Editing Vanabin

{{Short description|Family of vanadium-binding proteins}}
[[Image:1VFI.png|thumb|3-D representation of the structure of Vanabin2 from ''[[Ascisia sydneiensis]]'' var. ''samea'']]

'''Vanabins''' (also known as '''vanadium-associated proteins''' or '''vanadium chromagen''') are a group of [[vanadium]]-binding [[metalloprotein]]s. Vanabins are found almost exclusively in the [[blood cells]], or [[vanadocyte]]s, of some [[tunicate]]s (sea squirts), including the [[Ascidiacea]]. The vanabins extracted from tunicate vanadocytes are often called [[hemovanadin]]s. These organisms are able to concentrate vanadium from the surrounding [[seawater]], and vanabin proteins have been involved in collecting and accumulating this metal ion.<ref>{{cite journal |doi=10.1016/j.bbagen.2014.07.023|title=Participation of thioredoxin in the V(V)-reduction reaction by Vanabin2 |year=2014 |last1=Ueki |first1=Tatsuya |last2=Uwagaki |first2=Masayuki |last3=Yamamoto |first3=Sohei |last4=Michibata |first4=Hitoshi |journal=Biochimica et Biophysica Acta (BBA) - General Subjects |volume=1840 |issue=11 |pages=3238–3245 |pmid=25108063 |url=http://ir.lib.hiroshima-u.ac.jp/files/public/3/39966/20160513120110236009/BBAGenSub_1840_3238.pdf }}</ref> At present there is no conclusive understanding of why these organisms collect vanadium.

Vanadium has been reported in high concentrations in the blood of the [[sea cucumber]] ''[[Stichopus]]''.<ref>{{cite book | last = Phillips | first = Alexander |title = A Possible Source of Vanadium in Sedimentary Rocks | publisher =Princeton University | year = 1918 | url={{google books |plainurl=y |id=O6KxwrrC_LMC|page=473}}|page=473}}</ref> However, later research has found little or no vanadium in this and four other sea cucumber genera.<ref>{{cite journal | last1 = Ciereszko | first1 = L. |title = On the Occurrence of Vanadium in Holothurians | year = 1962 |doi=10.1016/0010-406X(62)90034-8  |last2 = Ciereszko | first2 = E | last3 = Harris | first3 = E | last4 =Lane | first4 = C | journal = Comparative Biochemistry and Physiology |volume = 7 | issue = 1–2 | pages = 127–9 | pmid = 14021342}}</ref>

==Possible function==
===Vanabins as oxygen carriers===
Because of the high concentration of vanadium in the blood, it has been assumed that vanabins are used for [[Respiration (physiology)|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 [[Millimolar|mM]], was found in the blood cells of ''[[Ascidia gemmata]]'' belonging to the [[Scientific classification|suborder]] [[Phlebobranchia]]. This concentration is 10,000,000 times higher than that in seawater.<ref name="PMID_12941642">{{cite journal|author1=Tatsuya Ueki |author2=Nobuo Yamaguchi |author3=Hitoshi Michibata |name-list-style=amp |title=Chloride channel in vanadocytes of a vanadium-rich ascidian ''Ascidia sydneiensis samea'' |journal=Comparative Biochemistry and Physiology |issue=1 |pages=91–98 |year=2003 |pmid=12941642 |url=http://ir.lib.hiroshima-u.ac.jp/metadb/up/74006214/CompBiochemPhys_B_136_91-98_2003.pdf |access-date=13 August 2013 |volume=136 |doi=10.1016/s1096-4959(03)00175-1 |url-status=dead |archive-url=https://web.archive.org/web/20140606215943/http://ir.lib.hiroshima-u.ac.jp/metadb/up/74006214/CompBiochemPhys_B_136_91-98_2003.pdf |archive-date=6 June 2014 }}</ref>

Vanabins accumulate vanadium in the blood cells and produce V(III) species and [[vanadyl]] ions (V(IV)) from orthovanadate ions (V(V)), with the use of [[Nicotinamide adenine dinucleotide|NADPH]] as a [[redox|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 (due to sulfuric acid in it), made possible by pumping hydrogen ions into the vacuoles by use of energy intensive H<sup>+</sup>-ATPase. All the use of NADPH and [[Adenosine triphosphate|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 [[predator]]s, [[parasite]]s and [[microorganism]]s.

==History==
German chemist [[Friedrich Wolfgang Martin Henze|Martin Henze]] discovered [[vanadium]] in [[ascidiacea]]ns in 1911.<ref name="henze1911">{{Cite journal |last=Henze |first=M. |date=1911 |title=Untersuchungen über das Blut der Ascidien. 1. Mitteilung. Die Vanadiumverbindung der Blutkörperchen |trans-title=Studies on the blood of sea squirts. I. Communication. The vanadium compound of the blood cells |journal=Biological Chemistry |language=de |volume=72 |issue=5–6 |pages=494–501 |doi=10.1515/bchm2.1911.72.5-6.494 |name-list-style=vanc|author-link=Friedrich Wolfgang Martin Henze|url=https://zenodo.org/record/1448780 }}</ref><ref name="michibata2002">Michibata H, Uyama T, Ueki T, Kanamori K (2002). Vanadocytes, cells hold the key to resolving the highly selective accumulation and reduction of vanadium in ascidians.  ''Microscopy Research and Technique.'' Volume 56 Issue 6, Pages 421 - 434</ref><ref>{{cite journal | doi = 10.1515/bchm2.1911.72.5-6.494 | title = Untersuchungen über das Blut der Ascidien. I. Mitteilung. Die Vanadiumverbindung der Blutkörperchen | year = 1911 | last1 = Henze | first1 = M. | journal = Hoppe-Seyler's Zeitschrift für physiologische Chemie | volume = 72 | issue = 5–6 | pages = 494–501| url = https://zenodo.org/record/1448780 }}</ref>

==References==
{{Reflist}}

==External links==
* {{Archive url|url=https://web.archive.org/web/20051218024255/http://www.sci.u-szeged.hu/kkk/vanadium4/Michibata.pdf|date=December 18, 2005|title=Characterization of Vanabins, Vanadium-Binding Proteins, Key Substances of Vanadium Accumulation in Ascidians}}
* [http://aem.asm.org/cgi/content/full/69/11/6442 Transferring vanabin genes to bacteria]

[[Category:Metalloproteins]]
[[Category:Vanadium compounds]]
[[Category:Ascidiacea]]