6.1.2. Aqueous V(III) Chemistry 877 6.1.3. Oxidation State of Vanadium in Tunicates 878 6.1.4. Uptake of Vanadate into Tunicates 879 6.1.5. Vanadium Binding Proteins: Vanabins 879 6.1.6. Model Complexes and Their Chemistry 880 6.1.7. Catechol-Based Model Chemistry 880 6.1.8. Vanadium Sulfate Complexes 881 6.2. Fan Worm Pseudopotamilla occelata 883 7. Vanadium Nitrogenase 883 7.1. Nitrogenases 883 7.2. Biochemistry of Nitrogenase 884 7.3. Clusters in Nitrogenase and Model Systems: Structure and Reactivity 885

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The chemistry and biochemistry of vanadium and the biological activities exerted by vanadium compounds.

Publisher Summary This chapter provides an overview of the structure and mechanism of the (Na, K)-ATPase. The (Na, K)-ATPase becomes concentrated into membrane fragments of unusually high density, which may be separated from other proteins and lipids by density gradient centrifugation. However, the major uncertainties currently debated are the size and subunit stoichiometry of the active pump, whether or not subunit interactions are necessary for transport, and whether or not Na+ and K+ can simultaneously bind to their respective sites on the protein. The Na+ ions leave the binding sites and are replaced by K+ ions from the outside of the cell. The presence of K+ ions near the phosphate anhydride catalyze dephosphorylation, allowing the aspartate residue to relax toward its original stable state. The large K+ ion, however, prevents complete relaxation and an intermediate is formed with the aspartate hindering K+ release to either side of the membrane. ATP rebinding to this state forces the aspartate into its original position, thereby blocking access from the extracellular side and allowing K+ release on the cytoplasmic side

Structure and Mechanism of the (Na,K)-ATPase

Vanadium, recent advancements and research prospects: A review.

Vanadium is an essential element. It is found widely distributed in minor and trace amounts throughout the lithosphere and biosphere. Vanadium is present in all mammalian tissues at concentrations of about 10 µM or less. Numerous physiological effects of this element are known at both the organismic and cellular levels, and in some instances these effects can be explained in terms of its biochemistry. Vanadate(V) and/or vanadyl(IV) ions are very potent inhibitors (K1 ≲ 10−6 M) of certain phosphatases, ATPases, phosphotransferases, nucleases, and kinases, among others. Vanadate(V) activates adenylate kinase. These ions have been quite useful as kinetic and spectroscopic (EPR) probes of complex cellular processes. Vanadium is an insulin mimetic agent. The ability of vanadium at low concentrations to influence enzymes is probably important to its function in vivo. Much of the biochemistry of vanadium is intricately related to the redox and coordination chemistry of this element. In particular, the parallel between the chemistries of vanadates and phosphates can account for the observed inhibition of various phosphohydrolases. However, there is ample evidence that vanadium in the body exists primarily as the vanadyl(IV) ion, VO2+, complexed to proteins and other cellular components; several biological reducing agents can convert vanadium(V) to this form. Serum transferrin appears to be involved in the metabolism of vanadium, perhaps as the transport protein for VO2+. Despite recent major advances in our understanding of the biochemistry of vanadium, the delineation of the physiological function(s) of this important element has remained an elusive goal.

The biochemistry of vanadium

The Na+,K+-transporting adenosine triphosphatase (Na+,K+-ATPase), also often known as the sodium pump or sodium-potassium pump, is an enzyme, found in nearly all animal-cell membranes, that uses energy from the hydrolysis of intracellular ATP to transport Na+ ions outwards and K+ ions inwards. It may be thought of as having three substrates (ATP, intracellular Na+ ions, and extracellular K+ ions) and four products (ADP, orthophosphate, extracellular Na+ ions, and intracellular K+ ions.) Because more Na+ ions are pumped out than K+ ions are pumped in, the activity of the enzyme generates an outward movement of positive charge, and this outward current may also be considered a “product” of the reaction.

Ian G. Macara

Papers

Glutathione reduces cytoplasmic vanadate. Mechanism and physiological implications