Protein ligands for molybdate. Specificity and charge stabilisation at the anion-binding sites of periplasmic and intracellular molybdate-binding proteins of Azotobacter vinelandii‡

Abstract

Electrostatic interactions are important in the binding of anions to proteins. In Gram negative bacteria, molybdate binds specifically to a periplasmic binding protein and a number of cytoplasmic binding proteins. The molybdate-binding site in an Azotobacter vinelandii periplasmic binding protein has been determined at the atomic level from the crystal structure of the protein with bound tungstate at 1.2 Å resolution. The periplasmic molybdate-binding protein is very similar to the sulfate-binding protein of Salmonella typhimurium. In both, the anions are completely buried and bound by seven hydrogen bonds donated by main-chain and neutral residues at the ends of α-helices. The specificity of the two proteins for binding their respective anions may be related to small differences in the sizes of the anions and the lengths 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-β structures with anti-parallel β-strands. Analysis of molybdate binding by the cytoplasmic binding proteins suggests that, unlike the α/β periplasmic binding proteins, molybdate binding in these proteins involves electrostatic interactions with positively charged residues. These findings are important in understanding anion-binding in proteins of different structural classes.