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.

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Crystal structure analysis and refinement at 2.0 A resolution of a rhombohedral crystal form of human annexin V at high calcium concentration revealed a domain motion compared to the previously analysed hexagonal crystal form. Five calcium ions were located on the convex face of the molecule. Three strongly bound calciums are liganded at protruding interhelical loops and Asp or Glu residues in homologous positions in repeats I, II and IV. Five proteinaceous oxygens and one solvent molecule form the coordination polyhedron in each case. The unoccupied seventh site is suggested as the phospholipid headgroup binding site. Two more weakly bound sites were identified by lanthanum labelling. The structural features suggest that annexin V attaches with its convex face to membranes by specified calcium mediated interactions with at least three phospholipids. The adjacent membrane bilayer may thus become locally disordered and permeable to allow calcium inflow through the central polar channel of the molecule.

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