The structure of Bacillus amyloliquefaciens ribonuclease (barnase), an extracellular 110-residue enzyme initially solved at 2.0 A resolution, has been refined at 1.5 A using synchrotron radiation and an imaging-plate scanner. Refinement with anisotropic atomic displacement parameters resulted in increased accuracy of the structure. The final model has a crystallographic R factor of 11.5% and an Rfree of 17.4%. The three independent molecules in the asymmetric unit, referred to as A, B and C, allowed detailed analysis of this final model and meaningful comparison with structures of barnase complexed either with nucleotide inhibitors or with its natural intracellular inhibitor, barstar. The analysis of the overall solvent structure revealed a similar number of water molecules associated with each barnase molecule; among these were 16 equivalent buried solvent molecules, the locations of which are discussed in detail and classified on the basis of their structural role. The importance of the water molecules' contribution to the barnase-barstar interaction is also highlighted. The high accuracy of the present analysis revealed the presence of a Zn2+ ion mediating the contacts between pairs of symmetry-related A, B or C molecules; such an ion had previously only been identified for pairs of C molecules.

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The crystal structures of a biologically and therapeutically active recombinant homotrimeric fragment of human lung surfactant protein D with a series of bound ligands have been determined. While the structures reveal various different binding modes, all utilise a similarly positioned pair of mannose-type O3′ and O4′ hydroxyls with no direct interaction between any non-terminal sugar and protein. The orientation, position, and interactions of the bound terminal sugar depend on the sugar itself, the presence and form of glycosidic linkage, and the environment in the crystal, which, via Asp325, places stereochemical and electronic constraints, different for the three different subunits in the homotrimer, on the ligand-binding site. As a direct consequence of this influence, the other binding-pocket flanking residue, Arg343, exhibits variable conformation and variable interactions with bound ligand and leaves open to question which orientation of terminal mannobiose, and of other terminal disaccharides, may be present in extended physiological ligands. The combined structural evidence shows that there is significant flexibility in recognition; that Asp325, in addition to Arg343, is an important determinant of ligand selectivity, recognition, and binding; and that differences in crystal contact interfaces exert, through Asp325, significant influence on preferred binding modes.

The symmetry elements detected by the self-rotation and the Patterson functions, associated with strong correlations between the positions of the molecules in the asymmetric unit, are used to reduce the effective number of independent bodies to be located by the molecular replacement method. A distinction is made between 'frustrated' crystallographic symmetries, i.e.those that are almost crystallographic ones, and "standard' non-crystallographic symmetries, which are taken into account by specific techniques. These have been successfully applied to many-body macromolecular crystal structures, with important savings in time and computational effort.