The crystal structure of the aldehyde oxido-reductase (Mop) from the sulfate reducing anaerobic Gram-negative bacterium Desulfovibrio gigas has been determined at 2.25 A resolution by multiple isomorphous replacement and refined. The protein, a homodimer of 907 amino acid residues subunits, is a member of the xanthine oxidase family. The protein contains a molybdopterin cofactor (Mo-co) and two different [2Fe-2S] centers. It is folded into four domains of which the first two bind the iron sulfur centers and the last two are involved in Mo-co binding. Mo-co is a molybdenum molybdopterin cytosine dinucleotide. Molybdopterin forms a tricyclic system with the pterin bicycle annealed to a pyran ring. The molybdopterin dinucleotide is deeply buried in the protein. The cis-dithiolene group of the pyran ring binds the molybdenum, which is coordinated by three more (oxygen) ligands.

The polypeptide fold of NAP and the arrangement of the cofactors is related to that of Escherichia coli formate dehydrogenase (FDH) and distantly resembles dimethylsulphoxide reductase. The close structural homology of NAP and FDH shows how small changes in the vicinity of the molybdenum catalytic site are sufficient for the substrate specificity.

The utilization of organized supramolecular assemblies to exploit the synergistic interactions afforded by close proximity, both for enzymatic synthesis and for the degradation of recalcitrant substrates, is an emerging theme in cellular biology. Anaerobic bacteria harness a multiprotein complex, termed the ''cellulosome,'' for efficient degradation of the plant cell wall. This megadalton catalytic machine organizes an enzymatic consortium on a multifaceted molecular scaffold whose ''cohesin'' domains interact with corresponding ''dockerin'' domains of the enzymes. Here we report the structure of the cohesin-dockerin complex from Clostridium thermocellum at 2.2-Å resolution. The data show that the ␤-sheet cohesin domain interacts predominantly with one of the helices of the dockerin. Whereas the structure of the cohesin remains essentially unchanged, the loop-helix-helix-loop-helix motif of the dockerin undergoes conformational change and ordering compared with its solution structure, although the classical 12-residue EF-hand coordination to two calcium ions is maintained. Significantly, internal sequence duplication within the dockerin is manifested in near-perfect internal twofold symmetry, suggesting that both ''halves'' of the dockerin may interact with cohesins in a similar manner, thus providing a higher level of structure to the cellulosome and possibly explaining the presence of ''polycellulosomes.'' The structure provides an explanation for the lack of cross-species recognition between cohesin-dockerin pairs and thus provides a blueprint for the rational design, construction, and exploitation of these catalytic assemblies.

Reducing CO 2 is a challenging chemical transformation that biology solves easily, with high efficiency and specificity. In particular, formate dehydrogenases are of great interest since they reduce CO 2 to formate, a valuable chemical fuel and hydrogen storage compound. The metal-dependent formate dehydrogenases of prokaryotes can show high activity for CO 2 reduction. Here, we report an expression system to produce recombinant W/Sec-FdhAB from Desulfovibrio vulgaris Hildenborough fully loaded with cofactors, its catalytic characterization and crystal structures in oxidized and reduced states. The enzyme has very high activity for CO 2 reduction and displays remarkable oxygen stability. The crystal structure of the formate-reduced enzyme shows Sec still coordinating the tungsten, supporting a mechanism of stable metal coordination during catalysis. Comparison of the oxidized and reduced structures shows significant changes close to the active site. The DvFdhAB is an excellent model for studying catalytic CO 2 reduction and probing the mechanism of this conversion.