, and therefore the natural formation of PBDDs has been suggested. This study of the bromoperoxidase mediated oxidative coupling of 2,4,6-tribromophenol (an abundant substance that is naturally formed in marine systems) identified the formation of ppb-level yields of 1,3,6,8-tetrabromodibenzo-p-dioxin (1,3,6,8-TeBDD) through direct condensation. Additional TeBDDs (1,3,7,9-TeBDD, 1,2,4,7-TeBDD, and/or 1,2,4,8-TeBDD) and tri-BDDs (1,3,7-TrBDD and 1,3,8-TrBDD) were frequently formed but at lower yields. The formation of these TeBDDs probably proceeds via bromine shifts or Smiles rearrangements, whereas the TrBDDs may result from subsequent debromination processes. Because all of the congeners formed by oxidative coupling and subsequent reactions are also found in Baltic Sea biota, the results support the theory that PBDDs are formed from natural precursors.

Modelling the site of bromide binding in vanadate-dependent bromoperoxidases

Treatment of Boc-protected (S)-serine (Ser) methyl ester with triphenylphosphine bromide Ph(3)PBr (intermittently generated from PPh(3) and N-bromosuccinimide) yields Boc-3-bromoalanine (R)-Boc-BrAlaMe and, after deprotection, bromoalanine methyl ester (R)-BrAlaMe in the form of its hydrobromide. Boc-BrAlaMe and BrAlaMe have been structurally characterised. The reaction between BrAlaMe, salicylaldehyde (sal) and VO(2+) results in the formation of Schiff base complexes of composition [VO(sal-BrAlaMe)solv](+) (solv = CH(3)OH: 3, THF: 5) and [VO(sal-BrAla)THF] 4. DFT calculations of the structures of 3, 4 and 5, based on the B3LYP functional and employing the triple zeta basis set 6-311++g(d,p), provide distances Br···V = 4.0 ± 0.1 Å, if some distortion of the dihedral angle ∠N-C-C-Br is allowed (affording a maximum energy of ca. 45 kJ mol(-1)), and thus model Br···V distances detected by X-ray methods in bromoperoxidases from the marine algae Ascophyllum nodosum and Corallina pilulifera. The DFT calculations have been validated by comparing calculated and found structures, including the new complex [V(V)O(Amp-sal)OMe(MeOH)] (1, Amp is the aminophenol moiety) and the known complex [VO(L-Ser-van)H(2)O] (van = vanillin). Additional validation has been undertaken by checking experimental against calculated (BHandHLYP) EPR spectroscopic hyperfine coupling constants. Complexes containing bromine as a substituent at the phenyl moiety of a Schiff base ligand do not allow for an appropriate simulation of the Br···V distance in peroxidases. The closest agreement, d(Br···V) = 4.87 Å, is achieved with [VO(3Br-salSer)THF] (6), where 3Brsal-Ser is the dianionic Schiff base formed between 3-Br-5-NO(2)-salicylaldehyde and serine.

Biocatalytic formation of synthetic melanin: the role of vanadium haloperoxidases, L-DOPA and iodide

The vanadium haloperoxidase (V-HPO) enzyme, extracted from the brown alga Laminaria saccharina, is able to catalyze the formation of a black precipitate, using as precursor the amino acid L-dopa in the presence of hydrogen peroxide and iodide, in one-pot synthesis. The L-dopa oxidation is a multistep reaction with a crucial role played by the iodide in the enzyme catalyzed peroxidative production of dopachrome, a well known intermediate in the synthesis of melanin. Dopachrome is then converted to a synthetic form of melanin through a polymerization reaction. Factors, such as buffer composition and pH, influence significantly the reaction first steps, but further steps of melanin production are hardly influenced. The biosynthetic melanin produced through the combination V-HPO/I/H(2)O(2), was characterized by several spectroscopic techniques (UV-vis and FT-IR) as well as XRD. Moreover, this biopolymer is light sensitive, decomposing into oligo- and monomeric units. Scanning electron microscopy (SEM) imaging showed different morphologies when compared with commercial available melanin. The biosynthetic production of melanin can have a wide range of applications from photosensitive cells to biomedicine with the advantage of being produced under eco-friendly and mild conditions.

A stereoselective vanadium-dependent chloroperoxidase in bacterial antibiotic biosynthesis

Halogenases catalyze reactions that introduce halogen atoms into electron-rich organic molecules. Vanadium-dependent haloperoxidases are generally considered to be promiscuous halogenating enzymes that have thus far been derived exclusively from eukaryotes, where their cellular function is often disputed. We now report the first biochemical characterization of a bacterial vanadium-dependent chloroperoxidase, NapH1 from Streptomyces sp. CNQ-525, which catalyzes a highly stereoselective chlorination-cyclization reaction in napyradiomycin antibiotic biosynthesis. This finding biochemically links a vanadium chloroperoxidase to microbial natural product biosynthesis.

Actinobacterial peroxidases: an unexplored resource for biocatalysis

Peroxidases are redox enzymes that can be found in all forms of life where they play diverse roles. It is therefore not surprising that they can also be applied in a wide range of industrial applications. Peroxidases have been extensively studied with particular emphasis on those isolated from fungi and plants. In general, peroxidases can be grouped into haem-containing and non-haem-containing peroxidases, each containing protein families that share sequence similarity. The order Actinomycetales comprises a large group of bacteria that are often exploited for their diverse metabolic capabilities, and with recent increases in the number of sequenced genomes, it has become clear that this metabolically diverse group of organisms also represents a large resource for redox enzymes. It is therefore surprising that, to date, no review article has been written on the wide range of peroxidases found within the actinobacteria. In this review article, we focus on the different types of peroxidases found in actinobacteria, their natural role in these organisms and how they compare with the more well-described peroxidases. Finally, we also focus on work remaining to be done in this research field in order for peroxidases from actinobacteria to be applied in industrial processes.

Mechanistic considerations of halogenating enzymes

In nature, halogenation is a strategy used to increase the biological activity of secondary metabolites, compounds that are often effective as drugs. However, halides are not particularly reactive unless they are activated, typically by oxidation. The pace of discovery of new enzymes for halogenation is increasing, revealing new metalloenzymes, flavoenzymes, S-adenosyl-L-methionine (SAM)-dependent enzymes and others that catalyse halide oxidation using dioxygen, hydrogen peroxide and hydroperoxides, or that promote nucleophilic halide addition reactions.