VANADIUM IN NATURALLY OCCURRING SYSTEM

Biological systems have developed haloperoxidases enzymes to catalyse the oxidation of chloride, bromide and iodide by hydrogen peroxide. On the basis of their cofactor requirement haloperoxidases are classified into the following three groups: heme-containing, vanadium-containing [56] and metal-free haloperoxidases [57]. Among them, vanadium haloperoxidase (VHPO) appears to be the most ubiquitous [58]. The bromoperoxidases involve in the polymerization of polyphenols holding the zygote to the membrane during the reproductive cycle of the cell [59]. The vanadium haloperoxidases represent a group of peroxidases that possess a single bound vanadate ion in a prosthetic group. These enzymes are able to oxidize a suitable electophilic halide species (X) to the corresponding hypohalous acid (HOX) in the presence of H2O2. The hypohalous acid species may further react to give a halogenated species. The presence of such enzyme in nature could do some way to explaining the formation of at least some of the wide diversity of halogenated compounds in environment [60].

The historical nomenclature convention of HPO is based on the most electronegative halide that the enzyme can oxidize (i.e., the chloroperoxidases (ClPO) can oxidize both Cl– and Br– and bromoperoxidases (BrPO) can oxidize Br– ). HPO does not have the driving force to oxidize the fluoride; however a fluorinating enzyme, fluorinase, has recently been isolated and is proposed to act by SN2 mechanism [61, 62]. In 1983, a naturally occurring vanadium-containing enzyme, vanadium bromoperoxidase (VBrPO), was discovered in the marine brown alga Ascophyllum nodosum [63]. It has trigonal bipyramidal coordination sphere including apical histdine and a meridionally bound oxo group; Figure 1.3 [64].
The oxidation state of the vanadium is +V and it does not change when hydrogen peroxide bind to give activated peroxointermidate species. Vanadium chloroperoxidases (VClPO), in the native and peroxo forms (Figure 1.4), have been isolated from the fungus Curvularia inaequalis [65, 66]. The enzyme contains α- helical with two four helix bundles as main structural motif, having 609 amino acids with molecular mass of 67488 Da. The active site is located at the top of the bundles [67]. A five-coordinated trigonal bipyrimidal V(V) moiety (Figure 1.4) is present in the native form which is coordinated by three nonprotein oxo groups in the equatorial plane and one His496 and a hydroxy group at the axial positions. The oxygens are hydrogen bonded to several amino acid residues of the protein chain. The nitrogen of His496 coordinates to vanadium and hence is the only direct bond from the protein to the metal center. The apical V-O bond length is 1.93 Å and the V-N bond is 1.96 Å, three equatorial V-O bonds are about 1.65 Å long.
In the peroxo form (Figure 1.5), the peroxide ligand is bound in a η2-manner in the equatorial plane. The apical oxygen ligand detaches and gives a distorted tetragonal pyramid coordination geometry around the vanadium centre with the two peroxo oxygens having V-O bond length ~ 1.87 Å and O-O is 1.47 Å. One oxygen (V-O bond length 1.93 Å) and the nitrogen (V-N bond length 2.19 Å) are in the basal plane while one oxygen (V-O bond length 1.60 Å) is in the apical position.
Carallina officinalis has been isolated from red algae and shows a high degree of amino acid homology in their active centre and has almost identical structural feature as have been reported for other enzymes [68].

In the last decade peroxidases, particularly chloroperoxidase have been shown to catalyse a variety of synthetically useful oxygen transfer reactions with H2O2, including enantioselective oxidation of sulfides [69, 70]. Vanadium haloperoxidases, such as vanadium chloroperoxidase from Curvularia inaequalis are much more stable. The vanadium-dependent bromoperoxidase from Corallina officinalis mediates the enantioselective oxidation of aromatic sulfides [71, 72]. The brown seaweed Ascophyllum nodosum mediate the formation of the (R)-enantiomer of the methyl phenyl sulfoxide with 91 % enantiomeric excess, whereas the red seaweed Corallina pilulifera mediates formation of the (S)-enantiomer (55 % enantiomeric excess) under optimal reaction conditions [73]. Recently Butler et al. have reported vanadium bromoperoxidase catalyzed biosynthesis of halogenated marine natural products [74].
In 1986, a vanadium nitrogenase enzyme was isolated from each of the two soil bacteria, Azotobacter chroococcum [75] and A. vinelandii [76, 77]. In vanadium-nitrogenases, vanadium is in a low to medium oxidation state as an integral part of an iron-sulphur cluster [78] which activates and reductively protonates various unsaturated substrates [79]. The coordination sphere around vanadium is probably octahedral (33 in Figure 1.6) and is similar to that of molybdenum in the structurally characterized molybdenum-nitrogenase [78, 80]. Thus, in vanadium-nitrogenase, vanadium is coordinated to histidine and the vicinal hydroxide and carboxylate groups of homocitrate in addition to three sulphide ions from iron-cluster.
The vanadium-iron protein of vanadium nitrogenases extracted from Azotobactor chroococcum contains an iron-vanadium cofactor. It is suggested that the nitrogen fixation requires specific interactions between Fe-V cofactor (Fe-Vaco) or Fe-Mo cofactor (Fe-Moco) and their respective polypeptides [81]. The vanadium nitrogenases [82] are multicomponent metalloenzyme complexes that are capable of reducing dinitrogen to ammonia, and hence to a form accessible by plants [83].
Many nitrogenases consist of a Fe-S cluster and a molybdenum-dependent component [84]. The first vanadium containing nitrogenase, i.e. a V-Fe-protein, was isolated and purified in 1986 from certain nitrogen-fixing bacteria [75].
Complexes have been designed that model parts of the structure of the V-Fe cofactor. The coordination atom is S in place of N and/or O coordination. Dixovanadium(V) complex (34 – 39 in Figure 1.7) having O,N, and S atoms have been prepared and structurally characterized in order to model in order to model nitrogenases [85 – 90].