Molecules of Escherichia coli MobB assemble into densely packed hollow cylinders in a crystal lattice with 75% solvent content

The crystal structure of Escherichia coli MobB, an enzyme involved in the final step of molybdenum-cofactor biosynthesis, forms intertwined dimers. Each molecule consists of two segments and requires the second monomer for stable folding. Dimerization buries a quarter of the solvent-accessible area of the monomer. These dimers assemble into a hexagonal lattice with P6₄22 symmetry and occupy only ∼25% of the unit-cell volume. The symmetry-related dimers associate tightly into a helical structure with a diameter of 250 Å and a pitch of 98 Å. Two such helices are intertwined, shifted by 49 Å along the sixfold axis. Within the crystal, these helices form thin-walled cylinders with an external diameter of 250 Å and an internal diameter of 190 Å. Their center is filled with solvent. These cylinders pack closely together, forming a hexagonal lattice with the highest possible packing density. This arrangement of dimers allows extensive intermolecular contacts with 75% solvent content in the crystal.