Chemistry Reference
In-Depth Information
It is interesting to point out that the amino acid sequence and structure of the active site of vanadium hal-
operoxidases is conserved within several families of phosphatases, with conservation of the amino acids involved
in vanadate binding in the one and phosphate binding in the other.
Information, particularly structural, concerning vanadium-dependent nitrogenases, is relatively limited. The
consensus is that they resemble the molybdenum nitrogenase in most aspects, except for the presence of a FeV
cofactor, and they will not be discussed further.
High levels of vanadium are found in the mushroom Amanita muscaria and in marine tunicates (sea-squirts).
In the former organism, a siderophore-like ligand which binds vanadium(IV) called amavidine is found.
Amavidine is a metal complex containing one equivalent of vanadium and two equivalents of the ligand S,S-2,2
0
-
hydroxyiminopropionic acid (
Figure 17.17
)
. The complex is very stable to hydrolysis, and has reversible one-
electron redox properties, suggestive of a possible role in biology as a one-electron redox mediator.
CH
3
CH
O
C
N
O
C
O
H
3
C
C
O
O
V
O
C
H
CH
3
O
O
O
N
C
O
CH
CH
3
FIGURE 17.17
Structure of amavadine.
Vanadium, as VOSO
4
, has been found to interfere with siderophore-mediated iron transport in bacteria and
plants. This seems to imply that vanadium can be transported by siderophores, and a number of studies focussing
on applications of hydroxamate V-complexes in biology have been initiated.
Tunicates (ascidians or sea-squirts) are invertebrate marine organisms which can accumulate vanadium at
concentrations approaching 350 mM (the concentration of vanadium in seawater is
35 nM!). This vanadium is
taken up as V(V) from seawater (
Figure 17.18
), reduced to oxidation state III or IVand stored in a soluble form in
the blood cells within very acidic vacuoles at concentrations a million fold higher than in their external
surroundings. Vanadium seems to be bound in the cytoplasm to vanadium binding proteins (vanabins, of
molecular weights 12
w
16 kD). However, the precise role of vanadium in these marine organisms remains
unknown. AV transporter of the DMT1 family of membrane metal transporters has been recently cloned from an
ascidian (
Ueki, Furano, & Michibata, 2011
)
.
Finally, we briefly consider the insulin-like effect of vanadium compounds. As was pointed out in Chapter 5,
the regulation of intermediary metabolism is a very complex phenomenon, and there are few examples less
complicated than the action of insulin, which through interaction with its receptor in a large number of target
tissues initiates a series of signalling cascades, which affect carbohydrate and lipid metabolism, but also have
many other metabolic repercussions. Vanadium compounds have been shown to enhance the effect of insulin by
stimulating the phosphorylation of the insulin receptor (in vitro!) and inhibiting protein phosphatases. They also
seem, particularly in the case of the vanadyl cation, to bind to transferrin, thereby facilitating their entry into cells
e
