The tungsten–iron–sulfur enzyme acetylene hydratase stands out from its class because it catalyzes a nonredox reaction, the hydration of acetylene to acetaldehyde. Sequence comparisons group the protein into the dimethyl sulfoxide reductase family, and it contains a bis-molybdopterin guanine dinucleotide-ligated tungsten atom and a cubane-type [4Fe:4S] cluster. The crystal structure of acetylene hydratase at 1.26 Å now shows that the tungsten center binds a water molecule that is activated by an adjacent aspartate residue, enabling it to attack acetylene bound in a distinct, hydrophobic pocket. This mechanism requires a strong shift of pK_aof the aspartate, caused by a nearby low-potential [4Fe:4S] cluster. To access this previously unrecognized W–Asp active site, the protein evolved a new substrate channel distant from where it is found in other molybdenum and tungsten enzymes.

The iron-sulfur flavoenzyme adenylylsulfate (adenosine 5′-phosphosulfate, APS) reductase catalyzes reversibly the reduction of APS to sulfite and AMP. The structures of APS reductase from the hyperthermophilic Archaeoglobus fulgidus in the two-electron reduced state and with sulfite bound to FAD are reported at 1.6- and 2.5-Å resolution, respectively. The FAD-sulfite adduct was detected after soaking the crystals with APS. This finding and the architecture of the active site strongly suggest that catalysis involves a nucleophilic attack of the N5 atom of reduced FAD on the sulfur atom of APS. In view of the high degree of similarity between APS reductase and fumarate reductase especially with regard to the FAD-binding α-subunit, it is proposed that both subunits originate from a common ancestor resembling archaeal APS reductase. The two electrons required for APS reduction are transferred via two [4Fe-4S] clusters from the surface of the protein to FAD. The exceptionally large difference in reduction potential of these clusters (−60 and −500 mV) can be explained by interactions of the clusters with the protein matrix.

The soluble tungsten, iron-sulfur enzyme acetylene hydratase (AH) from mesophilic Pelobacter acetylenicus is a member of the dimethyl sulfoxide (DMSO) reductase family. It stands out from its class as it catalyzes a nonredox reaction, the addition of H ₂ O to acetylene (H—C≡C—H) to form acetaldehyde (CH ₃ CHO). Caught in its active W(IV) state, the high-resolution three-dimensional structure of AH offers an excellent starting point to tackle its unique chemistry and to identify catalytic amino acid residues within the active site cavity: Asp13 close to W(IV) coordinated to two molybdopterin-guanosine-dinucleotide ligands, Lys48 which couples the [4Fe-4S] cluster to the W site, and Ile142 as part of a hydrophobic ring at the end of the substrate access channel designed to accommodate the substrate acetylene. A protocol was developed to express AH in Escherichia coli and to produce active-site variants which were characterized with regard to activity and occupancy of the tungsten and iron-sulfur centers. By this means, fusion of the N-terminal chaperone binding site of the E. coli nitrate reductase NarG to the AH gene improved the yield and activity of AH and its variants significantly. Results from site-directed mutagenesis of three key residues, Asp13, Lys48, and Ile142, document their important role in catalysis of this unusual tungsten enzyme.

The Mo enzyme transhydroxylase from the anaerobic microorganism Pelobacter acidigallici catalyzes the conversion of pyrogallol to phloroglucinol. Such trihydroxybenzenes and their derivatives represent important building blocks of plant polymers. None of the transferred hydroxyl groups originates from water during transhydroxylation; instead a cosubstrate, such as 1,2,3,5-tetrahydroxybenzene, is used in a reaction without apparent electron transfer. Here, we report on the crystal structure of the enzyme in the reduced Mo(IV) state, which we solved by single anomalous-diffraction technique. It represents the largest structure (1,149 amino acid residues per molecule, 12 independent molecules per unit cell), which has been solved so far by single anomalous-diffraction technique. Tranhydroxylase is a heterodimer, with the active Mo–molybdopterin guanine dinucleotide (MGD) ₂ site in the α-subunit, and three [4Fe—4S] centers in the β-subunit. The latter subunit carries a seven-stranded, mainly antiparallel β-barrel domain. We propose a scheme for the transhydroxylation reaction based on 3D structures of complexes of the enzyme with various polyphenols serving either as substrate or inhibitor.