title: Zinc/calcium- and cadmium/cadmium-substituted concanavalin A: interplay of metal binding, pH and molecular packing.
abstract: The crystal structures of cadmium/cadmium and zinc/calcium concanavalin A (con A) at pH 5.0 and pH 6.15, respectively, were determined. The structure of cadmium/cadmium con A confirms that the secondary Cd(2+)-binding site S3 is empty at pH 5. The metal-binding sites S1 and S2 are only very slightly affected by the substitution with cadmium. On the other hand, S1 and S2 and most of the protein surface of zinc/calcium con A at pH 6.15 differ from other fully metal-bound and carbohydrate-free structures. Most of these structural differences at the protein surface are a result of the interplay between metal binding, protonation and crystal packing. This interplay is expressed by relative rotations and translations of the con A units in alternative crystal packings and participation in space-group conversions inside crystals in situ. The particular crystal packing of zinc/calcium con A creates a novel zinc-binding site S4. The Zn(2+) ion in S4 ligates two aspartates from one tetramer and a histidine from a symmetry-related tetramer.
authors.affiliation: Laboratorium voor Ultrastructuur, Vlaams Interuniversitair Instituut voor Biotechnologie, Vrije Universiteit Brussel, Paardenstraat 65, B-1640 Sint-Genesius-Rode, Belgium. bouckaej@vub.ac.be
authors.forename: J
authors.initials: J
authors.lastname: Bouckaert
authors.forename: R
authors.initials: R
authors.lastname: Loris
authors.forename: L
authors.initials: L
authors.lastname: Wyns
mesh_terms: D001224:Aspartic Acid / Q000737:chemistry; D001665:Binding Sites; D002104:Cadmium / Q000737:chemistry*; D002118:Calcium / Q000737:chemistry*; D003208:Concanavalin A / Q000737:chemistry*; D007887:Fabaceae / Q000737:chemistry; D006632:Histamine / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D037121:Plant Lectins; D010946:Plants, Medicinal; D011487:Protein Conformation; D014961:X-Ray Diffraction; D015032:Zinc / Q000737:chemistry*
chemical_list: D037121:Plant Lectins; D002104:Cadmium; D003208:Concanavalin A; D001224:Aspartic Acid; D006632:Histamine; D015032:Zinc; D002118:Calcium