Vanadium and proteins: Uptake, transport, structure, activity and function

Abstract Vanadium is an element ubiquitously present in our planet's crust and thus there are several organisms that use vanadium for activity or function of proteins. Examples are the vanadium-dependent haloperoxidases and the vanadium-containing nitrogenases. Some organisms that use vanadium have extremely efficient and selective protein-dependent systems for uptake and transport of vanadium and are able to accumulate high levels of vanadium from seawater, vanabins being a unique family of vanadium binding proteins found in ascidians involved in this process. For all of the systems a discussion regarding the role of the V-containing proteins is provided, mostly centered on structural aspects of the vanadium site and, when possible or relevant, relating this to the mechanisms operating. Phosphate is very important in biological systems and is involved in an extensive number of biological recognition and bio-catalytic systems. Vanadate(V) is able to inhibit many of the enzymes involved in these processes, such as ATPases, phosphatases, ribonucleases, phosphodiesterases, phosphoglucomutase and glucose-6-phosphatase, and it appears clear that this is closely related to the analogous physicochemical properties of vanadate and phosphate. The ability of vanadium to interfere with the metabolic processes involving Ca2+ and Mg2+, connected with its versatility to undergo changes in coordination geometry, allow V to influence the function of a large variety of phosphate-metabolizing enzymes and vanadate(V) salts and compounds have been frequently used either as inhibitors of these enzymes, or as probes to study the mechanisms of their reactions and catalytic cycle. In this review we give an overview of the many examples so far reported, also disclosing that vanadate(IV) may also have an equally efficient inhibiting effect. The prospective application of vanadium compounds as therapeutics has also been an important topic of research. How vanadium may be transported in blood and up-taken by cells are particularly relevant issues, this being mainly dependent on transferrin (and albumin) present in blood plasma. The thousands of studies reported on the effects of vanadium compounds reflect the complexity of the interactions occurring. Although it is not easy to anticipate/determine if a particular effect observed in a test tube or in vitro is also going to take place in vivo, it is clear that vanadium ions may interfere with many metabolic processes at many distinct levels. Emphasis is given on structural and functional aspects of vanadium–protein interactions relevant for vanadium binding and/or for clarification of role of the metal center in the reaction mechanisms. The additional knowledge that the presence of vanadium can change the action of a protein, other than simply inhibiting it, may also be important to understand how vanadium affects biological systems. This possibility, together with the vanadate–phosphate analogy further potentiates the belief that vanadium probably has relevant functions in living beings, which may involve interaction or incorporation of the metal ion and/or its compounds with several proteins.