Vanabins

Vanabins (also known as vanadium-associated proteins or vanadium chromagen) are a specific group of vanadium-binding metalloproteins. Found only in the blood of some ascidians and tunicates (sea squirts), these organisms are able to concentrate vanadium to a level more than 100 times higher than in the surrounding seawater. Vanabin proteins seem to be involved in collecting and accumulating this metal ion. At present there is no conclusive understanding of why these organisms collect vanadium, and it remains a biological mystery.

Theories

Vanabins as oxygen carriers

Because of the high concentration of vanadium in the blood, it has been assumed that vanabins are used for oxygen transport like iron-based hemoglobin or copper-based hemocyanin. Unfortunately no scientific evidence can be found to back this hypothesis.

The highest concentration of vanadium found so far, 350 mM, was found in the blood cells of "Ascidia gemmata" belonging to the suborder Phlebobranchia. This concentration is 10,000,000 times higher than that in seawater.

Vanabins accumulate vanadium in the blood cells and produce V(III) species and vandanyl ions (V(IV)) from orthovandate ions (V(V)), with the use of NADPH as a reducing agent. Vanabins also transport the reduced vanadium species into the vacuoles of vanadocytes (vanadium-containing blood cells). The vacuoles are kept at a very acidic pH of 1.9, made possible by pumping hydrogen ions into the vacuoles by use of energy intensive H⁺-ATPase. All the use of NADPH and ATP to collect and maintain the vanadium is extremely energy intensive, unusual for oxygen transporting proteins.

Other oxygen transporting proteins have a very low dissociation constant with their metal prosthetic group and bind these groups tightly. Vanabins on the other hand have a moderate dissociation constant and do not tightly bind vanadium. Most importantly, because of this moderate dissociation constant, vanadium is usually found free-floating and separated from any proteins inside the vacuoles. This is completely different from other oxygen transporting proteins.

Vanabins not oxygen carriers

The most convincing evidence against vanadium in use for oxygen transport is that many ascidians and tunicates also have hemocyanin in their blood that could be assumed to handle all oxygen transport.

The use of vanabins and vanadium for oxygen transport in ascidians and tunicates is doubtful. Another hypothesized reason for these organisms collecting vanadium is to make themselves toxic to predators, parasites and microorganisms.

Technological use of vanabins

The world oceans carry diluted concentrations of rare minerals. Total amount of gold, silver, manganese and platinum (among others) in all the world's oceans rivals that of terrestrial mines, yet because of the low concentration of these minerals in sea water, cost effective extraction has been considered improbable. Understanding the mechanisms behind how some organisms concentrate rare minerals from sea water could make it biotechnologically possible to grow and farm organisms to filter sea water and extract these minerals, similar to bioleaching.

References

*cite journal
author = Ueki T, Adachi T, Kawano S, Aoshima M, Yamaguchi N, Kanamori K, Michibata H.
title = Vanadium-binding proteins (vanabins) from a vanadium-rich ascidian Ascidia sydneiensis samea
journal = Biochim Biophys Acta
volume = 1626
issue = 1-3
pages = 43–50
year = 2003
doi = 10.1016/S0167-4781(03)00036-8
pmid = 12697328
* cite journal
author = Michibata H., Uyama T., Ueki T.
title = Vanadocytes, cells hold the key to resolving the highly selective accumulation and reduction of vanadium in ascidians
journal = Microscopy Research and Technique
volume = 56
issue = 6
pages = 421–434
year = 2002
doi = 10.1002/jemt.10042
* cite journal
author = Michibata H.
title = The mechanism of accumulation of vanadium by ascidians: Some progress towards an understanding of this unusual phenomenon
journal = Zoological Science
volume = 13
issue = 4
pages = 489–502
year = 1996
* cite journal
author = Ishit T., Nakai I., Okoshi K.
title = Biochemical Significance of Vanadium in a Polychaete Worm
journal = Metal Ions in Biological Systems
volume = 31
pages = 491–509
year = 1995
* cite journal
author = Martoja R., Gouzerh P., Monniot F.
title = Cytochemical studies of vanadium, tunichromes and related substances in ascidians: possible biological significance
journal = Oceanography and Marine Biology
volume = 32
pages = 531–556
year = 1994

External links

* http://www.sci.u-szeged.hu/kkk/vanadium4/Michibata.pdf
* [http://aem.asm.org/cgi/content/full/69/11/6442 Transferring vanabin genes to bacteria]