Section 23.4.4.1.2. Legume lectin family

Whereas the study on serine proteases described above focused on the buried water molecules, the study on the legume lectin family included all of the conserved water molecules in the first hydration sphere. A total of 11 crystal structures were superimposed, many of them containing two independently refined monomers, making a total of 21 crystallographically independent monomers (Loris et al., 1994). The six different proteins in the family (lentil lectin, pea lectin, Lythyrus lectin, Griffonia isolectin IV, Erythrina lectin and concanavalin A) have sequence identities ranging from 100% to 40%. Water molecules in two superimposed crystal structures were considered to occupy the same site if they were within a predefined distance of 1 Å from each other. Seven water sites were found to be conserved in all of the family members included in the study. Four of these interact with the manganese and calcium ions, and one is in the ligand-binding site. The other two stabilize secondary structures: a β-hairpin turn and a β-bulge. In all cases, the protein composition of the site was strictly conserved. A larger number of water molecules are conserved within groups of closely related members of the family. The majority of these sites are found in the interface between the two monomers that come together to form a continuous 12-stranded β-pleated sheet and around the metal and monosaccharide binding regions (Fig. 23.4.4.2). Three crystal forms of lentil lectin were available for the study, and it was observed that of the 33 water molecules conserved between the corresponding three structures, none are involved in crystal contacts.

Figure 23.4.4.2| top | pdf | View of the 33 conserved hydration sites in the lentil lectin crystal structures superimposed on the backbone of the lentil lectin dimer. In order to emphasize the twofold symmetry, the waters at the dimer interface are shown for both lectin monomers. Reprinted with permission from Loris et al. (1994). Copyright (1994) The American Society for Biochemistry & Molecular Biology.

If one could generalize from the two studies described above, the conclusion would be that the water molecules strictly conserved across families of homologous proteins are found either at the binding site, at the interface between domains, or bridging secondary-structure elements which would otherwise not be part of the well defined protein architecture. Furthermore, it is clear that evolutionary pressure exists to maintain the composition of the amino-acid residues with which these crucial water molecules interact at their respective protein binding sites. A more recent study of conserved water molecules in a large family of microbial ribonucleases confirms the conclusions obtained in the two studies presented here (Loris et al., 1999).