PROTEIN LIGANDS FOR MOLYBDATE - SPECIFICITY AND CHARGE STABILIZATION AT THE ANION-BINDING SITES OF PERIPLASMIC AND INTRACELLULAR MOLYBDATE-BINDING PROTEINS OF AZOTOBACTER-VINELANDII

Electrostatic interactions are important in the binding of anions to p roteins. In Gram negative bacteria, molybdate binds specifically to a periplasmic binding protein and a number of cytoplasmic binding protei ns. The molybdate-binding site in an Azotobacter vinelandii periplasmi c binding protein has been determined at the atomic level from the cry stal structure of th-protein with bound tungstate at 1.2 Angstrom reso lution. The periplasmic molybdate-binding protein is very similar to t he sulfate-binding protein of Salmonella typhimurium. In both, the ani ons are completely buried and bound by seven hydrogen bonds donated by main-chain and neutral residues at the ends of alpha-helices. The spe cificity of the two proteins for binding their respective anions may b e related to small differences in the sizes of the anions and the leng ths of the bonds formed. In the cytoplasm three distinct proteins have similar 7 kDa molybdate-binding domains. Secondary structure analysis indicates that the domains are all-beta structures with anti-parallel beta-strands. Analysis of molybdate binding by the cytoplasmic bindin g proteins suggests that, unlike the alpha/beta periplasmic binding pr oteins, molybdate binding in these proteins involves electrostatic int eractions with positively charged residues. These findings are importa nt in understanding anion-binding in proteins of different structural classes.