The Desulfovibrio gigas aldehyde oxidoreductase contains molybdenum bound to a pterin cofactor and [2Fe-2S] centers. The enzyme was characterized by SDSPAGE, gel-filtration and analytical ultracentrifugation experiments. It was crystallized at !"C, pH 7.2, using isopropanol and MgC1, as precipitants. The crystals diffract beyond 0.3-nm (3.0-A) resolution and belong to space group P6,22 or its enantiomorph, with cell dimensions u = b = 14.45 nm and c = 16.32 nm. There is one subunit/asymmetric unit which gives a packing density of 2.5 X nm'/Da (2.5 A3/Da), consistent with the experimental crystal density, p = 1.14 g/cm3. One dimer (approximately 2 X 100 kDa) is located on a crystallographic twofold axis.The detailed knowledge of the structure and the catalytic mechanism of molybdenum-containing enzymes has considerably increased during the last decade [I -61 and, in particular, the biochemistry of aldehyde oxidoreductases has been extensively studied [4, 51. Molybdenum is a relevant transition metal in biological systems. Two groups of molybdo-proteins have been described. In the first group Mo is associated with iron in a complex cluster type structure (FeMocofactor) in the nitrogenase enzyme. Recently, a structural model of the FeMoco was proposed based on crystallographic analysis, consisting of [4Fe,3S] and [lMo,3Fe,3S] clusters bridged by three nonprotein ligands [7, 81. In the second group Mo is contained in an organic structural component (pterin), designated as Mocofactor (molybdopterin), generally associated with ironsulfur centers and/or a flavin or heme center, as in the case of molybdenum oxotransferases [6].From sulfate-reducing bacteria (strict anaerobes) enzymes related to the second group have been isolated and characterized [9-171. They represent unique situations for the presence of such a group of enzymes in the prokaryotic world, since most often these proteins are studied in eukaryotes (e.g. plants, insects and higher animals).The molybdenum iron-sulfur protein (MOP) isolated from D. gigas has analogies with the molybdenum hydrox- An important advance on these studies was the possibility to isolate the enzyme from 57Fe-enriched media with obvious interest for an iron-sulfur-center site labelling, enhanced sensitivity of the Mossbauer studies (an advantage with respect to mammalian systems) and the possibility of a direct measurement of substrate binding [14]. Previously described molybdenum (V) resting-type and slow-type EPR signals and the extended-X-ray-absorption fine-structure (EXAFS) spectrum of the molybdenum center indicated close similarities to desulfo-xanthine oxidase (inactive) [lo-121. In addition, it was later demonstrated for MOP, a new Mo(V) EPR signal (rapid type 2) centered at EPR average g-value (gav) of 1.9742, similar to those observed for xanthine and aldehyde oxidases [12]. These rapid EPR signals have been shown, in these proteins, to be physiologically significant, as they develop within the enzymeturnover time scale. A molybdenum cofactor, liberated from the protei...