The history and changing function of tungsten as the heaviest element in biological systems is given. It starts from an inhibitory element/anion, especially for the iron molybdenum-cofactor (FeMoCo)-containing enzyme nitrogenase involved in dinitrogen fixation, as well as for the many "metal binding pterin" (MPT)-, also known as tricyclic pyranopterin- containing classic molybdoenzymes, such as the sulfite oxidase and the xanthine dehydrogenase family of enzymes. They are generally involved in the transformation of a variety of carbon-, nitrogen- and sulfur-containing compounds. But tungstate can serve as a potential positively acting element for some enzymes of the dimethyl sulfoxide (DMSO) reductase family, especially for CO(2)-reducing formate dehydrogenases (FDHs), formylmethanofuran dehydrogenases and acetylene hydratase (catalyzing only an addition of water, but no redox reaction). Tungsten even becomes an essential element for nearly all enzymes of the aldehyde oxidoreductase (AOR) family. Due to the close chemical and physical similarities between molybdate and tungstate, the latter was thought to be only unselectively cotransported or cometabolized with other tetrahedral anions, such as molybdate and also sulfate. However, it has now become clear that it can also be very selectively transported compared to molybdate into some prokaryotic cells by two very selective ABC-type of transporters that contain a binding protein TupA or WtpA. Both proteins exhibit an extremely high affinity for tungstate (K(D) < 1 nM) and can even discriminate between tungstate and molybdate. By that process, tungsten finally becomes selectively incorporated into the few enzymes noted above.

The Gram-positive anaerobe Eubacterium acidaminophilum contains at least two tungsten-dependent enzymes: viologen-dependent formate dehydrogenase and aldehyde dehydrogenase.185 W-Labeled tungstate was taken up by this organism with a maximum rate of 0.53 pmol min ؊1 mg ؊1 of protein at 36°C. The uptake was not affected by equimolar amounts of molybdate. The genes tupABC coding for an ABC transporter specific for tungstate were cloned in the downstream region of genes encoding a tungsten-containing formate dehydrogenase. The substrate-binding protein, TupA, of this putative transporter was overexpressed in Escherichia coli, and its binding properties toward oxyanions were determined by a native polyacrylamide gel retardation assay. Only tungstate induced a shift of TupA mobility, suggesting that only this anion was specifically bound by TupA. If molybdate and sulfate were added in high molar excess (>1000-fold), they were also slightly bound by TupA. The K d value for tungstate was determined to be 0.5 M. The genes encoding the tungstate-specific ABC transporter exhibited highest similarities to putative transporters from Methanobacterium thermoautotrophicum, Haloferax volcanii, Vibrio cholerae, and Campylobacter jejuni. These five transporters represent a separate phylogenetic group of oxyanion ABC transporters as evident from analysis of the deduced amino acid sequences of the binding proteins. Downstream of the tupABC genes, the genes moeA, moeA-1, moaA, and a truncated moaC have been identified by sequence comparison of the deduced amino acid sequences. They should participate in the biosynthesis of the pterin cofactor that is present in molybdenum-and tungstencontaining enzymes except nitrogenase.

The mop gene, encoding the molybdate-binding protein from Eubacterium acidaminophilum, was cloned using Clostridium pasteurianum mopI as a probe for heterologous hybridization. mop encodes a 69-amino-acid protein ( M(r) 7,328) with high sequence similarities to members of the molbindin protein family, which have been implicated in molybdenum storage and homeostasis. Northern blot analysis showed three mRNA transcripts (1.0, 1.6, and 2.6 kb) for mop. This result was obtained independent of the availability of tungstate in the growth medium. mop was overexpressed in Escherichia coli as a C-terminal Strep-tag fusion protein. On the basis of gel filtration, the native protein was a homohexamer of 48 kDa. The specificity of oxyanion binding was examined by protein mobility shift assay. Molybdate, tungstate, and chromate strongly changed the mobility of the protein in a native polyacrylamide gel, indicating the binding of these oxyanions to Mop. Other oxyanions, such as sulfate and phosphate, had no effect on Mop mobility. Mutational analysis revealed that the positive charge of the Arg-6, located in the conserved SARN region of Mop, was not directly involved in oxyanion binding.

2,4-Dichlorophenol hydroxylase (EC 1.14.13.20) was purified to apparent homogeneity from the bacterial strain S1, a member of the alpha-2 subgroup of the Proteobacteria. The molecular masses of the native enzyme and the subunit were determined to be 256 and 64 kDa, respectively, suggesting a homotetrameric structure. The enzyme converted 2,4-dichlorophenol to 3,5-dichlorocatechol. The apparent K(m) values for 2,4-dichlorophenol, NADPH and NADH were 3, 240 and 420 microM, respectively. The enzyme hydroxylated a broad range of halogenated phenols. 3-Chloro-, 2,6-dichloro- and 2,4,6-trichlorophenol acted as 'non-substrate' effectors. The N-terminal sequence revealed 72% identity with the amino acid sequence deduced from the pJP4-encoded tfdB gene.