A novel vanadium reductase, Vanabin2, forms a possible cascade involved in electron transfer

Abstract

The unusual ascidian ability to accumulate high levels of vanadium ions at concentrations of up to 350 mM, a 10(7)-fold increase over that found in seawater, has been attracting interdisciplinary attention for a century. Accumulated V(V) is finally reduced to V(III) via V(IV) in ascidian vanadocytes. Reducing agents must therefore participate in the reduction. Previously, we identified a vanadium-binding protein, Vanabin2, in which all 18 cysteines form nine disulfide bonds. Here, we report that Vanabin2 is a novel vanadium reductase because partial cleavage of its disulfide bonds results in the reduction of V(V) to V(IV). We propose that Vanabin2 forms a possible electron transfer cascade from the electron donor, NADPH, via glutathione reductase, glutathione, and Vanabin2 to the acceptor, and vanadium ions conjugated through thiol-disulfide exchange reactions.

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

“…The 15 N HSQC spectra provided information regarding the amino acids implicated in the V IV binding as their resonances are missing or altered when compared to the native spectrum ( Fig. 3) [47]. In fact, the obtained results show that V IV ions, which are exclusively localized on the same face of the molecule, are coordinated by amine-N donors from lysines, arginines and histidines, corroborating the EPR and ESEEM data previously collected [44] [45].…”

“…and have also been recognized as V IV binding proteins [47]. Structural modeling of vanabins 1, 3, and 4, based on the NMR data of vanabin2, resulted in fairly good homologous structures.…”

Hyperaccumulation of vanadium in the Antarctic polychaete Perkinsiana littoralis as a natural chemical defense against predation

“…The family of vanabins now includes at least five closely related small proteins (vanabin1 to vanabin 4 and vanabin P) of approximately 90 a.a. (Kawakami et al 2009). Novel vanadium binding proteins not homologous to vanabins have been recently isolated from blood cells of A. sydneiensis samea, including VBP129, AsGST-I, and AsGST-II having a striking homology to glutathione S-transferases (Yoshihara et al 2008).…”

“…These processes have been well characterized in ascidian vanadocytes where vanadium accumulated from seawater is bound to vanabins in the cytosol and reduced from V(V) to V(IV) in the presence of GSH, forming an electron transfer cascade from NADPH, via glutathione reductase and vanabin2, to the vanadium ions conjugated through thiolsulfide exchange reactions (Michibata et al 2002;Ueki et al 2003;Kawakami et al 2009). The V(IV) would be transported into vacuoles and further reduced to V(III) by unknown reductants.…”

Metal-binding domains and the metal selectivity of the vanadium(IV)-binding protein VBP-129 in blood plasma

“…Recently, our group reported that Vanabin2 can act as a V V -reductase in a thiol-disulfide exchange reaction cascade that included GSH and NADPH [27]. To examine the possibility that such a reduction can occur in blood plasma, we performed ESR analysis to examine V V -reductase activity of VBP-129 as described previously.…”