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 anomalousdiffraction 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 anomalousdiffraction technique. Tranhydroxylase is a heterodimer, with the active Mo-molybdopterin guanine dinucleotide (MGD)2 site in the ␣-subunit, and three [4FeO4S] centers in the ␤-subunit. The latter subunit carries a seven-stranded, mainly antiparallel ␤-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.

Acetylene hydratase is a tungsten-containing hydroxylase that converts acetylene to acetaldehyde in a unique reaction that requires a strong reductant. The subsequent disproportionation of acetaldehyde yields acetate and ethanol. Crystals of the tungsten/iron±sulfur protein acetylene hydratase from Pelobacter acetylenicus strain WoAcy 1 (DSM 3246) were grown by the vapour-diffusion method in an N 2 /H 2 atmosphere using polyethylene glycol as precipitant. Growth of crystals suitable for X-ray analysis strictly depended on the presence of Ti III citrate or dithionite as reducing agents.