Bacterioferritin of Escherichia coli, also known as cytochrome b1, is a hollow, nearly spherical shell made up of 24 identical protein subunits and 12 haems. We have solved this structure in a tetragonal crystal form at 2.9 A resolution. We find that each haem is bound in a pocket formed by the interface between a pair of symmetry-related subunits. The quasi-twofold axis of the haem is closely aligned with the local twofold axis relating these subunits. The axial ligands of the haem are sulphurs of two equivalent methionyl residues (Met 52) from the symmetry-related subunits. A cluster of four water molecules is trapped in the gap between the upper edge of the haem and two extended protein loops which close off the haem from the outer aqueous environment. This is the first structure of a bis-methionine ligated haem-binding site and the first case of a twofold symmetric haem-binding site.

A complex of concanavalin A with methyl alpha‐D‐mannopyranoside has been crystallized in space group P212121 with a = 123.9 A, b = 129.1 A and c = 67.5 A. X‐ray diffraction intensities to 2.9 A resolution have been collected on a Xentronics/Nicolet area detector. The structure has been solved by molecular replacement where the starting model was based on refined coordinates of an I222 crystal of saccharide‐free concanavalin A. The structure of the saccharide complex was refined by restrained least‐squares methods to an R‐factor value of 0.19. In this crystal form, the asymmetric unit contains four protein subunits, to each of which a molecule of mannoside is bound in a shallow crevice near the surface of the protein. The methyl alpha‐D‐mannopyranoside molecule is bound in the C1 chair conformation 8.7 A from the calcium‐binding site and 12.8 A from the transition metal‐binding site. A network of seven hydrogen bonds connects oxygen atoms O‐3, O‐4, O‐5 and O‐6 of the mannoside to residues Asn14, Leu99, Tyr100, Asp208 and Arg228. O‐2 and O‐1 of the mannoside extend into the solvent. O‐2 is hydrogen‐bonded through a water molecule to an adjacent asymmetric unit. O‐1 is not involved in any hydrogen bond and there is no fixed position for its methyl substituent.

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The three-dimensional structure of the complex between methyl a-D-mannopyranoside and concanavalin A has been refined at 2.0 A resolution. Diffraction data were recorded from a single crystal (space group P21212~, a = 123.7, b = 128.6, c = 67.2 A) using synchrotron radiation at a wavelength of 1.488 A. The final model has good geometry and an R factor of 19.9% for 58871 reflections (82% complete), within the resolution limits of 8 to 2 A, with F > 1.0tr(F). The asymmetric unit contains four protein subunits arranged as a dimer of dimers with approximate 222 point symmetry. Each monomer binds one saccharide molecule. Each sugar is bound to the protein by hydrogen bonds and van der Waals contacts. Although the four subunits are not crystallographically equivalent, the protein-saccharide interactions are nearly identical in each of the four binding sites. The differences that do occur between the four sites are in the structure of the water network which surrounds each saccharide; these networks are involved in crystal packing. The structure of the complex is compared with a refined saccharide-free concanavalin A structure. The saccharide-free structure is composed of crystallographically identical subunits, again assembled as a f To whom all correspondence should be addressed.(" 1994 International Union or" Crystallography Printed in Great Britain -all rights reserved dimer of dimers, but with exact 222 symmetry. In the saccharide complex the tetramer association is different in that the monomers tend to separate resulting in fewer intersubunit interactions. The average temperature factor of the mannoside complex is considerably higher than that of the saccharide-free protein. The binding site in the saccharide-free structure is occupied by three ordered water molecules and the side chain of Asp71 from a neighbouring molecule in the crystal. These occupy positions similar to those of the four saccharide hydroxyls which are hydrogen bonded to the site. Superposition of the saccharide-binding site from each structure shows that the major changes on binding involve expulsion of these ordered solvents and the reorientation of the side chain of Tyrl00. Overall the surface accessibility of the saccharide decreases from 370 to 100 A 2 when it binds to the protein. This work builds upon the earlier studies of Derewenda et al.